A Taxonomy for Mechanical Ventilation: 10 ...

A Taxonomy for Mechanical Ventilation: 10 Fundamental Maxims Robert L Chatburn MHHS RRT-NPS FAARC, Mohamad El-Khatib PhD MD RRT FAARC, and Eduardo Mireles-Cabodevila MD

Introduction What Is a Mode of Mechanical Ventilation? The 10 Maxims Application of the Taxonomy Discussion The Problem of Growing Complexity The Problem of Identifying Unique Modes The Problem of Teaching Mechanical Ventilation The Problem of Implementation Conclusions

The American Association for Respiratory Care has declared a benchmark for competency in mechanical ventilation that includes the ability to “apply to practice all ventilation modes currently available on all invasive and noninvasive mechanical ventilators.” This level of competency presupposes the ability to identify, classify, compare, and contrast all modes of ventilation. Unfortunately, current educational paradigms do not supply the tools to achieve such goals. To fill this gap, we expand and refine a previously described taxonomy for classifying modes of ventilation and explain how it can be understood in terms of 10 fundamental constructs of ventilator technology: (1) defining a breath, (2) defining an assisted breath, (3) specifying the means of assisting breaths based on control variables specified by the equation of motion, (4) classifying breaths in terms of how inspiration is started and stopped, (5) identifying ventilator-initiated versus patient-initiated start and stop events, (6) defining spontaneous and mandatory breaths, (7) defining breath sequences (8), combining control variables and breath sequences into ventilatory patterns, (9) describing targeting schemes, and (10) constructing a formal taxonomy for modes of ventilation composed of control variable, breath sequence, and targeting schemes. Having established the theoretical basis of the taxonomy, we demonstrate a step-by-step procedure to classify any mode on any mechanical ventilator. Key words: taxonomy; ontology; mechanical ventilation; mechanical ventilator; modes of ventilation; classification; ventilator; survey; standardized nomenclature; controlled vocabulary. [Respir Care 2014;59(11):1747–1763. © 2014 Daedalus Enterprises]

Introduction The American Association for Respiratory Care (AARC) has sponsored a number of conferences to outline the com-

petencies of the registered respiratory therapist (RRT) of the future.1-3 One of the competencies in the area of critical care was declared as the ability to “apply to practice

Dr El-Khatib is affiliated with the Department of Anesthesiology, American University of Beirut Medical Center, Beirut, Lebanon. Mr Chatburn and Dr Mireles-Cabodevila are affiliated with the Respiratory Institute, Cleveland Clinic, Cleveland, Ohio and the Lerner College of Medicine of Case Western Reserve University, Cleveland, Ohio.

Supplementary material related to this paper is available at http:// www.rcjournal.com.

RESPIRATORY CARE • NOVEMBER 2014 VOL 59 NO 11

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MECHANICAL VENTILATION

all ventilation modes currently available on all invasive and noninvasive mechanical ventilators, as well as all adjuncts to the operation of modes.”2 Kacmarek4 recently published a paper discussing the expectations of this future RRT regarding mechanical ventilation competencies. (Of course, these competencies apply to any clinician responsible for managing ventilated patients, as many countries do not have RRTs.) He states that: The RT of 2015 and beyond must be a technical expert on every aspect of the mechanical ventilator. They should be able to discuss all of the technical nuances of the mechanical ventilator. They should be able to compare the capabilities of one ventilator to the other. They should be able to discuss in detail the mechanism of action of all of the modes and adjuncts that exist on the mechanical ventilator.

He further says that “The RT of 2015 and beyond should be capable of defining the operational differences between each of these modes.” These statements seem reasonable at first glance, but further consideration reveals some major challenges. The number of modes of ventilation has grown exponentially in the last 3 decades. Consider just one popular textbook on respiratory care equipment5 that includes 174 unique names of modes on 34 different ventilators. The level of complexity in terms of the real number of unique modes is much greater: most ICU ventilators allow the operator to activate various features that modify a given mode and actually transform it into anther mode without any naming convention to signify the transition. The result is that there are many more unique modes (in terms of different patterns of patient-ventilator interaction) than there are names indicated on the ventilators, in operators’ manuals, or in textbooks. This growing complexity has generated an urgent need for a classification system (taxonomy) for modes of mechanical ventilation to facilitate the identification and comparison of the technical capabilities of ventilators. The purpose of this article is to describe a formal taxonomy for modes of mechanical ventilation (ie, a classification of modes into groups based on similar character-

Table 1.

Ten Basic Maxims for Understanding Ventilator Operation

(1) A breath is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flow vs time curve. (2) A breath is assisted if the ventilator provides some or all of the work of breathing. (3) A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system. (4) Breaths are classified according to the criteria that trigger (start) and cycle (stop) inspiration. (5) Trigger and cycle events can be either patient-initiated or ventilator-initiated. (6) Breaths are classified as spontaneous or mandatory based on both the trigger and cycle events. (7) Ventilators deliver 3 basic breath sequences: CMV, IMV, and CSV. (8) Ventilators deliver 5 basic ventilatory patterns: VC-CMV, VCIMV, PC-CMV, PC-IMV, and PC-CSV. (9) Within each ventilatory pattern, there are several types that can be distinguished by their targeting schemes (set-point, dual, biovariable, servo, adaptive, optimal, and intelligent). (10) A mode of ventilation is classified according to its control variable, breath sequence, and targeting schemes. CMV ⫽ continuous mandatory ventilation IMV ⫽ intermittent mandatory ventilation CSV ⫽ continuous spontaneous ventilation VC ⫽ volume control PC ⫽ pressure control

DOI: 10.4187/respcare.03057

istics) using a simple structured approach to teaching and learning the fundamental principles of ventilator operation. This taxonomy has recently been adopted by the ECRI (formerly the Emergency Care Research Institute) for describing and comparing ventilators.6 We do not discuss clinical application, but rather the technology that is the foundation for clinical application. This is a topic that we believe is not sufficiently discussed in current textbooks. We have developed this system over many years of clinical experience and instruction of medical students, physicians, and respiratory therapists in both the hospital and university environments. It is based on what we consider to be 10 fundamental theoretical constructs or maxims (Table 1) that are recognizable to most people familiar with mechanical ventilation.7 We demonstrate how these 10 maxims form the basis of the taxonomy. We also show how the taxonomy is a practical tool for dealing with the complexity represented by the many mode names mentioned above. Figure 1 illustrates a hierarchy of skills we believe must be mastered before one is fully able to use mechanical ventilation technology as suggested by the AARC competency statements. Note that this hierarchy is consistent with Bloom’s revised taxonomy of learning objectives (a classification of levels of intellectual behavior important in learning).8

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Mr Chatburn is a paid consultant for Philips Respironics, Covidien, Dra¨ger, Hamilton Medical, and ResMed. The other authors have disclosed no conflicts of interest. Correspondence: Robert L Chatburn MHHS RRT-NPS FAARC, Cleveland Clinic, M-56, 9500 Euclid Avenue, Cleveland, OH 44195. E-mail: [email protected].

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The following sections describe the 10 theoretical constructs that we believe form the basis of a practical syllabus for learning mechanical ventilation technology. They also provide the context for some basic definitions of terms used to construct a standardized vocabulary (see the supplementary materials at http://www.rcjournal.com). We start with very simple, intuitively obvious ideas and then build on these concepts to form a theoretical framework for understanding and using ventilators. (1) A breath is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flowtime curve. A breath is defined in terms of the flow-time curve (Fig. 3). By convention, positive flow (ie, values of flow above zero) is designated as inspiration. Negative flow (values below zero) indicates expiration. Inspiratory time is defined as the period from the start of positive flow to the start of negative flow. Expiratory time is defined as

the period from the start of negative flow to the start of positive flow. Total cycle time (also called the ventilatory period) is the sum of inspiratory and expiratory times. It is also equal to the inverse of breathing frequency (total cycle time ⫽ 1/frequency, usually expressed as 60 s/breaths/ min). The inspiratory-expiratory ratio is defined as the ratio of inspiratory time to expiratory time. The duty cycle (or percent inspiration) is defined as the ratio of inspiratory time to total cycle time. The tidal volume (VT) is the integral of flow with respect to time. For constant flow inspiration, this simply reduces to the product of flow and inspiratory time. (2) A breath is assisted if the ventilator provides some or all of the work of breathing. An assisted breath is one for which the ventilator does some portion of the work of breathing. This work may be defined, for example, as the integral of inspiratory transrespiratory pressure with respect to inspired volume. Graphically, this corresponds to airway pressure increasing above baseline during inspiration. Increased work of breathing per breath, as a result of increased resistive and/or elastic work, is characterized by increased transrespiratory pressure (for a definition of transrespiratory pressure, see the supplementary materials at http://www. rcjournal.com). In contrast, a loaded breath is one for which transrespiratory pressure decreases below baseline during inspiration9 and is interpreted as the patient doing work on the ventilator (eg, to start inspiration). A ventilator provides all of the mechanical work of inspiration (ie, full support) only if the patient’s inspiratory muscles are inactive (eg, drug-induced neuromuscular blockade). An unassisted breath is one for which the ventilator simply provides flow at the rate required by the patient’s inspiratory effort, and transrespiratory system pressure stays constant throughout the breath. An example of this would be CPAP delivered with a demand valve. A ventilator can assist expiration by making the transrespiratory pressure fall below baseline during expiration. An example of this is automatic tube compensation on the Evita XL ventilator (Dra¨ger, Lu¨beck, Germany). When tube compensation is activated, the ventilation pressure in the breathing circuit is increased during inspiration or decreased during expiration. The airway pressure is adjusted to the tracheal level if 100% compensation of the tube resistance has been selected. Another example is the use of a cough-assist device (eg, CoughAssist mechanical insufflator-exsufflator, Philips Respironics, Murrysville, Pennsylvania). In this case, transrespiratory pressure goes negative during expiration because pressure on the body surface is increased while pressure at the mouth remains at atmospheric pressure. (3) A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system. The theoretical framework for understanding control variables is the equation of motion


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Fig. 1. Pyramid of skills required to master ventilator technology. The terms in green are from Bloom’s revised taxonomy of learning objectives.

What Is a Mode of Mechanical Ventilation? A mode of mechanical ventilation may be defined, in general, as a predetermined pattern of patient-ventilator interaction. It is constructed using 3 basic components: (1) the ventilator breath control variable, (2) the breath sequence, and (3) the targeting scheme (Fig. 2) To understand each of these components, we use the maxims that form the basis for the taxonomy of mechanical ventilation. These 10 maxims describe, in a progressive manner, the rationale of how we classify modes by understanding what a mode does. Maxims 1–3 explain the ventilator breath control variable. Maxims 4 – 8 explain the breath sequence. Maxim 9 explains the targeting schemes. Maxim 10 pulls together the previous maxims to formulate the complete taxonomy. The 10 Maxims

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Fig. 2. Building blocks for constructing a mode. CMV ⫽ continuous mandatory ventilation; IMV ⫽ intermittent mandatory ventilation; CSV ⫽ continuous spontaneous ventilation.

˙ (t). for the passive respiratory system: P(t) ⫽ EV(t) ⫹ RV This equation relates pressure (P), volume (V), and flow ˙ ) as continuous functions of time (t) with the parameters (V of elastance (E) and resistance (R). If any one of the func˙ ) is predetermined, the other two may be tions (P, V, or V derived. The control variable refers to the function that is controlled (predetermined) during a breath (inspiration). This form of the equation assumes that the patient makes no inspiratory effort and that expiration is complete (no auto-PEEP). Volume control (VC) means that both volume and flow are pre-set prior to inspiration. Setting the VT is a necessary but not sufficient criterion for declaring volume control because some modes of pressure control allow the operator to set a target VT but allow the ventilator to determine the flow (see adaptive targeting scheme below). Similarly, setting flow is also a necessary but not sufficient criterion. Some pressure control modes allow the operator

to set the maximum inspiratory flow, but the VT depends on the inspiratory pressure target and respiratory system mechanics. Pressure control (PC) means that inspiratory pressure as a function of time is predetermined. In practice, this currently means pre-setting a particular pressure waveform (eg, P(t) ⫽ constant), or inspiratory pressure is set to be proportional to patient inspiratory effort, measured by various means. For example, P(t) ⫽ NAVA level ⫻ EAdi(t), where NAVA stands for neurally adjusted ventilatory assist, and EAdi stands for electrical activity of the diaphragm (see servo targeting scheme below). In a passive patient, after setting the form of the pressure function (ie, the waveform), volume and flow depend on elastance and resistance.10 Time control is a general category of ventilator modes for which inspiratory flow, inspiratory volume, and inspiratory pressure are all dependent on respiratory system mechanics. As no parameters of the pressure, volume, or flow waveforms are pre-set, the only control of the breath is the timing (ie, inspiratory and expiratory times). Examples of this are high-frequency oscillatory ventilation (3100 ventilator, CareFusion, San Diego, California) and volumetric diffusive respiration (Percussionaire, Sagle, Idaho). (4) Breaths are classified according to the criteria that trigger (start) and cycle (stop) inspiration. Inspiration starts (or is triggered) when a monitored variable (trigger variable) achieves a pre-set threshold (the trigger event). The simplest trigger variable is time, as in the case of a pre-set breathing frequency (recall that the period between breaths is 1/frequency). Other trigger variables include a minimum level of minute ventilation, a pre-set apnea interval, or various indicators of inspiratory effort (eg, changes in baseline pressure or flow or electrical signals derived from diaphragm movement). Inspiration stops (or is cycled off) when a monitored variable (cycle variable) achieves a pre-set threshold (cycle event). The simplest cycle variable is a pre-set inspiratory time. Other cycle variables include pressure (eg, peak airway pressure), volume (eg, VT), flow (eg, percent of peak inspiratory flow), and electrical signals derived from diaphragm movement.

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Fig. 3. A breath is defined in terms of the flow-time curve. Important timing parameters related to ventilator settings are labeled.

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(5) Trigger and cycle events can be either patient-initiated or ventilator-initiated. Inspiration can be patienttriggered or patient-cycled by a signal representing inspiratory effort (eg, changes in baseline airway pressure, changes in baseline bias flow, or electrical signals derived from diaphragm activity, as with neurally adjusted ventilatory assist11 or a calculated estimate of muscle pressure12). Furthermore, the ventilator can be triggered and cycled solely by the patient’s passive respiratory system mechanics (elastance and resistance).13 For example, an increase in elastance or resistance in some modes will increase airway pressure beyond the alarm threshold and cycle inspiration. Inspiration may be ventilator-triggered or ventilator-cycled by pre-set thresholds. Patient triggering means starting inspiration based on a patient signal, independent of a ventilator-generated trigger signal. Ventilator triggering means starting inspiratory flow based on a signal (usually time) from the ventilator, independent of a patient-triggered signal. Patient cycling means ending inspiratory time based on signals representing the patient-determined components of the equation of motion (ie, elastance or resistance and including effects due to inspiratory effort). Flow cycling is a form of patient cycling because the rate of flow decay to the cycle threshold (and hence, the inspiratory time) is determined by patient mechanics (ie, the time constant and effort). Ventilator cycling means ending inspiratory time independent of signals representing the patient-determined components of the equation of motion. As a further refinement, patient triggering can be defined as starting inspiration based on a patient signal occurring in a trigger window, independent of a ventilatorgenerated trigger signal. A trigger window is the period composed of the entire expiratory time minus a short refractory period required to reduce the risk of triggering a breath before exhalation is complete (Fig. 4). If a signal from the patient (ie, some measured variable indicating an inspiratory effort) occurs within this trigger window, inspiration starts and is defined as a patient-triggered event. A synchronization window is a short period, at the end of a pre-set expiratory or inspiratory time, during which a patient signal may be used to synchronize the beginning or ending of inspiration to the patient’s actions. If the patient signal occurs during an expiratory time synchronization window, inspiration starts and is defined as a ventilatortriggered event initiating a mandatory breath. This is because the mandatory breath would have been time-triggered regardless of whether the patient signal had appeared or not and because the distinction is necessary to avoid logical inconsistencies in defining mandatory and spontaneous breaths (see below), which are the foundation of the mode taxonomy. Trigger and synchronization windows are another way to distinguish between continuous mandatory ventilation (CMV) and intermittent mandatory ven-

tilation (IMV) (see below). Sometimes a synchronization window is used at the end of the inspiratory time of a pressure control, time-cycled breath. If the patient signal occurs during such an inspiratory time synchronization window, expiration starts and is defined as a ventilatorcycled event, ending a mandatory breath. Some ventilators offer the mode called airway pressure release ventilation (or something similar with a different name), which may use both expiratory and inspiratory synchronization windows. This mode is an example of the importance of distinguishing between trigger/cycle windows (allowing for patient-triggered breaths) and synchronization windows (allowing for patient-synchronized, ventilator-triggered breaths). Airway pressure release ventilation is intended to provide a set number of so-called releases or drops from a high-pressure level to a lowpressure level. Spontaneous breaths are possible at the high-pressure and low-pressure levels (although there may not be enough time to accomplish this if the duration of the low pressure is too short). Using the standardized vocabulary we have been discussing, these releases (paired with their respective rises) are actually mandatory breaths because they are time-triggered and time-cycled. On some ventilators, synchronization windows were added to both the expiratory time (to synchronize the transition to high pressure with a patient inspiratory effort) and the inspiratory time (to synchronize cycling with the expiratory phase of a spontaneous breath taken during the high-pressure level). If both triggering and cycling occurred with patient signals in the synchronization window, and if we called


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Fig. 4. Trigger and synchronization windows. If a patient signal occurs within the trigger window, inspiration is patient-triggered. If a patient signal occurs within a synchronization window, inspiration is ventilator-triggered (or cycled if at the end of inspiration) and patient-synchronized. Note that, in general, a trigger window is used with continuous mandatory ventilation, a synchronization window is used with intermittent mandatory ventilation.

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Fig. 5. Rubric for classifying trigger and cycle events. Courtesy Mandu Press.

these events patient-triggered and patient cycled, then we would end up with the ambiguous possibility of having spontaneous breaths (ie, synchronized) occurring during spontaneous breaths (unsynchronized breaths during the high-pressure level). Another example occurs with a ventilator such as the CareFusion Avea, which allows the operator to set a flow cycle criterion for pressure control PC-IMV. Thus, every inspiration is patient-cycled, and if we said that any synchronized breaths (synchronized IMV) were patient-triggered, we would be implying that these mandatory breaths were really spontaneous breaths. This would be misleading because the pre-set mandatory breathing frequency would then be larger than what we count as

mandatory breaths when observing the patient. On modes that are classified as forms of IMV (such as airway pressure release ventilation), we need to distinguish between the mandatory minute ventilation and the spontaneous minute ventilation (to gauge the level of mechanical support), and we cannot do this if the definitions of mandatory and spontaneous breaths are in any way ambiguous. Figure 5 shows the decision rubric for classifying trigger and cycle events. (6) Breaths are classified as spontaneous or mandatory based on both the trigger and cycle events. A spontaneous breath is a breath for which the patient retains control over timing. This means that the start and end of inspiration are

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determined by the patient, independent of any ventilator settings for inspiratory and expiratory times. That is, the patient both triggers and cycles the breath. A spontaneous breath may occur during a mandatory breath (eg, airway pressure release ventilation). A spontaneous breath may be assisted or unassisted. Indeed, the definition of a spontaneous breath applies to normal breathing as well as mechanical ventilation. Some authors use the term spontaneous breath to refer only to unassisted breaths, but that is an unnecessary limitation that prevents the word from being used as a key term in the mode taxonomy. A mandatory breath is a breath for which the patient has lost control over timing (ie, frequency or inspiratory time). This is a breath for which the start or end of inspiration (or both) is determined by the ventilator, independent of the patient: the ventilator triggers and/or cycles the breath. A mandatory breath can occur during a spontaneous breath (eg, high-frequency jet ventilation). A mandatory breath is, by definition, assisted. (7) Ventilators deliver 3 basic breath sequences: CMV, IMV, and continuous spontaneous ventilation CSV. A breath sequence is a particular pattern of spontaneous and/or mandatory breaths. The 3 possible breath sequences are CMV, IMV, and CSV. CMV, commonly known as assist control, is a breath sequence for which spontaneous breaths are not possible between mandatory breaths because every patient-triggered signal in the trigger window produces a ventilator-cycled inspiration (ie, a mandatory breath). IMV is a breath sequence for which spontaneous breaths are possible between mandatory breaths. Ventilator-triggered mandatory breaths may be delivered at a pre-set frequency. The mandatory breathing frequency for CMV may be higher than the set frequency but never below it (ie, the set frequency is a minimum value). In some pressure control modes on ventilators with an active exhalation valve, spontaneous breaths may occur during mandatory breaths, but the defining characteristic of CMV is that spontaneous breaths are not permitted between mandatory breaths. In contrast, the set frequency of mandatory breaths for IMV is the maximum value because every patient signal between mandatory breaths initiates a spontaneous breath. There are 3 variations of IMV. (1) Mandatory breaths are always delivered at the set frequency (eg, SIMV volume control mode on the PB840 ventilator, Covidien, Mansfield, Massachusetts). In general, if a synchronization window is used, the actual ventilatory period for a mandatory breath may be shorter than the set period. Some ventilators will add the difference to the next mandatory period to maintain the set mandatory breathing frequency (eg, Dra¨ger Evita XL ventilator). (2) Mandatory breaths are delivered only when the spontaneous breathing frequency falls below the set frequency (eg, BiPAP [bi-level positive airway pressure] S/T mode on the Philips Respironics V60 ventilator). In other words, spontaneous breaths

may suppress mandatory breaths. (3) Mandatory breaths are delivered only when the measured minute ventilation (ie, product of breathing frequency and VT) drops below a pre-set threshold (examples include Dra¨ger’s mandatory minute volume ventilation mode and Hamilton Medical’s adaptive support ventilation mode). Again, in this form of IMV, spontaneous breaths may suppress mandatory breaths. (8) Ventilators deliver 5 basic ventilatory patterns: volume control VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV. A ventilatory pattern is a sequence of breaths (CMV, IMV, or CSV) with a designated control variable (volume or pressure) for the mandatory breaths (or the spontaneous breaths for CSV). Thus, with 2 control variables and 3 breath sequences, there are 5 possible ventilatory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV. The VC-CSV combination is not possible because volume control implies ventilator cycling, and ventilator cycling makes every breath mandatory, not spontaneous (maxim 6). For completeness, we should also include the possibility of a time control ventilatory pattern such as time control IMV. Although this is uncommon and nonconventional, it is possible, as demonstrated by modes such as high-frequency oscillatory ventilation and intrapulmonary percussive ventilation. Because any mode of ventilation can be associated with one and only one ventilatory pattern, the ventilatory pattern serves as a simple mode classification system. (9) Within each ventilatory pattern, there are several types that can be distinguished by their targeting schemes (set-point, dual, bio-variable, servo, adaptive, optimal, and intelligent). A targeting scheme is a model14 of the relationship between operator inputs and ventilator outputs to achieve a specific ventilatory pattern, usually in the form of a feedback control system. A target is a predetermined goal of ventilator output. Targets can be viewed as the goals of the targeting scheme. Targets can be set for parameters during a breath (within-breath targets). These parameters relate to the pressure, volume, and flow waveforms. Examples of within-breath targets include peak inspiratory flow and VT or inspiratory pressure and rise time (set-point targeting); pressure, volume, and flow (dual targeting); and constant of proportionality between inspiratory pressure and patient effort (servo targeting). Targets can be set between breaths to modify the within-breath targets and/or the overall ventilatory pattern (between-breath targets). These are used with more advanced targeting schemes, where targets act over multiple breaths. Examples of between-breath targets and targeting schemes include average VT (for adaptive targeting using pressure control); work rate of breathing and minute ventilation (for optimal targeting); and combined end-tidal PCO2, volume, and frequency values describing a zone of comfort (for intelligent targeting, eg, SmartCare/PS [Dra¨ger Evita In-


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finity V500] or IntelliVent-ASV [S1 ventilator, Hamilton Medical, Reno, Nevada]). The targeting scheme (or combination of targeting schemes) is what distinguishes one ventilatory pattern from another. There are currently 7 basic targeting schemes that comprise the wide variety seen in different modes of ventilation. (1) Set-point is a targeting scheme for which the operator sets all of the parameters of the pressure waveform (pressure control modes) or volume and flow waveforms (volume control modes). (2) Dual is a targeting scheme that allows the ventilator to switch between volume control and pressure control during a single inspiration. (3) Bio-variable is a targeting scheme that allows the ventilator to automatically set the inspiratory pressure (or VT) randomly to mimic the variability observed during normal breathing. (4) Servo is a targeting scheme for which the output of the ventilator (eg, inspiratory pressure) automatically follows a varying input (eg, inspiratory effort). (5) Adaptive is a targeting scheme that allows the ventilator to automatically set one target (eg, pressure within a breath) to achieve another target (eg, average VT over several breaths). (6) Optimal is a targeting scheme that automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some overall performance characteristic (eg, work rate of breathing). (7) Intelligent is a targeting scheme that automatically adjusts the targets of the ventilatory pattern using artificial intelligence programs such as fuzzy logic, rule-based expert systems, and artificial neural networks. (10) A mode of ventilation is classified according to its control variable, breath sequence, and targeting scheme(s). The preceding 9 maxims create a theoretical foundation for the taxonomy of mechanical ventilation. Taxonomy is the science of classification. A full explanation of how taxonomies are created, as it applies to mechanical ventilation, has been published previously.15 In short, the first step is to create a standardized set of definitions. We have refined such a vocabulary over the last 20 years (see the supplementary materials at http://www.rcjournal.com). Selected terms in the vocabulary are used to create a hierarchical classification system (essentially an outline) that forms the structure of the taxonomy. The taxonomy has 4 hierarchical levels (analogous to order, class, genus, and species used in biological taxonomies): (1) control variable (pressure or volume, for the primary breath), (2) breath sequence (for CMV, IMV, or CSV), (3) primary breath targeting scheme (for CMV, IMV, or CSV) , and (4) secondary breath targeting scheme (for IMV). The primary breath is either the only breath that occurs (mandatory breaths in CMV and spontaneous breaths in CSV) or the mandatory breath in IMV. We consider it primary because if the patient becomes apneic, it is the only thing keeping the patient alive.

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The targeting schemes can be represented by single lowercase letters: set-point ⫽ s, dual ⫽ d, servo ⫽ r, biovariable ⫽ b, adaptive ⫽ a, optimal ⫽ o, and intelligent ⫽ i. For example, on the Covidien PB840 ventilator, there is a mode called A/C volume control (volume assist control). This mode is classified as VC-CMV with set-point targeting, represented by VC-CMVs. Minor differences in a species of modes (such as unique operational algorithms) can be accommodated by adding a fifth level we could call variety (as is done in biology). As an example, there are 3 varieties of PC-CSV using servo targeting. One makes inspiratory pressure proportional to the square of inspiratory flow (automatic tube compensation), one makes it proportional to the electrical signal from the diaphragm (neurally adjusted ventilatory assist), and one makes it proportional to the patient-generated volume and flow (proportional assist ventilation). The first can support only the resistive load of breathing, whereas the other two can support both the elastic and resistive loads. Application of the Taxonomy Translating a name of a mode into a mode classification using the taxonomy is a simple 3-step procedure. In step 1, the primary breath control variable is identified. Simply put, if inspiratory pressure is set or if pressure is proportional to inspiratory effort, then the control variable is pressure. In contrast, if the VT and inspiratory flow are set, then the control variable is volume. Figure 6 shows the decision rubric with a few refinements to accommodate dual targeting. In step 2, the breath sequence is identified. Figure 7 shows the decision rubric. In step 3, the targeting schemes for the primary and, if applicable, secondary breaths are identified (Table 2). Examples To demonstrate these steps, we will classify some of the most commonly used modes in ICUs, starting with A/C volume control (Covidien PB840 ventilator). For this mode, both inspiratory volume and flow are pre-set, so the control variable is volume (see Fig. 6). Every breath is volume-cycled, which is a form of ventilator cycling. Any breath for which inspiration is ventilator-cycled is classified as a mandatory breath. Hence, the breath sequence is CMV (see Fig. 7). Finally, the operator sets all of the parameters of the volume and flow waveforms, so the targeting scheme is set-point (see Table 2). Thus, the mode is classified as VC-CMV with set-point targeting (VCCMVs). Another common mode is SIMV volume control plus (Covidien PB840 ventilator). For this mode, the operator sets the VT, but not inspiratory flow. Because setting vol-


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Fig. 6. Rubric for determining the control variable of a mode. Paw ⫽ airway pressure, SIMV ⫽ synchronized intermittent mandatory ventilation; VT ⫽ tidal volume; P ⫽ pressure; E ⫽ elastance; V ⫽ volume; R ⫽ resistance; V˙ ⫽ inspiratory flow. Courtesy Mandu Press.

ume alone (like setting flow alone) is a necessary but not sufficient criterion for volume control, the control variable is pressure (see Fig. 6). Spontaneous breaths are allowed between mandatory breaths, so the breath sequence is IMV (see Fig. 7). The ventilator adjusts inspiratory pressure for mandatory breaths to achieve an average pre-set VT, so the targeting scheme for the mandatory breaths is adaptive (see Table 2). For spontaneous breaths, inspiratory pressure is set by the operator (eg, pressure support), so the

targeting scheme for these breaths is set-point. The mode tag is PC-IMVa,s. A very common mode for spontaneous breathing trials (or for assistance of spontaneous breaths in IMV modes) is pressure support or pressure support ventilation (note that although ubiquitous, pressure support is a name, not a classification). For this mode, the operator sets an inspiratory pressure, so the control variable is pressure. All breaths are patient-triggered and patient-cycled (note that flow cy-


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Fig. 7. Rubric for determining the breath sequence of a mode. Courtesy Mandu Press.

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TAXONOMY Table 2.

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Targeting Schemes

Name (Abbreviation) Set-point (s)

Dual (d)

Servo (r)

Adaptive (a)

Description The operator sets all parameters of the pressure waveform (pressure control modes) or volume and flow waveforms (volume control modes). The ventilator can automatically switch between volume control and pressure control during a single inspiration. The output of the ventilator (pressure/volume/flow) automatically follows a varying input. The ventilator automatically sets target(s) between breaths in response to varying patient conditions.

Bio-variable (b)

The ventilator automatically adjusts the inspiratory pressure or VT randomly.

Optimal (o)

The ventilator automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some overall performance characteristic (eg, work rate of breathing). This is a targeting scheme that uses artificial intelligence programs such as fuzzy logic, rule-based expert systems, and artificial neural networks.

Intelligent (i)

Disadvantage

Example Mode Name

Ventilator (Manufacturer)

Simplicity

Changing patient conditions may make settings inappropriate.

Volume control CMV

Evita Infinity V500 (Dräger)

It can adjust to changing patient conditions and ensure either a pre-set VT or peak inspiratory pressure, whichever is deemed most important. Support by the ventilator is proportional to inspiratory effort.

It may be complicated to set correctly and may need constant readjustment if not automatically controlled by the ventilator. It requires estimates of artificial airway and/or respiratory system mechanical properties. Automatic adjustment may be inappropriate if algorithm assumptions are violated or if they do not match physiology. Manually set range of variability may be inappropriate to achieve goals.

Volume control

Servo-i (Maquet)

Proportional assist ventilation plus

PB840 (Covidien)

Pressure-regulated volume control

Servo-i

Variable pressure support

Evita Infinity V500

Automatic adjustment may be inappropriate if algorithm assumptions are violated or if they do not match physiology.

ASV

G5 (Hamilton Medical)

Automatic adjustment may be inappropriate if algorithm assumptions are violated or if they do not match physiology.

SmartCare/PS IntelliVent-ASV

Evita Infinity V500 S1 (Hamilton Medical)

Advantage

It can maintain stable VT delivery with pressure control for changing lung mechanics or patient inspiratory effort. It simulates the variability observed during normal breathing and may improve oxygenation or mechanics. It can adjust to changing lung mechanics or patient inspiratory effort.

It can adjust to changing lung mechanics or patient inspiratory effort.

CMV ⫽ continuous mandatory ventilation ASV ⫽ adaptive support ventilation VT ⫽ tidal volume

cling is a form of patient cycling, as discussed above), so the breath sequence is CSV. Because the ventilator does not automatically adjust any of the parameters of the breath, the targeting scheme is set-point, and the tag is PC-CSVs. If carefully applied, the taxonomy has the power to clarify and unmask hidden complexity in a mode that has a cryptic name. Take, for example, the mode called CMV ⫹ AutoFlow on the Dra¨ger Evita XL ventilator. Although CMV on this ventilator is a mode equivalent to volume assist control (described above), adding the AutoFlow feature changes it to a completely different mode. For CMV ⫹ AutoFlow, the operator sets a target VT, but not inspiratory flow. Indeed, inspiratory flow is highly variable because the ventilator automatically sets the inspiratory pressure within a breath. Thus, according to the equation of motion, the control variable is pressure. Every inspiration is time-cycled and hence mandatory, and the breath sequence is CMV. The ventilator adjusts the inspiratory pressure between breaths to achieve an average

VT equal to the pre-set value using an adaptive targeting scheme. Thus, the mode is classified as PC-CMV with adaptive targeting (PC-CMVa). On the other hand, the taxonomy can unmask the complexity in an apparently simple mode. The mode called volume control (Servo-i, Maquet, Wayne, New Jersey) allows setting the VT and inspiratory time. Setting both volume and inspiratory time is equivalent to setting mean inspiratory flow (flow ⫽ volume/time); hence, the control variable is volume. Every breath is normally time-cycled and hence mandatory, so our initial thought is that the breath sequence is CMV. The tricky part is the targeting scheme. The operator’s manual states that “. . . if a pressure drop of 3 cm H2O is detected during inspiration, the ventilator (switches) to Pressure Support with a resulting increase in inspiratory flow.” This indicates dual targeting as described above (see Table 2). Noting that the breath may switch to pressure support alerts us that the breath sequence is not what it first seemed to be. A breath may be


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patient-triggered with a patient inspiratory effort, and if the effort is large enough and long enough, inspiration is flow-cycled, not time-cycled. Flow cycling (at a certain percent of peak inspiratory pressure) is, as we described above, a form of patient cycling because the time constant of the patient’s respiratory system determines when the cycle threshold is met for passive inhalation (hence, inspiratory time is determined by the patient). Alternatively, the patient may make an expiratory effort that cycles inspiration off. Either way, a patient-triggered and patientcycled breath is a spontaneous breath. Thus, spontaneous breaths may occur between mandatory breaths, and the breath sequence is actually IMV. Finally, the tag for this mode is VC-IMVd,d. Note that with dual targeting modes, we need to identify which control variable is in effect at the start of inspiration, and in this case, it is volume. In contrast, the mode called pressure A/C with machine volume (CareFusion Avea) is also dual targeting but starts out in pressure control and may switch to volume control. This convention is used because if the criterion that causes the switch between control variables is never met during a breath, the original control variable remains in effect throughout the inspiratory time. Finally, some modes are composed of compound targeting schemes. For example, some ventilators offer tube compensation, a feature that increases inspiratory pressure in proportion to flow to support the resistive load of breathing through an artificial airway. This is a form of servo targeting. On the Dra¨ger Evita XL ventilator, tube compensation can be added to CMV ⫹ AutoFlow to obtain a mode classified as PC-CMVar, where ar represents the compound targeting scheme composed of adaptive plus servo (note the absence of a comma between a and r because we are referring only to the primary breaths, and no secondary breaths exist). A mode classified as PC-IMV with set-point control for both primary (mandatory) and secondary (spontaneous) breaths would have the tag PCIMVs,s (note the comma indicating set-point for primary breaths and set-point for secondary breaths). If tube compensation is used for the spontaneous breaths (eg, Covidien PB840 ventilator), the tag would change to PCIMVs,r. If it is added to both mandatory and spontaneous breaths (eg, Dra¨ger Evita XL ventilator), the tag would change to PC-IMVsr,sr. Another example is IntelliVentASV (Hamilton Medical S1 ventilator), which uses optimal targeting to minimize the work rate and intelligent targeting to establish lung-protective limits and adjust PEEP and FIO2. The tag for this mode is PC-CMVoi,oi. The utility of this taxonomy becomes evident when comparing modes on different ventilators (eg, for making a purchase decision), as shown in a recent issue of Health Devices.6 We have extended this type of comparison to include several common ICU ventilators (Table 3). Table 3 is sorted by manufacturer and model, control variables,

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breath sequences, and targeting schemes (simple to complex). Table 4 shows the most commonly used modes for the ventilators described in Table 3, sorted by classification (tag). Table 4 illustrates how modes that are essentially the same or very similar are given very different names. We have constructed a table of all of the modes on 30 different ventilators from 11 different manufacturers (not shown). The table lists 290 unique names of modes representing 45 different classifications (tags). Dealing with this level of complexity is not unlike the challenge facing clinicians when applying clinical diagnostic reasoning. The elements of the mode taxonomy can thus be seen as analogous to the discriminating and defining features of a set of diagnostic hypotheses,16 as shown in Figure 8. Discussion The Problem of Growing Complexity We mentioned in the introduction how modes of mechanical ventilation have evolved to a high level of complexity. If we agree that the goal is to be able to appropriately use all modes of ventilation (even if we restrict this goal to a single type of ventilator that might be available), then this implies the ability to compare and contrast their advantages and disadvantages. The urgency for dealing with the complexity of mechanical ventilation is ultimately based on the growing concern for patient safety. The ECRI “has repeatedly stressed the need for users to understand the operation and features of ventilators . . . The fact that ventilators are such an established technology by no means guarantees that these issues are clearly understood . . . We continue to receive reports of hospital staff misusing ventilators because they’re unaware of the device’s particular operational considerations.”17 To deal with this complexity, researchers are designing even more complex systems that attempt to better serve the 3 goals of mechanical ventilation (ie, safety, comfort, and liberation).18 For example, Tehrani19 has recently described a system designed to automatically control the support level in proportional assist ventilation to guarantee delivery of a patient’s required ventilation (serving the goal of safety). This system can also be used to control the proportional assist ventilation support level based on the patient’s work of breathing (serving the goal of comfort). Blanch et al20 have developed the Better Care system, which reliably detects ineffective respiratory efforts during expiration (ie, inability of a patient to trigger breaths) with accuracy similar to that of expert intensivists and the EAdi signal (serving the comfort goal). According to Kilic and Kilic,21 conventional weaning predictors ignore important dimensions of weaning outcome. They describe a fuzzy logic system that provides an approach that can handle multi-attribute decision making as a tool to over-


TAXONOMY Table 3.

FOR


Classification of Modes on Common ICU Ventilators Mode Name

Covidien PB840 A/C volume control SIMV volume control with pressure support SIMV volume control with tube compensation A/C pressure control A/C volume control plus SIMV pressure control with pressure support SIMV pressure control with tube compensation Bi-level with pressure support Bi-level with tube compensation SIMV volume control plus with pressure support SIMV volume control plus with tube compensation Spontaneous pressure support Spontaneous tube compensation Spontaneous proportional assist Spontaneous volume support Dräger Evita XL CMV CMV with pressure-limited ventilation SIMV SIMV with automatic tube compensation SIMV with pressure-limited ventilation SIMV with pressure-limited ventilation and automatic tube compensation Mandatory minute volume ventilation Mandatory minute volume ventilation with automatic tube compensation Mandatory minute volume with pressure-limited ventilation Mandatory minute volume with pressure-limited ventilation and automatic tube compensation Pressure control ventilation plus assisted CMV with AutoFlow CMV with AutoFlow and tube compensation Pressure control ventilation plus/pressure support APRV Mandatory minute volume with AutoFlow SIMV with AutoFlow Mandatory minute volume with AutoFlow and tube compensation SIMV with AutoFlow and tube compensation Pressure control ventilation plus/pressure support and tube compensation APRV with tube compensation CPAP/pressure support SmartCare CPAP/pressure support with tube compensation Hamilton Medical G5 Synchronized controlled mandatory ventilation SIMV SIMV with tube-resistance compensation Pressure control CMV Adaptive pressure ventilation CMV Adaptive pressure ventilation CMV with tube-resistance compensation Pressure control CMV with tube-resistance compensation Pressure SIMV NIV-spontaneous timed Nasal CPAP-pressure support APRV DuoPAP

Tag VC-CMVs* VC-IMVs,s VC-IMVs,r PC-CMVs PC-CMVa PC-IMVs,s PC-IMVs,r PC-IMVs,s PC-IMVs,r PC-IMVa,s PC-IMVa,r PC-CSVs PC-CSVr PC-CSVr PC-CSVa VC-CMVs VC-CMVd VC-IMVs,s VC-IMVs,sr VC-IMVd,s VC-IMVd,sr VC-IMVa,s VC-IMVa,sr VC-IMVda,s VC-IMVda,sr PC-CMVs PC-CMVa PC-CMVar PC-IMVs,s PC-IMVs,s PC-IMVa,s PC-IMVa,s PC-IMVar,sr PC-IMVar,sr PC-IMVsr,sr PC-IMVsr,sr PC-CSVs PC-CSVi PC-CSVsr VC-CMVs VC-IMVs,s CV-IMVs,sr PC-CMVs PC-CMVa PC-CMVar PC-CMVsr PC-IMVs,s PC-IMVs,s PC-IMVs,s PC-IMVs,s PC-IMVs,s (continued)


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TAXONOMY Table 3.

FOR


Continued Mode Name

Tag

Adaptive pressure ventilation SIMV Adaptive pressure ventilation SIMV with tube-resistance compensation ASV IntelliVent-ASV ASV with tube-resistance compensation IntelliVent-ASV with tube-resistance compensation Pressure SIMV with tube-resistance compensation APRV with tube-resistance compensation Spontaneous with tube-resistance compensation Spontaneous NIV Maquet Servo-i Volume control SIMV (volume control) Automode (volume control to volume support) Pressure control Pressure-regulated volume control SIMV (pressure control) BiVent Automode (pressure control to pressure support) SIMV pressure-regulated volume control Automode (pressure-regulated volume control to volume support) Spontaneous/CPAP Pressure support Neurally adjusted ventilatory assist Volume support

PC-IMVa,s PC-IMVar,sr PC-IMVoi,oi PC-IMVoi,oi PC-IMVoir,oir PC-IMVoir,oir PC-IMVsr,sr PC-IMVsr,sr PC-CSVr PC-CSVs PC-CSVs VC-IMVd,d VC-IMVd,d VC-IMVd,a PC-CMVs PC-CMVa PC-IMVs,s PC-IMVs,s PC-IMVs,s PC-IMVa,s PC-IMVa,a PC-CSVs PC-CSVs PC-CSVr PC-CSVa

* Targeting schemes are represented by single lowercase letters: s ⫽ set-point, r⫽ servo, a ⫽ adaptive, d ⫽ dual, i ⫽ intelligent, and o ⫽ optimal. Combinations include: sr ⫽ set-point with servo, da ⫽ dual with adaptive, as ⫽ adaptive with set-point, ar ⫽ adaptive with servo, oi ⫽ optimal with intelligent, and ois ⫽ optimal with intelligent and servo. A/C ⫽ assist control SIMV ⫽ synchronized intermittent mandatory ventilation CMV ⫽ continuous mandatory ventilations CSV ⫽ continuous spontaneous ventilation IMV ⫽ intermittent mandatory ventilation NIV ⫽ noninvasive ventilation APRV ⫽ airway pressure release ventilation DuoPAP ⫽ dual positive airway pressure ASV ⫽ adaptive support ventilation VC ⫽ volume control PC ⫽ pressure control

come the weaknesses of currently used weaning predictors (serving the goal of liberation). Wysocki et al22 provide a very good overview of technical and engineering considerations regarding closed-loop controlled ventilation, as well as tangible clinical evidence that such systems make mechanical ventilation safer and more efficient.

Before we can compare modes, we must have a list of available modes. To generate such a list, we cannot just copy the names of modes found in ventilator operators’ manuals for 2 reasons. First, there is no consistency among manufacturers regarding how modes are named: some names are the same but the modes are different, and some

names are different but the modes are the same. Second, complex ICU ventilators offer features that, when activated, result in modes that are not named by the manufacturer and, in many cases, not even recognized as different modes. Such ambiguity makes learning about how ventilators work very difficult. Thus, before we can generate a list of modes to compare, we first have to find some way to deal with this confusion. One way is to distinguish between a mode name, which is determined by the manufacturer, and a mode tag or general classification. However, before we can do that, we must have a practical taxonomy. And before a taxonomy can be constructed, there must be a standardized glossary (or controlled vocabulary, as it is called by taxonomists) of relevant terms.15,23 The need for a standardized

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The Problem of Identifying Unique Modes

TAXONOMY Table 4.

FOR


Most Common Modes in Table 3 Sorted by Tag to Show Which Mode Names Have Equivalent Mode Classifications

Tag

Covidien PB840

Dräger Evita XL

VC-CMVs*

A/C volume control

CMV

VC-IMVs,s

SIMV

PC-CMVs

SIMV volume control with pressure support A/C pressure control

PC-CMVa

A/C volume control plus

PC-IMVs,s

SIMV-pressure control with pressure support Bi-level with pressure support

PC-IMVa,s

SIMV volume control plus with pressure support

PC-CSVs PC-CSVa

Spontaneous pressure support Spontaneous volume support

Hamilton G5

Pressure control ventilation plus assisted CMV with AutoFlow Pressure control ventilation plus/pressure support APRV

Mandatory minute volume with AutoFlow SIMV with AutoFlow CPAP/pressure support NA

Maquet Servo-i

Synchronized controlled mandatory ventilation SIMV

NA†

Pressure control CMV

Pressure control

Adaptive pressure ventilation CMV Pressure SIMV

Pressure-regulated volume control SIMV (pressure control)

NIV-spontaneous timed Nasal CPAP-pressure support

BiVent Automode (pressure control to pressure support)

NA‡

APRV DuoPAP Adaptive pressure ventilation SIMV

SIMV pressure-regulated volume control

Spontaneous NA

Spontaneous/CPAP Volume support

* Targeting schemes are represented by single lowercase letters: s ⫽ set-point, and a ⫽ adaptive. † Volume control continuous mandatory ventilation (VC-CMV) is not available because the mode called volume control allows some breaths to be patient-triggered and patient-cycled; hence, they are spontaneous, making the breath sequence intermittent mandatory ventilation (IMV) rather than CMV. ‡ VC-IMV is available only with dual targeting, called SIMV (volume control) with the tag VC-IMVd,d. PC ⫽ pressure control CSV ⫽ continuous spontaneous ventilation A/C ⫽ assist control SIMV ⫽ synchronized intermittent mandatory ventilation NA ⫽ not available APRV ⫽ airway pressure release ventilation DuoPAP ⫽ dual positive airway pressure NIV ⫽ noninvasive ventilation

We believe that there is a growing and under-recognized problem regarding training in the art and science of mechanical ventilation that frequently leads to operational errors. The reason is that technology is expanding faster than our educational resources. Not even a 4-year respiratory care program can afford the time to ensure the abovementioned mechanical ventilation competencies for RRTs. The challenge for physicians may be even greater because they generally rely on their residency (rather than medical school) to learn mechanical ventilation. But according to at least one study, “. . . internal medicine residents are not gaining important evidence-based knowledge needed to

provide effective care for patients who require mechanical ventilation.”24 So how do instruction programs deal with the challenge? We recently conducted an informal survey of members of the AARC specialty section on education. We asked program directors to send us their outlines for teaching mechanical ventilation. On the basis of what we found, we contend there are 3 categories of organizational structure: (1) simple lists of skills needed to operate specific ventilators, (2) lists of topics ranging from indications for ventilation to weaning (and everything in between) with no apparent order, and (3) topic organization identical to or closely following the table of contents in textbooks on mechanical ventilation. Having written ventilator-related content in such textbooks, we can safely say that such material was never designed to be used as the basis for a class syllabus. In most of our previous writings focusing on modes of mechanical ventilation,7,14,25,26 the emphasis has been on descriptions of modes rather than the background technical knowledge required to understand them. That knowledge was assumed on the part of the reader


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vocabulary is the reason we included one in the supplementary materials (http://www.rcjournal.com). This vocabulary has been carefully developed by the authors over the last 20 years with the specific purpose of establishing basic concepts that are logically consistent across all applications. The Problem of Teaching Mechanical Ventilation

TAXONOMY

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Fig. 8. Venn diagram illustrating how the mode taxonomy can be viewed in terms of discriminating features and defining features. VC ⫽ volume control; PC ⫽ pressure control; CMV ⫽ continuous mandatory ventilation; IMV ⫽ intermittent mandatory ventilation; CSV ⫽ continuous spontaneous ventilation; PETCO2 ⫽ end-tidal partial pressure of carbon dioxide; a ⫽ adaptive targeting; s⫽ set-point targeting. Courtesy Mandu Press.

(and instructor). This seems to be a universal theme among authors of both articles and book chapters on how ventilators work, but as the technology grows more complex, the gap between assumed and actual knowledge becomes wider. We trust that this article helps to narrow that gap. The Problem of Implementation

ganization for Standardization and the Integrating the Healthcare Enterprise Rosetta Terminology Mapping project are working toward such a goal. Alternatively, the AARC might be an appropriate venue for maintaining the taxonomy as they do for clinical practice guidelines. Conclusions

The first problem regarding implementation of any taxonomy (and its underlying standardized vocabulary) is, of course, reaching a tipping point in acceptance by stakeholders. We believe that such acceptance is achievable only if the tools for implementing the taxonomy are simple, practical, and efficient in permitting both the identification and comparison of modes. The acceptance of this taxonomy by the ECRI is a step in the right direction. We hope that the detailed definitions, descriptions, and algorithms provided here will address the deficiencies in the available textbook references. Indeed, such tools are amenable to dissemination using mobile computing technology such as tablet computers and smartphones. The second problem is ongoing maintenance of the taxonomy. Terms and concepts necessarily change as technology evolves. Ideally, a professional organization should take responsibility for this function. The International Or-

The rapid increase in the number and complexity of mechanical ventilators, and specifically the modes they offer, has outpaced development of tools for describing them. A key problem has been the lack of a practical classification system or taxonomy. Partial solutions to that problem have been offered in our previous publications. In this paper, we developed a full taxonomy and standardized vocabulary for modes of mechanical ventilation. We showed how the taxonomy is based on 10 fundamental constructs, or maxims, describing ventilator technology. Finally, we showed how to use the taxonomy to classify modes of ventilation found on common ICU ventilators. Identifying and classifying modes are necessary steps before being able to compare their relative merits and ultimately to choose the most appropriate mode to serve the goals of care for a particular patient.18 The tools offered in this paper (including the standardized vocabulary and

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2 summary handouts in the supplementary materials at http://www.rcjournal.com) serve as a means for achieving the mechanical ventilation competencies of the respiratory therapist in 2015 and beyond.4 Indeed, they serve the needs of all stakeholders, including clinicians (to understand treatment options), researchers (to evaluate treatment options), educators (to prepare the next generation of ventilator experts), administrators (to make purchase decisions), and, perhaps most important of all, manufacturers (to explain the technical capabilities of their products and serve the needs of the other stakeholders).

1. Kacmarek RM, Durbin CG Jr, Barnes TA, Kageler WV, Walton JR, O’Neil EH. Creating a vision for respiratory care in 2015 and beyond. Respir Care 2009;54(3):375-389. 2. Barnes TA, Gale DD, Kacmarek RM, Kageler WV. Competencies needed by graduate respiratory therapists in 2015 and beyond. Respir Care 2010;55(5):601-616. 3. Barnes TA, Kacmarek RM, Kageler WV, Morris MJ, Durbin CG Jr. Transitioning the respiratory therapy workforce for 2015 and beyond. Respir Care 2011;56(5):681-690. 4. Kacmarek RM. Mechanical ventilation competencies of the respiratory therapist in 2015 and beyond. Respir Care 2013;58(6):10871096. 5. Cairo JM, Pilbeam SP. Mosby’s respiratory care equipment, 8th edition. St. Louis, MO: Mosby/Elsevier; 2009. 6. ECRI. Take a breath: our review of three portable ventilators. Health Devices 2013;42(8):248-261. 7. Chatburn RL, Volsko TA, Hazy J, Harris LN, Sanders S. Determining the basis for a taxonomy of mechanical ventilation. Respir Care 2012;57(4):514-524. 8. Overbaugh RC, Schultz L. Bloom’s taxonomy. http://ww2.odu.edu/ educ/roverbau/Bloom/blooms_taxonomy.htm. Accessed August 17, 2013. 9. Coutinho Myrrha MA, Vieira DS, Moraes KS, Lage SM, Parreira VF, Britto RR. Chest wall volumes during inspiratory loaded breathing in COPD patients. Respir Physiol Neurobiol 2013;188(1):15-20. 10. Marini JJ, Crooke PS 3rd, Truwit JD. Determinants and limits of pressure-preset ventilation: a mathematical model of pressure control. J Appl Physiol 1989;67(3):1081-1092.

11. Sinderby C, Beck JC. Neurally adjusted ventilatory assist. In: Tobin MJ, editor. Principles and practice of mechanical ventilation, 3rd edition. New York: McGraw-Hill, 2013;351-375. 12. Kondili E, Alexopoulou C, Xirouchaki N, Vaporidi K, Georgopoulos D. Estimation of inspiratory muscle pressure in critically ill patients. Intensive Care Med 2010;36(4):648-655. 13. Babic MD, Chatburn RL, Stoller JK. Laboratory evaluation of the Vortran Automatic Resuscitator Model RTM. Respir Care 2007; 52(12):1718-1727. 14. Chatburn RL, Mireles-Cabodevila E. Closed-loop control of mechanical ventilation: description and classification of targeting schemes. Respir Care 2011;56(1):85-102. 15. Rabec C, Langevin B, Rodenstein D, Perrin C, Leger P, Pepin JL, Janssens JP, Gonzalez-Bermejo J, SomnoNIV Group. Chatburn RL. Ventilatory Modes. What’s in a name? Respir Care 2012;57(12): 2138-2139; author reply 2139-2150. 16. Bowen JL. Educational strategies to promote clinical diagnostic reasoning. N Engl J Med 2006;355(21):2217-2225. 17. ECRI. Health Devices 2002;31(7). 18. Mireles-Cabodevila E, Hatipog˘lu U, Chatburn RL. A rational framework for selecting modes of ventilation. Respir Care 2013;58(2): 348-366. 19. Tehrani FT. A control system for mechanical ventilation of passive and active subjects. Comput Methods Programs Biomed 2013;110(3): 511-518. 20. Blanch L, Sales B, Montanya J, Lucangelo U, Garcia-Esquirol O, Villagra A, et al. Validation of the Better Care system to detect ineffective efforts during expiration in mechanically ventilated patients: a pilot study. Intensive Care Med 2012;38(5):772-780. 21. Kilic YA, Kilic I. A novel fuzzy logic inference system for decision support in weaning from mechanical ventilation. J Med Syst 2010; 34(6):1089-1095. 22. Wysocki M, Jouvet P, Jaber S. Closed loop mechanical ventilation. J Clin Monit Comput 2014;28(1):49-56. 23. Hedden, H. The accidental taxonomist. Medford, NJ: Information Today; 2010. 24. Cox CE, Carson SS, Ely EW, Govert JA, Garrett JM, Brower RG, et al. Effectiveness of medical resident education in mechanical ventilation. Am J Respir Crit Care Med 2003;167(1):32-38. 25. Chatburn RL. Classification of ventilator modes: update and proposal for implementation. Respir Care 2007;52(3):301-323. 26. Chatburn RL. Understanding mechanical ventilators. Expert Rev Respir Med 2010;4(6):809-819.


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REFERENCES

HANDOUT 1 – HOW TO IDENTIFY MODES Robert L. Chatburn, MHHS, RRT-NPS, FAARC 6/5/2014

Background Issues Discussions about mechanical ventilator technology are hampered by the lack of a standardized vocabulary related to ventilator function, and in particular, to modes of ventilation. In general, a “mode” is a predetermined pattern of interaction between the ventilator and the patient. Manufacturers give certain patterns names and ignore other patterns. The result is that many ventilators have functionality that is not explicitly recognized as distinct modes. Some names of modes are so commonly used that they are virtually default classifications, eg, “Assist/Control” or “Pressure Support” albeit without any consensus on their exact meanings. Others are so rare that they are used as marketing devices, eg, “SmartCarePS” or “Neurally Adjusted Ventilatory Assist”. The most popular textbook on equipment for respiratory therapy lists 174 unique names of modes but offers no way to classify them. I have personally identified almost 300 unique mode names using just the terms created by manufacturers in operators’ manuals. Clearly, a classification system (formal taxonomy) is required in order to recognize, compare, and contrast modes or to contemplate future mode designs. What follows is a very brief overview of a standardize vocabulary and a taxonomy I have developed and described in the literature over the last 20 years. I have recently created implementation tools; see Respir Care 2012;57(12):2138-2150.

A Taxonomy for Mechanical Ventilation The taxonomy is a logical classification system based on 10 maxims of ventilator design: 1. A breath is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flow-time curve. Inspiratory time is defined as the period from the start of positive flow to the start of negative flow. Expiratory time is defined as the period from the start of expiratory flow to the start of inspiratory flow. The flow-time curve is the basis for many variables related to ventilator settings. 2. A breath is assisted if the ventilator does work on the patient. An assisted breath is one for which the ventilator does some portion of the work of breathing. For constant flow inflation, work is defined as inspiratory pressure multiplied by tidal volume. Therefore, an assisted breath is identified as a breath for which airway pressure (displayed on the ventilator) rises above baseline during inspiration. An unassisted breath is one for which the ventilator simply provides the inspiratory flow demanded by the patient and pressure stays constant throughout the breath. 3. A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system. Providing assistance means doing work on the patient, which is accomplished by controlling either pressure or volume. A simple mathematical model describing this fact is known as the equation of motion for the passive respiratory system. Pressure = (Elastance × Volume) + (Resistance × Flow) Volume control means that both volume and flow are preset prior to inspiration. In other words, the right hand side of the equation of motion remains constant while pressure changes with changes in elastance and resistance. Pressure control means that inspiratory pressure is preset as either a constant value or it is proportional to the patient’s inspiratory effort. In other words, the left hand side of the equation of motion remains constant while volume and flow change with changes in elastance and resistance. 4. Breaths are classified by the criteria that trigger (start) and cycle (stop) inspiration. The start of inspiration is called the trigger event. The end of inspiration is called the cycle event. 5. Trigger and cycle events can be initiated by the patient or the machine. Inspiration can be patient triggered or patient cycled by a signal representing inspiratory effort. Inspiration may also be machine triggered or machine cycled by preset ventilator thresholds. Patient triggering means starting inspiration based on a patient signal independent of a machine trigger signal. Machine triggering means starting inspiratory flow based on a signal from the ventilator, independent of a patient trigger signal. Patient cycling means ending inspiratory time based on signals representing the patient determined components of the equation of motion, (ie, elastance or resistance and including effects due to inspiratory effort). Flow cycling is a form of patient cycling because the rate of flow decay to the cycle threshold is determined by patient mechanics. Machine cycling means ending inspiratory time independent of signals representing the patient determined components of the equation of motion. 6. Breaths are classified as spontaneous or mandatory based on both the trigger and cycle events. A spontaneous breath is a breath for which the patient both triggers and cycles the breath. A spontaneous breath may occur during a mandatory breath (eg Airway Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted. A mandatory breath is a breath for which the machine triggers and/or cycles the breath. A mandatory breath can occur during a spontaneous breath (eg, High Frequency Jet Ventilation). A mandatory breath is, by definition, assisted. 7. There are 3 breath sequences: Continuous mandatory ventilation (CMV), Intermittent Mandatory Ventilation (IMV), and Continuous Spontaneous Ventilation (CSV). A breath sequence is a particular pattern of spontaneous and/or mandatory breaths. The 3 possible breath sequences are: continuous mandatory ventilation, (CMV, spontaneous breaths are not allowed between mandatory breaths), intermittent mandatory ventilation (IMV, spontaneous breaths may occur between mandatory breaths), and continuous spontaneous ventilation (CSV, all breaths are spontaneous). 8. There are 5 basic ventilatory patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV. The combination VC-CSV is not possible because volume control implies machine cycling and machine cycling makes every breath mandatory, not spontaneous.

9. Within each ventilatory pattern there are several variations that can be distinguished by their targeting scheme(s). A targeting scheme is a description of how the ventilator achieves preset targets. A target is a predetermined goal of ventilator output. Examples of within-breath targets include inspiratory flow or pressure and rise time (set-point targeting), tidal volume (dual targeting) and constant of proportionality between inspiratory pressure and patient effort (servo targeting). Examples of between-breath targets and targeting schemes include average tidal volume (for adaptive targeting), percent minute ventilation (for optimal targeting) and combined PCO2, volume, and frequency values describing a “zone of comfort” (for intelligent targeting, eg, SmartCarePS or IntelliVent-ASV). The targeting scheme (or combination of targeting schemes) is what distinguishes one ventilatory pattern from another. There are 7 basic targeting schemes that comprise the wide variety seen in different modes of ventilation: Set-point: A targeting scheme for which the operator sets all the parameters of the pressure waveform (pressure control modes) or volume and flow waveforms (volume control modes). Dual: A targeting scheme that allows the ventilator to switch between volume control and pressure control during a single inspiration. Bio-variable: A targeting scheme that allows the ventilator to automatically set the inspiratory pressure or tidal volume randomly to mimic the variability observed during normal breathing. Servo: A targeting scheme for which inspiratory pressure is proportional to inspiratory effort. Adaptive: A targeting scheme that allows the ventilator to automatically set one target (eg, pressure within a breath) to achieve another target (eg, average tidal volume over several breaths). Optimal: A targeting scheme that automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some overall performance characteristic (eg, minimize the work rate done by the ventilatory pattern). Intelligent: A targeting scheme that uses artificial intelligence programs such as fuzzy logic, rule based expert systems, and artificial neural networks. 10. A mode of ventilation is classified according to its control variable, breath sequence, and targeting scheme(s). The preceding 9 maxims create a theoretical foundation for a taxonomy of mechanical ventilation. The taxonomy is based on these theoretical constructs and has 4 hierarchical levels: Control Variable (Pressure or Volume, for the primary breath) Breath Sequence (CMV, IMV, or CSV) Primary Breath Targeting Scheme (for CMV or CSV) Secondary Breath Targeting Scheme (for IMV) The “primary breath” is either the only breath there is (mandatory for CMV and spontaneous for CSV) or it is the mandatory breath in IMV. The targeting schemes can be represented by single, lower case letters: set-point = s, dual = d, servo = r, bio-variable = b, adaptive = a, optimal = o, intelligent = i. A tag is an abbreviation for a mode classification, such as PC-IMVs,s. Compound tags are possible, eg, PC-IMVoi,oi.

How to Classify a Mode Step 1: Identify the primary breath control variable. If inspiration starts with a preset inspiratory pressure, or if pressure is proportional to inspiratory effort, then the control variable is pressure. If inspiration starts with a preset tidal volume and inspiratory flow, then the control variable is volume. If neither is true, the control variable is time. Step 2: Identify the breath sequence. Determine whether trigger and cycle events are patient or machine determined. Then, use this information to determine the breath sequence. Step 3: Identify the targeting schemes for the primary breaths and (if applicable) secondary breaths.

Example Mode Classification Mode Name: A/C Volume Control (Covidien PB 840) Step 1: Inspiratory volume and flow are preset, so the control variable is volume. Step 2: Every breath is volume cycled, which is a form of machine cycling. Any breath for which inspiration is machine cycled is classified as a mandatory breath. Hence, the breath sequence is continuous mandatory ventilation. Step 3: The operator sets all the parameters of the volume and flow waveforms so the targeting scheme is set-point. Thus, the mode is classified as volume control continuous mandatory ventilation with set-point targeting (VC-CMVs). Mode Name: SIMV Volume Control Plus (Covidien PB 840) Step 1: The operator sets the tidal volume but not the inspiratory flow. Because setting volume alone (like setting flow alone) is a necessary but not sufficient criterion for volume control, the control variable is pressure. Step 2: Spontaneous breaths are allowed between mandatory breaths so the breath sequence is IMV. Step 3: The ventilator adjusts inspiratory pressure between breaths to achieve an average preset tidal volume, so the targeting scheme is adaptive. The mode tag is PC-IMVa,s. Mode Name: Pressure Support Step 1: Inspiratory pressure is preset, so the control variable is pressure. Step 2: All breaths are patient triggered and patient cycled (note what was said about flow cycling above in Maxim 5) so the breath sequence is CSV. Step 3: Because the ventilator does not adjust any of the parameters of the breath, the targeting scheme is set-point and the tag is PC-CSVs.

Standardized Vocabulary assisted breath A breath during which all or part of inspiratory (or expiratory) flow is generated by the ventilator doing work on the patient. In simple terms, if the airway pressure rises above end expiratory pressure during inspiration, the breath is assisted (as in the Pressure Support mode). breath A positive change in airway flow (inspiration) paired with a negative change in airway flow (expiration), associated with ventilation of the lungs. This definition allows the superimposition of, for example, a spontaneous breath on a mandatory breath or vice versa. The flows are paired by size, not necessarily by timing. breath sequence A particular pattern of spontaneous and/or mandatory breaths. The 3 possible breath sequences are: continuous mandatory ventilation, (CMV), intermittent mandatory ventilation (IMV), and continuous spontaneous ventilation (CSV). compliance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in volume to the associated change in the pressure difference across the system. continuous mandatory ventilation Commonly known as “Assist/Control”; CMV is a breath sequence for which spontaneous breaths are not possible between mandatory breaths because every patient trigger signal in the trigger window produces a machine cycled inspiration (ie, a mandatory breath). Machine triggered mandatory breaths may be delivered at a preset rate. Therefore, in contrast to IMV, the mandatory breath frequency may be higher than the set frequency but never below it. In some pressure controlled modes on ventilators with an active exhalation valve, spontaneous breaths may occur during mandatory breaths, but the defining characteristic of CMV is that spontaneous breaths are not permitted between mandatory breaths. continuous spontaneous ventilation A breath sequence for which all breaths are spontaneous. elastance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in volume. Elastance is the reciprocal of compliance. control variable The variable (ie, pressure or volume in the equation of motion) that the ventilator uses as a feedback signal to manipulate inspiration. For simple set-point targeting, the control variable can be identified as follows: If the peak inspiratory pressure remains constant as the load experienced by the ventilator changes, then the control variable is pressure. If the peak pressure changes as the load changes but tidal volume remains constant, then the control variable is volume. Some primitive ventilators cannot maintain either constant peak pressure or tidal volume and thus control only inspiratory and expiratory times (ie, they are time controllers). cycle variable The variable (usually pressure, volume, flow, or time) that is used to end the inspiratory time (and begin expiratory flow). To “cycle” inspiration means to end the inspiratory time and start expiratory flow. equation of motion for the respiratory system A relation among pressure difference, volume, and flow (as variable functions of time) that describes the mechanics of the respiratory system. The simplest and most useful form is a differential equation with constant coefficients describing the respiratory system as a single deformable compartment including the lungs and chest wall connected in series to a single flow conducting tube:

PTR (t ) Pmus (t ) EV (t ) RV (t ) autoPEEP where PTR(t) = the change in transrespiratory pressure difference (i.e., airway opening pressure minus body surface pressure) as a function of time (t), measured relative to end expiratory airway pressure. This is the pressure generated by a ventilator, Pvent(t), during an assisted breath. Pmus(t) = ventilatory muscle pressure difference as a function of time (t); the theoretical chestwall transmural pressure difference that would produce movements identical to those produced by the ventilatory muscles during breathing maneuvers (positive during inspiratory effort, negative during expiratory effort) V(t) = volume change relative to end expiratory volume as a function of time (t)

V (t ) = flow as a function of time (t), the first derivative of volume with respect to time E = elastance (inverse of compliance; E = 1/C) R = resistance autoPEEP = end expiratory alveolar pressure above end expiratory airway pressure For the purposes of classifying modes of mechanical ventilation the equation is often simplified to:

Pvent

EV

RV

where Pvent = the transrespiratory pressure difference (ie, “airway pressure”) generated by the ventilator during an assisted breath expiratory time The period from the start of expiratory flow to the start of inspiratory flow. inspiratory hold (pause) time The period from the cessation of inspiratory flow (into the airway opening) to the start of expiratory flow during mechanical ventilation. inspiratory time The period from the start of inspiratory flow to the start of expiratory flow. Inspiratory time equals inspiratory flow time plus inspiratory pause time. intermittent mandatory ventilation Breath sequence for which spontaneous breaths are permitted between mandatory breaths. For most ventilators, a short “window” is opened before the scheduled machine triggering of mandatory breaths to allow synchronization with any detected inspiratory effort on the part of the patient. This is referred to as synchronized IMV (or SIMV). Three common variations of IMV are: (1) Mandatory breaths are always delivered at the set frequency; (2) Mandatory breaths are delivered only when the spontaneous breath frequency falls below the set frequency; (3) Mandatory breaths are delivered only when

the spontaneous minute ventilation (ie, product of spontaneous breath frequency and spontaneous breath tidal volume) drops below a preset or computed threshold (aka Mandatory Minute Ventilation). Therefore, in contrast to CMV, with IMV the mandatory breath frequency can never be higher than the set rate but it may be lower. For some modes (eg, Airway Pressure Release Ventilation), a short window is also opened at the end of the inspiratory time. Because spontaneous breaths are allowed during the mandatory pressure controlled breath, this window synchronizes the end of the mandatory inspiratory time with the start of spontaneous expiratory flow, if detected. With these technological developments, potential confusion arises as to whether inspiration that is synchronized (either start or stop) is considered patient triggered/cycled or machine triggered/cycled. If we say synchronized breaths are patient triggered and cycled, we have the awkward possibility of a spontaneous breath occurring during another spontaneous breath. This is avoided by distinguishing between a trigger window and a synchronization window. There are some modes where the idea of IMV may be vague: With Airway Pressure Release Ventilation, relatively high frequency spontaneous breaths are superimposed on low frequency mandatory breaths. However, the expiratory time between mandatory breaths is often set so short that a spontaneous breath is unlikely to occur between them. Other ambiguous modes are High Frequency Oscillation, High Frequency Jet Ventilation, Intrapulmonary Percussive Ventilation and Volumetric Diffusive Respiration. With these modes, high frequency mandatory breaths are superimposed on low frequency spontaneous breaths and again, there is no possibility of a spontaneous breath actually occurring between mandatory breaths. Nevertheless, we classify all these modes as forms of IMV because spontaneous breaths can occur along with mandatory breaths and because spontaneous efforts do not affect the mandatory breath frequency. mandatory breath A breath for which the patient has lost control over timing. This means a breath for which the start or end of inspiration (or both) is determined by the ventilator, independent of the patient. That is, the machine triggers and/or cycles the breath. A mandatory breath can occur during a spontaneous breath (eg, High Frequency Jet Ventilation). A mandatory breath is, by definition, assisted. primary breaths Mandatory breaths during CMV or IMV or spontaneous breaths during CSV. resistance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in flow. secondary breaths Spontaneous breaths during IMV. spontaneous breath A breath for which the patient retains substantial control over timing. This means the start and end of inspiration may be determined by the patient, independent of any machine settings for inspiratory time and expiratory time. That is, the patient both triggers and cycles the breath. Note that use of this definition for determining the breath sequence (ie, CMV, IMV, CSV) assumes normal ventilator operation. For example, coughing during VC-CMV may result in patient cycling for a patient triggered breath due to the pressure alarm limit. While inspiration for that breath is both patient triggered and patient cycled, this is not normal operation and the sequence does not turn into IMV. A spontaneous breath may occur during a mandatory breath (eg Airway Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted. synchronization window A short period, at the end of a preset expiratory time or at the end of a preset inspiratory time, during which a patient signal may be used to synchronize a mandatory breath trigger or cycle event to a spontaneous breath. If the patient signal occurs during an expiratory time synchronization window, inspiration starts and is defined as a machine triggered event. This is because the mandatory breath would have been time triggered regardless of whether the patient signal had appeared or not and because the distinction is necessary to avoid logical inconsistencies in defining mandatory and spontaneous breaths which are the foundation of the mode taxonomy. If inspiration is triggered in a synchronization window, the actual ventilatory period for the previous breath will be shorter than the set ventilatory period (determined by the set mandatory breath frequency). Some ventilators add the lost time to the next mandatory breath period to maintain the set frequency. Sometimes a synchronization window is used at the end of the inspiratory time of a pressure controlled, time cycled breath. If the patient signal occurs during such an inspiratory time synchronization window, expiration starts and is defined as a machine cycled event. Some ventilators offer the mode called Airway Pressure Release Ventilation (or something similar with a different name) that makes use of both expiratory and inspiratory synchronization windows. target A predetermined goal of ventilator output. Targets can be viewed as the goals of the targeting scheme. Within-breath targets are the parameters of the pressure, volume, or flow waveform. Examples of within-breath targets include inspiratory flow or pressure and rise time (set-point targeting), tidal volume (dual targeting) and constant of proportionality between inspiratory pressure and patient effort (servo targeting). Note that preset values within a breath that end inspiration, such as tidal volume, inspiratory time, or percent of peak flow, are also cycle variables. Between-breath targets serve to modify the within-breath targets and/or the overall ventilatory pattern. Between-breath targets are used with more advanced targeting schemes, where targets act over multiple breaths. Examples of between-breath targets and targeting schemes include average tidal volume (for adaptive targeting), percent minute ventilation (for optimal targeting) and combined PCO 2, volume, and frequency values describing a “zone of comfort” (for intelligent targeting). targeting scheme A model of the relationship between operator inputs and ventilator outputs to achieve a specific ventilatory pattern, usually in the form of a feedback control system. The targeting scheme is a key component of a mode description. tidal volume The volume of gas, either inhaled or exhaled, during a breath. The maximum value of the volume vs time waveform. trigger variable The variable that the ventilator uses to start or “trigger” the inspiratory time. Common variables are time (pressure control modes) and tidal volume (volume control modes). To “trigger” inspiration means to start inspiratory flow. trigger window The period comprised of the entire expiratory time minus a short “refractory” period required to reduce the risk of triggering a breath before exhalation is complete. If a signal from the patient (indicating an inspiratory effort) occurs within this trigger window, inspiration starts and is defined as a patient triggered event.

HANDOUT 2 – HOW TO COMPARE MODES Robert L. Chatburn, MHHS, RRT-NPS, FAARC 6/5/2014

Background Issues The appropriate use of current modes, or the development of new modes, relies on the ability to compare and contrast their relative advantages (assuming that we can identify and understand the functionality of modes in the first place; see Handout 1). In the larger context of medicine, patients are linked to their data by the process of assessing their needs (diagnosis). They are also linked to treatment options (biomedical innovation). But the fundamental responsibility of caregivers is to appropriately match patient needs to available treatments (planning). In the more restricted context of mechanical ventilation, patient needs can be expressed as three fundamental goals of mechanical ventilation (safety, comfort, and liberation). Treatment options can be viewed as the technological capabilities of various modes to serve these goals. Thus, appropriate matching of technology to needs reduces to identifying which of the available modes best serves the immediate clinical goals; see Respir Care. 2013;58(2):348-66

Why Compare Modes? We need to compare modes because there are so many of them and because they differ enough in technological capability that they cannot possibly all offer the same benefits to the patient. Hence, there is a need for comparison and choice. The issue is whether the comparisons are based on logic and information or on personal bias. Unfortunately, the amount of good animal and clinical data is relatively small. Thus, we tend to use mechanical ventilation based on tradition and the available technology rather than on evidencebased medicine. In fact, after decades of clinical research, the only thing we seem to know is that smaller tidal volumes are better than larger ones.

Which Modes Should be Compared? As with any technology of sufficient complexity, the ability to compare and contrast objects requires a shift of focus away from names to tags, using a formal classification system, or taxonomy. To briefly recap our discussion of taxonomy, all modes can be divided into two broad orders, volume control and pressure control. Within these orders are families based on the breath sequences (possible combinations of mandatory and spontaneous breaths). There are only 3 possible sequences of breaths a mode can generate: all spontaneous breaths, called continuous spontaneous ventilation (CSV), mandatory breaths with the possibility of spontaneous breaths between them, called intermittent mandatory ventilation (IMV), and mandatory breaths with no possibility of spontaneous breaths between them, called continuous mandatory ventilation (CMV). Within the families are genus and species, identified by the targeting schemes used for primary breaths (for CMV and CSV) and secondary breaths (for IMV). Major benefits accrue from using this classification system; It allows us to start with a relatively large set of unique mode names on common ventilators and greatly reduce it to a more manageable set of mode tags (classifications). In that set, redundancies are easily recognized and eliminated, leaving only unique mode tags (at least to four or five levels of discrimination) that are amenable to comparison.

How Can Modes be Compared? Despite the availability of a wide variety of modes, only the simplest set-point targeting schemes (mainly volume control continuous mandatory ventilation) are used most of the time in daily practice. Such practice may be justified by the uncomplicated reliability of these modes and the lack of evidence that any other mode is better in terms of major clinical outcomes. Yet we could also argue that “lack of evidence is not evidence of lack of differential effectiveness”. And it takes little effort to understand why there will never be enough clinical evidence to appropriately compare modes. Consider, for example, randomized controlled trials of 50 modes (approximately the number of unique modes currently available), would require 1,225 head-to-head comparisons (ie, combinations of 50 modes taken 2 at time). Using the ARDSnet experience to estimate the resource cost per study of about 4 years and 38 million dollars (in 1999), gathering evidence would take 4,900 labor years and over 46 billion US dollars! Thus, a complete set of clinical evidence required to compare all modes of ventilation does not exist, and never will. Thus, to rationally compare the relative merits of various modes, we must resort to deductive reasoning from first principles. We posit that a mode of mechanical ventilation has certain design features that implement a general technological capability. These capabilities (identified in the next section) are defined on the basis of an extensive analysis of all modes such that they can be used as unique identifiers whose benefits are intuitively obvious (again, we have no data to prove their merits). Each technological capability serves a clinical aim. Each clinical aim, in turn, serves specific objectives and general goals of mechanical ventilation based on the clinician’s assessment of the patient (Figure 1). Using this rubric, any current or proposed feature of a mode should have a direct and logical link to specific patient needs. Figure 1. Hierarchy of priorities showing how specific features of modes ultimately serve the goals of ventilation for the patient.

The utility of this hierarchical approach is that we can start on familiar ground (the general goals of mechanical ventilation) and progress deductively to a linkage with specific ventilator capabilities and features, some of which might seem questionable without such a line of reasoning to justify their existence. More to the point, the capabilities form the basis for comparing the relative benefits of modes to guide appropriate selection for a given patient at a given time. The capabilities as described here are, by definition, beneficial (given that the underlying assumptions of the targeting schemes are not violated). It follows that the more capabilities a mode has, the better it serves the specific goals of mechanical ventilation that are judged to be most important in any given clinical situation. Note that this approach explicitly ignores the issue of how modes are used. This conceptual distinction is essential because of the huge variation in outcomes that can be attributed to the different knowledge base and skill levels of clinicians. Few would argue that given current technology, a highly skilled clinician using a technologically simple mode would likely achieve better results than, for example, a naïve clinician using a complex mode.

The Three goals of Mechanical Ventilation Any number of indications for mechanical ventilation may be found in the literature, but they can all be condensed into three goals and their associated objectives: 1. Promote Safety a. Optimize ventilation/perfusion of the lung (maximize ventilation and oxygenation) b. Optimize pressure/volume curve (minimize risk of atelectrauma and volutrauma) 2. Promote Comfort a. Optimize patient-ventilator synchrony (minimize occurrence of trigger, flow, and cycle asynchronies) b. Optimize work demand versus work delivered (minimize inappropriate shifting of work from vent to patient) 3. Promote Liberation a. Optimize the weaning experience (minimize duration of ventilation and risk of adverse events)

Technical Capabilities of Modes The procedure for identifying the most appropriate mode for a particular clinical goal starts with a list of available modes (eg, on ventilators owned by a particular institution) identified by applying the mode taxonomy. Next, we construct a matrix that allows the identification of the presence or absence of the technological capabilities that fulfill a clinical goal as described above. Finally, we simply tabulate the capabilities for each mode. Three of the most common modes used in the world for adults are Volume Assist/Control (classified as VC-CMVs), Pressure Control SIMV (classified as PC-IMVs,s) and Pressure Support (classified as PCCSVs). We will contrast these modes to more sophisticated modes, AutoMode PRVC-VS (classified as PC-IMVa,a ) and IntelliVent, (classified as PC-IMVoi,oi; not available in the US). Ideally, this type of analysis should be applied to all unique modes for a complete comparison. Note that there are some capabilities that are not matched to the modes in this example but do match other modes. Goal Safety

Comfort

Liberation

Technical Capability Automatic minute ventilation target adjustment Automatic Support adjustment with changing lung mechanics Automatic frequency and/or tidal volume adjustment Manual frequency and tidal volume settings Automatic FiO2 adjustment Automatic PEEP adjustment Automatic lung protection limits Minimizes tidal volume All breaths can be spontaneous Trigger and cycle on diaphragm movement Coordination of mandatory and spontaneous breaths Automatic limits to avoid autoPEEP Unrestricted inspiratory flow Automatic adjustment of flow based on frequency Automatic adjustment of support based on breathing pattern Automatic adjustment of support to meet inspiratory effort Ventilator initiated weaning Ventilator initiated spontaneous breathing trial Automatic reduction of support with increased inspiratory effort

A/C

PC-SIMV

PS

AutoMode

x x x

x

IntelliVent

x x x x x x

x x x

x

x

x

x x

x x x

x

x x x

From this type of analysis we can identify a logical reason for preferring one mode over others on the basis of how well it serves the clinical goal of mechanical ventilation for a particular patient at a particular point in time.

Standardized Vocabulary for Mechanical Ventilation Version 7.9.14

2012 by Mandu Press Ltd. active exhalation valve A mechanism for holding pressure in the breathing circuit by delivering the flow required to allow the patient breathe spontaneously. This feature is especially prominent in modes like Airway Pressure Release Ventilation that are intended to allow unrestricted spontaneous breathing during a prolonged mandatory (i.e., time triggered and time cycled) pressure controlled breath. asynchrony (dyssynchrony) Regarding the timing of a breath, asynchrony means triggering or cycling of an assisted breath that either leads or lags the patient’s inspiratory effort. Regarding the size of a breath, asynchrony means the inspiratory flow or tidal volume does not match the patient’s demand. Also, some ventilators allow a patient to inhale freely during a pressure controlled mandatory breath but not to exhale, thus inducing asynchrony. Asynchrony may lead to increased work of breathing and discomfort. adaptive targeting scheme A control system that allows the ventilator to automatically set some (or conceivably all) of the targets between breaths to achieve other preset targets. One common example is adaptive pressure targeting (e.g., Pressure Regulated Volume Control mode on the Maquet Servo-i ventilator) where a static inspiratory pressure is targeted within a breath (i.e., pressure controlled inspiration) but this target is automatically adjusted by the ventilator between breaths to achieve an operator set tidal volume target (aka, volume-targeted pressure control). airway pressure The pressure at the airway opening measured relative to atmospheric pressure during mechanical ventilation. airway pressure release ventilation (APRV) A form of pressure control intermittent mandatory ventilation that is designed to allow unrestricted spontaneous breathing throughout the breath cycle. APRV is applied using I:E ratios much greater than 1:1 and usually relying on short expiratory times and gas trapping to maintain end expiratory lung volume rather than a preset PEEP. This is in contrast to Bilevel Positive Airway Pressure (BIPAP) which is also pressure control intermittent mandatory ventilation but with I:E ratios closer to 1:1, expiratory times that do not create significant gas tapping and preset PEEP levels above zero. assisted breath A breath during which all or part of inspiratory (or expiratory) flow is generated by the ventilator doing work on the patient. In simple terms, if the airway pressure rises above end expiratory pressure during inspiration, the inspiration is assisted (as in the Pressure Support mode). It is also possible to assist expiration by dropping airway pressure below end expiratory pressure (such as Automatic Tube Compensation on the Dräger Evita 4 ventilator). In contrast, spontaneous breaths during CPAP are

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unassisted because the ventilator attempts to maintain a constant airway pressure during inspiration. autoPEEP The positive difference between end-expiratory alveolar pressure (total or intrinsic PEEP) and the end-expiratory airway pressure (set or extrinsic PEEP; Am J Respir Crit Care Med 2011;184:756-762). When autoPEEP exists, a positive pressure difference drives flow throughout exhalation until the subsequent breath interrupts deflation. AutoPEEP is caused when expiratory time (either set by the patient’s brain or a ventilator) is short relative to the expiratory time constant of the respiratory system (possibly including the expiratory resistance of the breathing circuit). automatic tube compensation A feature that allows the operator to enter the size of the patient’s endotracheal tube and have the ventilator calculate the tube’s resistance and then generate just enough pressure (in proportion to inspiratory or expiratory flow) to compensate for the added resistive load. (See servo control.) autotrigger A condition in which the ventilator repeatedly triggers itself because the sensitivity is set too high(sometimes called “autocycling”). For pressure triggering, the ventilator may autotrigger due to a leak in the system dropping airway pressure below a pressure trigger threshold. When sensitivity is set too high, even the heartbeat can cause inadvertent triggering. Autotriggering is a form of patient-ventilator asynchrony. bio-variable targeting scheme A control system that allows the ventilator to automatically set the inspiratory pressure or tidal volume randomly to mimic the variability observed during normal breathing. Currently this “biologically variable” targeting scheme is only available in one mode, Variable Pressure Support, on the Dräger V500 ventilator. The operator sets a target inspiratory pressure and a percent variability from 0% to 100%. A setting 0% means the preset inspiratory pressure will be delivered for every breath. A 100 % variability setting means that the actual inspiratory pressure varies randomly from PEEP/CPAP level to double the preset pressure support level. blower A blower is a machine for generating relatively large flows of gas as the direct ventilator output with a relatively moderate increase of pressure (e.g., 2 psi). Blowers are used on home care and transport ventilators. (see compressor) breath A positive change in airway flow (inspiration) paired with a negative change in airway flow (expiration), associated with ventilation of the lungs. This definition excludes flow changes caused by hiccups or cardiogenic oscillations. However, it allows the superimposition of, for example, a spontaneous breath on a mandatory breath or vice versa. The flows are paired by size, not necessarily by timing. For example, in Airway Pressure Release Ventilation there is a large inspiration (transition from low pressure to high pressure) possibly followed by a few small inspirations and expirations, followed finally by a large expiration (transition from high pressure to low pressure). These comprise several small spontaneous breaths superimposed on one large mandatory breath. In contrast, during High Frequency Oscillatory Ventilation, small mandatory breaths are superimposed on larger spontaneous breaths.

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breathing circuit System of tubing connecting the patient to the ventilator. breath sequence A particular pattern of spontaneous and/or mandatory breaths. The 3 possible breath sequences are: continuous mandatory ventilation, (CMV), intermittent mandatory ventilation (IMV), and continuous spontaneous ventilation (CSV). compliance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in volume to the associated change in the pressure difference across the system. Compliance is the reciprocal of elastance. compressor A compressor is a machine for moving a relatively low flow of gas to a storage container at a higher level of pressure (e.g, 20 psi). Compressors are generally found on intensive care ventilators whereas blowers are used on home care and transport ventilators. Compressors are typically larger and consume more electrical power than blowers, hence the use of the latter on small, portable devices. (see blower) CMV See continuous mandatory ventilation continuous mandatory ventilation Commonly known as “Assist/Control”; CMV is a breath sequence for which spontaneous breaths are not permitted between mandatory breaths because every patient trigger signal in the trigger window produces a machine cycled inspiration (ie, a mandatory breath). Machine triggered mandatory breaths may be delivered at a preset rate. Therefore, in contrast to IMV, the mandatory breath frequency may be higher than the set frequency but never below it. In some pressure controlled modes on ventilators with an active exhalation valve, spontaneous breaths may occur during mandatory breaths, but the defining characteristic of CMV is that spontaneous breaths are not permitted between mandatory breaths. See mandatory breath, intermittent mandatory ventilation, trigger window continuous spontaneous ventilation A breath sequence for which all breaths are spontaneous. control variable The variable (ie, pressure or volume in the equation of motion) that the ventilator uses as a feedback signal to manipulate inspiration. For simple set-point targeting, the control variable can be identified as follows: If the peak inspiratory pressure remains constant as the load experienced by the ventilator changes, then the control variable is pressure. If the peak pressure changes as the load changes but tidal volume remains constant, then the control variable is volume. Volume control implies flow control and vice versa, but it is possible to distinguish the two on the basis of which signal is used for feedback control. Some modes (eg, High Frequency Oscillation and Intrapulmonary Percussive Ventilation) do not maintain either constant peak pressure or tidal volume and thus control only inspiratory and expiratory times (ie, they may be called time controllers). CPAP Continuous positive airway pressure; the set or measured mean value of transrespiratory system pressure during unassisted breathing or between assisted breaths. While this term is sometimes used synonymously for PEEP, historically, PEEP came

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first. PEEP mechanisms originally required the patient to drop transrespiratory system pressure to below atmospheric pressure to inhale, imposing a load and causing an increased work of breathing. CPAP mechanisms were developed so that the patient only had to drop pressure below the set CPAP level to inhale, thus decreasing the imposed load. See PEEP. CSV See continuous spontaneous ventilation; all breaths are spontaneous. See spontaneous breath. cycle (cycling) To end the inspiratory time (and begin expiratory flow) cycle variable The variable (usually pressure, volume, flow, or time) that is used to end inspiratory time (and begin expiratory flow). driving pressure The pressure causing delivery of the tidal volume during pressure control modes (ie the change in transrespiratory pressure associated with tidal volume delivery). Driving pressure may be estimated either from ventilator settings (ie, driving pressure = set inspiratory pressure above total PEEP) or from the airway pressure waveform (ie, driving pressure = end inspiratory pressure above total PEEP). See airway pressure, inspiratory pressure, peak inspiratory pressure dual targeting scheme A control system that allows the ventilator to switch between volume control and pressure control during a single inspiration. Dual targeting is a more advanced version of set-point targeting. It gives the ventilator the decision of whether the breath will be volume or pressure controlled according to the operator set priorities. The breath may start out in pressure control and automatically switch to volume control, as in the Bird “VAPS” mode or, the reverse, as in the Dräger “Pressure Limited” mode feature. The Maquet Servo-i ventilator has a mode called “Volume Control” and the operator presets both inspiratory time and tidal volume as would be expected with any conventional volume control mode. However, if the patient makes an inspiratory effort that decreases inspiratory pressure by 3 cm H2O, the ventilator switches to pressure control and, if the effort lasts long enough, flow cycles the breath. Indeed, if the tidal volume and inspiratory time are set relatively low and the inspiratory effort is relatively large, the resultant breath delivery is indistinguishable from Pressure Support. As a result, the tidal volume may be much larger than the expected, preset value. This highlights the need to understand dual targeting. Because both pressure and volume are the control variables during dual targeting, we identify the control variable as the one with which the breath initiates. This is because the alternate control variable may never be implemented during the breath, depending on the other factors in the targeting scheme. duty cycle The ratio of inspiratory time to total cycle time, usually expressed as a percent. dynamic compliance The slope of the pressure-volume curve drawn between two points of zero flow (eg, at the start and end of inspiration).

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dynamic hyperinflation The increase in lung volume that occurs whenever insufficient exhalation time prevents the respiratory system from returning to its normal resting end-expiratory equilibrium volume between breath cycles. Inappropriate operator set expiratory time may lead to dynamic hyperinflation, inability of the patient to trigger breaths, and an increased work of breathing. elastance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in volume. Elastance is the reciprocal of compliance. elastic load The pressure difference applied across a system (e.g., a container) that sustains the system's volume relative to some reference volume, and/or the amount of its compressible contents relative to some reference amount. (For a linear system: elastance volume, or, volume/compliance; for a container, the overall effective elastance (compliance) includes the elastances (compliances) of its structural components and the compressibility of the fluid [gas or liquid] within it.) equation of motion for the respiratory system A relation among pressure difference, volume, and flow (as variable functions of time) that describes the mechanics of the respiratory system. The simplest and most useful form is a differential equation with constant coefficients describing the respiratory system as a single deformable compartment including the lungs and chest wall connected in series to a single flow conducting tube:

PTR (t ) Pmus (t ) EV (t ) RV (t ) autoPEEP where PTR(t) = the change in transrespiratory pressure difference (i.e., airway opening pressure minus body surface pressure) as a function of time (t), measured relative to end expiratory airway pressure. This is the pressure generated by a ventilator, Pvent(t), during an assisted breath. Pmus(t) = ventilatory muscle pressure difference as a function of time (t); the theoretical chestwall transmural pressure difference that would produce movements identical to those produced by the ventilatory muscles during breathing maneuvers (positive during inspiratory effort, negative during expiratory effort) V(t) = volume change relative to end expiratory volume as a function of time (t)

V (t ) = flow as a function of time (t), the first derivative of volume with respect to time E = elastance (inverse of compliance; E = 1/C)

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R = resistance autoPEEP = end expiratory alveolar pressure above end expiratory airway pressure For the purposes of classifying modes of mechanical ventilation the equation is often simplified to:

Pvent

EV

RV

where Pvent = the transrespiratory pressure difference (ie, “airway pressure”) generated by the ventilator during an assisted breath expiratory flow time The period from the start of expiratory flow to the instant when expiratory flow stops, usually expressed in seconds. By convention, expiratory flow is in the negative direction (below zero) in graphs. expiratory pause time The period from cessation of expiratory flow to the start of inspiratory flow. expiratory time The period from the start of expiratory flow to the start of inspiratory flow, usually expressed in seconds. Expiratory time equals expiratory flow time plus expiratory pause time. feedback control Closed loop control accomplished by using the output as a signal that is fed back (compared) to the operator-set input. The difference between the two is used to drive the system toward the desired output (ie, negative feedback control). For example, pressure controlled modes use airway pressure as the feedback signal to manipulate gas flow from the ventilator to maintain an inspiratory pressure setpoint. flow control Maintenance of an invariant inspiratory flow waveform despite changing respiratory system mechanics flow triggering The starting of inspiratory flow due to a patient inspiratory effort that generates inspiratory flow above a preset threshold (ie, the trigger sensitivity setting). flow target Inspiratory flow reaches a preset value that may be maintained before inspiration cycles off. flow cycling The ending of inspiratory time due to inspiratory flow decay below a preset threshold (aka, the cycle sensitivity). IMV See intermittent mandatory ventilation. I:E The ratio of inspiratory time to expiratory time, TI/TE.

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inspiratory flow The flow into the airway opening during the inspiratory time. By convention, inspiratory flow is in the positive direction (above zero) in graphs. inspiratory flow time The period from the start of inspiratory flow (into the airway opening) to the cessation of inspiratory flow, usually expressed in seconds. inspiratory hold An intentional maneuver during mechanical ventilation whereby exhalation is delayed for a preset time (inspiratory hold time) after an assisted breath. This maneuver is used to assess static respiratory system mechanics and also to increase mean airway pressure during volume control ventilation in an attempt to improve gas exchange. inspiratory hold (pause) time The period from the cessation of inspiratory flow (into the airway opening) to the start of expiratory flow during mechanical ventilation, usually expressed in seconds. inspiratory pressure General term for the pressure at the patient connection during the inspiratory phase. inspiratory pressure change The change in transrespiratory system pressure associated with delivery of the tidal volume as described in the equation of motion for the respiratory system. For pressure control modes, if inspiratory pressure is set relative to atmospheric pressure, the term “peak inspiratory pressure” is used to describe the setting. If inspiratory pressure is set relative to PEEP, the term “inspiratory pressure change” is used. See equation of motion for the respiratory system, peak inspiratory pressure inspiratory time The period from the start of inspiratory flow to the start of expiratory flow, usually expressed in seconds. Inspiratory time equals inspiratory flow time plus inspiratory pause time. intelligent targeting scheme A ventilator control system that uses artificial intelligence programs such as fuzzy logic, rule based expert systems, and artificial neural networks. Examples include the rule based system used by SmartCare (Dräger Evita XL ventilator) and IntelliVent-ASV (Hamilton S1 ventilator). intermittent mandatory ventilation Breath sequence for which spontaneous breaths are permitted between mandatory breaths. For most ventilators, a short “window” is opened before the scheduled machine triggering of mandatory breaths to allow synchronization with any detected inspiratory effort on the part of the patient. This is referred to as synchronized IMV (or SIMV). Three common variations of IMV are: (1) Mandatory breaths are always delivered at the set frequency; (2) Mandatory breaths are delivered only when the spontaneous breath frequency falls below the set frequency; (3) Mandatory breaths are delivered only when the spontaneous minute ventilation (ie, product of spontaneous breath frequency and spontaneous breath tidal volume) drops below a preset or computed threshold (aka

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Mandatory Minute Ventilation). Therefore, in contrast to CMV, with IMV the mandatory breath frequency can never be higher than the set rate but it may be lower. For some modes (eg, Airway Pressure Release Ventilation), a short window is also opened at the end of the inspiratory time. Because spontaneous breaths are allowed during the mandatory pressure controlled breath, this window synchronizes the end of the mandatory inspiratory time with the start of spontaneous expiratory flow, if detected. With these technological developments, potential confusion arises as to whether inspiration that is synchronized (either start or stop) is considered patient triggered/cycled or machine triggered/cycled. If we say synchronized breaths are patient triggered and cycled, we have the awkward possibility of a spontaneous breath occurring during another spontaneous breath. This is avoided by distinguishing between a trigger window and a synchronization window. There are some modes where the idea of IMV may be vague: With Airway Pressure Release Ventilation, relatively high frequency spontaneous breaths are superimposed on low frequency mandatory breaths. However, the expiratory time between mandatory breaths is often set so short that a spontaneous breath is unlikely to occur between them. Other ambiguous modes are High Frequency Oscillation, High Frequency Jet Ventilation, Intrapulmonary Percussive Ventilation and Volumetric Diffusive Respiration. With these modes, high frequency mandatory breaths are superimposed on low frequency spontaneous breaths and again, there is no possibility of a spontaneous breath actually occurring between mandatory breaths. Nevertheless, we classify all these modes as forms of IMV because spontaneous breaths can occur along with mandatory breaths and because spontaneous efforts do not affect the mandatory breath frequency. See machine triggering, patient triggering, synchronization window, trigger window, continuous mandatory ventilation load The pressure required to generate inspiration (see elastic load and resistive load). loaded breath A breath during which all or part of inspiratory (or expiratory) flow is generated by the patient doing work on the ventilator. In simple terms, if the airway pressure falls below end expiratory pressure during inspiration, the inspiration is loaded. If pressure rises above baseline on expiration, then expiration is loaded. machine cycling Ending inspiratory time independent of signals representing the patient determined components of the equation of motion (Pmus, elastance, or resistance). Common examples are cycling due to a preset tidal volume or inspiratory time. If a patient signal (indicating expiration) occurs during an inspiratory time synchronization window, inspiration stops and is defined as a machine cycled event that ends a mandatory breath. See machine triggering, patient triggering, synchronization window, trigger window, continuous mandatory ventilation, intermittent mandatory ventilation machine triggering Starting inspiratory flow based on a signal (usually time) from the ventilator, independent of a patient trigger signal. Examples include triggering based on a preset frequency (which sets the ventilatory period), or based on a preset minimum

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minute ventilation (determined by tidal volume divided by the ventilatory period). If a signal from the patient (indicating an inspiratory effort) occurs within a synchronization window, the start of inspiration is defined as a machine trigger event that begins a mandatory breath. See machine cycling, patient triggering, synchronization window, trigger window, continuous mandatory ventilation, intermittent mandatory ventilation mandatory breath A breath for which the patient has lost control over timing. This means a breath for which the start or end of inspiration (or both) is determined by the ventilator, independent of the patient. That is, the machine triggers and/or cycles the breath. A mandatory breath can occur during a spontaneous breath (eg, High Frequency Jet Ventilation). A mandatory breath is, by definition, assisted. See assisted breath, spontaneous breath mandatory minute ventilation A form of intermittent mandatory ventilation (IMV) in which the ventilator monitors the exhaled minute ventilation as a target variable. If the exhaled minute ventilation falls below the operator set value, the ventilator will trigger mandatory breaths or increase the inspiratory pressure until the target is reached. mechanical ventilator An automatic machine designed to provide all or part of the work required to generate enough breaths to satisfy the body’s respiratory needs. minute ventilation The product of tidal volume times ventilatory frequency, usually expressed in L/min. mode of ventilation A predetermined pattern of interaction between a patient and a ventilator, specified as a particular combination of control variable, breath sequence, and targeting schemes for primary and secondary breaths. negative pressure ventilation A type of assisted breathing for which transrespiratory pressure difference is generated by keeping airway pressure equal to atmospheric pressure and making body surface pressure less than atmospheric pressure. Examples would be ventilation with an “iron lung” or “chest cuirass”. Neurally Adjusted Ventilatory Assist The name of a mode using a servo targeting scheme in which the controller sets airway pressure to be proportional to patient effort based on the voltage recorded from diaphragmatic activity from sensors embedded in an orogastric tube:

P( t )

KEdi( t )

where P(t) is inspiratory pressure relative to end expiratory pressure as a function of time, t, K is the NAVA support level (an amplification factor), Edi (t) is the electrical signal from the diaphragm as a function of time. The operator inputs the constant of proportionality between voltage and pressure (gain). Then the controller sets airway pressure to equal the product of gain and the Edi.

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optimal targeting scheme A ventilator control system that automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some overall performance characteristic. One example is Adaptive Support Ventilation (Hamilton Medical G5 ventilator) in which the ventilator adjusts the mandatory tidal volume and frequency (for a passive patient) is such a way as to minimize the work rate of ventilation. partial ventilatory support the ventilator and the respiratory muscles each provide some of the work of breathing; muscle pressure adds to ventilator pressure in the equation of motion. patient cycling Ending inspiratory time based on signals representing the patient determined components of the equation of motion, (Pmus , elastance, or resistance). Common examples of cycling variables are peak inspiratory pressure and percent inspiratory flow. See machine triggering, machine cycling, patient triggering, synchronization window, trigger window, continuous mandatory ventilation, intermittent mandatory ventilation patient triggering Starting inspiration based on a patient signal occurring in a trigger window, independent of a machine trigger signal. The signal is related to one of the patient determined components of the equation of motion (Pmus, elastance, or resistance). Common examples of patient trigger variables are airway pressure drop below baseline and inspiratory flow due to patient effort. See machine triggering, machine cycling, synchronization window, trigger window, continuous mandatory ventilation, intermittent mandatory ventilation PC-CMV Pressure controlled continuous mandatory ventilation. PC-IMV Pressure controlled intermittent mandatory ventilation. PC-CSV Pressure controlled continuous spontaneous ventilation. peak airway pressure The maximum airway pressure during a mechanically assisted inspiration, measured relative to atmospheric pressure. peak inspiratory pressure The inspiratory pressure change that is set relative to atmospheric pressure during pressure control modes. See inspiratory pressure change PEEP Positive end expiratory pressure; the value of transrespiratory system pressure at end expiration. See CPAP positive pressure ventilation A type of assisted breathing for which transrespiratory pressure difference is generated by raising airway pressure above body surface pressure (usually equal to atmospheric pressure). Examples would be ventilation with intensive care or transport ventilators. pressure A measure of force per unit of area at a particular point in space. 10

pressure change The difference between pressure (or pressure gradient) measured at one point in time and the same pressure measured at a previous point in time. pressure gradient The difference between pressure measured at one point in space and another point in space. Examples include the pressure difference across a cell membrane causing gas diffusion into the cell and the pressure difference across the respiratory system causing flow into the lungs. See transairway pressure, transalveolar pressure, transchestwall pressure, transpulmonary pressure, transrespiratory pressure, transthoracic pressure pressure control A general category of ventilator modes for which pressure delivery is predetermined by a targeting scheme such that inspiratory pressure is either proportional to patient effort or has a particular waveform regardless of respiratory system mechanics. When inspiratory pressure is preset, we further specify that inspiration must start out with the preset pressure to avoid confusion with dual targeting that may switch from a preset flow to a preset pressure (eg, Pmax feature used with volume control modes on the Dräger Evita Infinity V500 ventilator). See dual targeting scheme. According to the equation of motion, pressure control means that inspiratory pressure is predetermined as the independent variable so that volume and flow become the dependent variables. See volume control and equation of motion. pressure cycling Inspiration ends (ie, expiratory flow starts) when airway pressure reaches a preset threshold. Pressure Support: The name of a mode using a set-point targeting scheme in which all breaths are pressure or flow triggered, pressure targeted, and flow cycled. pressure triggering The starting of inspiratory flow due to a patient inspiratory effort that generates an airway pressure drop below end expiratory pressure larger than a preset threshold (ie, the trigger sensitivity setting). pressure target Inspiratory pressure reaches a preset value before inspiration cycles off. primary breaths Mandatory breaths during CMV or IMV or spontaneous breaths during CSV. Proportional Assist Ventilation (PAV) The name of a mode using a servo targeting scheme based on the equation of motion for the respiratory system in the form: P(t )

K 1V (t ) K 2V (t )

where inspiratory pressure relative to end expiratory pressure as a function of time P(t) is the sum of two components. The first is the “volume assist” or the amount of elastic load supported, ie, K1 times volume as a function of time V(t). The second component is the “flow assist” or the amount of resistive load supported, ie, K2, times flow as a function of time, V (t ). The values of K1 and K2 are preset by the operator and represent the

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supported elastance and resistance, respectively, whereas volume and flow are generated by the patient. Because volume and flow are initiated by the patient’s inspiratory effort created by muscle pressure, Pmus, the pressure generated by PAV can be thought of as an amplifier of Pmus. ramp A mathematical function whose value rises or falls at a constant rate. Ascending (rising) or descending (falling) functions are sometimes used for inspiratory flow in volume control modes. resistance A mechanical property of a structure such as the respiratory system; a parameter of a lung model, or setting of a lung simulator; defined as the ratio of the change in the pressure difference across the system to the associated change in flow. resistive load The pressure difference applied across a system (e.g., a container) that is related to a rate of change of the system's volume and/or the flow of fluid within or through the system. (For a linear system: resistance x flow, or, resistance x rate of change of volume; for a container, the effective resistance includes the mechanical (usually viscous) resistances of its structural components and the flow resistance of the fluid [gas or liquid] within it.) secondary breaths Spontaneous breaths during IMV. sensitivity The sensitivity setting of the ventilator is a threshold value for the trigger variable which, when met, starts inspiration. In other words, the sensitivity is the amount the trigger variable must change to start inspiratory flow. Sensitivity is sometimes used to refer to the cycle threshold. servo targeting A control system for which the output of the ventilator automatically follows a varying input. In practice, this means that inspiratory pressure is proportional to inspiratory effort. For example, the Automatic Tube Compensation feature on the Dräger Evita 4 ventilator tracks flow and forces pressure to be equal to flow squared and multiplied by a constant (representing endotracheal tube resistance). Other examples include Proportional Assist Ventilation (Covidien PB 840 ventilator; pressure is proportional to spontaneous volume and flow) and Neurally Adjusted Ventilatory Assist (Maquet Servo-i ventilator; pressure is proportional to diaphragmatic electrical activity). For all three of these example modes airway pressure is effectively proportional to the patient’s inspiratory effort. set-point targeting A control system for which the operator sets all the parameters of the pressure waveform (pressure control modes) or volume and flow waveforms (volume control modes). Advanced volume control modes actually allow the ventilator to make small adjustments to the set inspiratory flow to compensate for such factors as patient circuit compliance. From an engineering point of view, this is adaptive feedback control, but from a ventilator mode taxonomy point of view, such adjustments are better seen as a way of implementing operator preset values, and thus classified as set-point targeting.

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sinusoid A mathematical function having a magnitude that varies as the sine of an independent variable (eg time). A sinusoidal function is sometimes used for inspiratory flow in volume control modes. spontaneous breath A breath for which the patient retains substantial control over timing. This means the start and end of inspiration may be determined by the patient, independent of any machine settings for inspiratory time and expiratory time. That is, the patient both triggers and cycles the breath. Note that use of this definition for determining the breath sequence (ie, CMV, IMV, CSV) assumes normal ventilator operation. For example, coughing during VC-CMV may result in patient cycling for a patient triggered breath due to the pressure alarm limit. While inspiration for that breath is both patient triggered and patient cycled, this is not normal operation and the sequence does not turn into IMV. A spontaneous breath may occur during a mandatory breath (eg Airway Pressure Release Ventilation). A spontaneous breath may be assisted or unassisted. See assisted breath, mandatory breath synchronized IMV (SIMV) A form of IMV in which mandatory breath delivery is coordinated with patient effort. A synchronized breath is considered to be machine triggered. See intermittent mandatory ventilation synchronization window A short period, at the end of a preset expiratory time or at the end of a preset inspiratory time, during which a patient signal may be used to synchronize a mandatory breath trigger or cycle event to a spontaneous breath. If the patient signal occurs during an expiratory time synchronization window, inspiration starts and is defined as a machine triggered event. This is because the mandatory breath would have been time triggered regardless of whether the patient signal had appeared or not and because the distinction is necessary to avoid logical inconsistencies in defining mandatory and spontaneous breaths which are the foundation of the mode taxonomy. If inspiration is triggered in a synchronization window, the actual ventilatory period for the previous breath will be shorter than the set ventilatory period (determined by the set mandatory breath frequency). Some ventilators add the lost time to the next mandatory breath period to maintain the set frequency. Sometimes a synchronization window is used at the end of the inspiratory time of a pressure controlled, time cycled breath. If the patient signal occurs during such an inspiratory time synchronization window, expiration starts and is defined as a machine cycled event. Some ventilators offer the mode called Airway Pressure Release Ventilation (or something similar with a different name) that makes use of both expiratory and inspiratory synchronization windows. See intermittent mandatory ventilation, machine triggering, patient triggering, trigger window. tag A mode classification. A tag can be an acronym. For example the mode named Volume A/C is classified as volume control (VC) continuous mandatory ventilation (CMV) with set-point targeting (s) and can be represented as VC-CMVs. Another example (using both primary and secondary breaths) would be PRVC SIMV classified as PC-IMVa,s, where the primary breath uses adaptive targeting (a) and the secondary breath uses set-point targeting (s). The mode named Adaptive Support Ventilation has

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multiple targeting for each type of breath (ie, both optimal, o, and intelligent, i). It is classified as PC-IMVoi,oi. target A predetermined goal of ventilator output. Targets can be viewed as the goals of the targeting scheme. Within-breath targets are the parameters of the pressure, volume, or flow waveform. Examples of within-breath targets include inspiratory flow or pressure and rise time (set-point targeting), tidal volume (dual targeting) and constant of proportionality between inspiratory pressure and patient effort (servo targeting). Note that preset values within a breath that end inspiration, such as tidal volume, inspiratory time, or percent of peak flow, are also cycle variables. Between-breath targets serve to modify the within-breath targets and/or the overall ventilatory pattern. Between-breath targets are used with more advanced targeting schemes, where targets act over multiple breaths. Examples of between-breath targets and targeting schemes include average tidal volume (for adaptive targeting), percent minute ventilation (for optimal targeting) and combined PCO2, volume, and frequency values describing a “zone of comfort” (for intelligent targeting). targeting scheme A model of the relationship between operator inputs and ventilator outputs to achieve a specific ventilatory pattern, usually in the form of a feedback control system. The targeting scheme is a key component of a mode description. taxonomy A hierarchical classification system. A taxonomy for modes of ventilation has four levels: 1) the control variable, 2) the breath sequence; 3) the targeting scheme for primary breaths and ; 4) the targeting scheme for secondary breaths. These levels correspond to the levels of Family, Class, Genus, and Species of the Linnaean taxonomy used in biology. TC-IMV Time controlled intermittent mandatory ventilation (eg, High Frequency Oscillatory Ventilation or Intrapulmonary Percussive Ventilation). tidal volume The volume of gas, either inhaled or exhaled, during a breath. The maximum value of the volume vs time waveform. time cycling Inspiratory time ends after a preset time interval has elapsed. The most common examples are a preset inspiratory time or a preset inspiratory pause time. time constant The time at which an exponential function attains 63% of its steady state value in response to a step input; the time necessary for inflated lungs to passively empty by 63%; the time necessary for the lungs to passively fill 63% during pressure control ventilation with a rectangular pressure waveform. The time constant for a passive mechanical system is calculated as the product of resistance and compliance and has units of time (usually expressed in seconds). Passive inhalation or exhalation is virtually complete after 5 time constants. time control A general category of ventilator modes for which inspiratory flow, inspiratory volume, and inspiratory pressure are all dependent on respiratory system mechanics. As no parameters of the pressure or flow waveform are preset, the only

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control of the breath is the timing, ie, inspiratory and expiratory times. Examples of this are high frequency oscillatory ventilation (CareFusion 3100 ventilator) and Volumetric Diffusive Respiration (Percussionaire). tidal pressure the change in trans-alveolar pressure (i.e., pressure in the alveolar region minus pressure in the pleural space, equivalent to elastance times volume in the equation of motion) associated with the inhalation or exhalation of a tidal volume. total cycle time Same as ventilatory period, the sum of inspiratory time and expiratory time, usually expressed in seconds. total PEEP The sum of autoPEEP and intentionally applied PEEP or CPAP. Synonymous with intrinsic PEEP. time triggering The starting of inspiratory flow due to a preset time interval. The most common example is a preset ventilatory frequency. total ventilatory support The ventilator provides all the work of breathing; muscle pressure in the equation of motion is zero. This is normally only possible if the patient is paralyzed or heavily sedated. transairway pressure Pressure at the airway opening minus pressure in the lungs (i.e., alveolar pressure). transalveolar pressure Pressure in the lungs minus pressure in the pleural space. Equal to transpulmonary pressure only under static conditions. transchestwall pressure Pressure in the pleural space minus pressure on the body surface. transpulmonary pressure Pressure at the airway opening minus pressure in the pleural space. transrespiratory pressure Pressure at the airway opening minus pressure on the body surface; equal to the sum of transairway pressure plus transalveolar pressure plus transchestwall pressure. transthoracic pressure Pressure in the lungs minus pressure on the body surface; equal to the sum of transalveolar pressure plus transchestwall pressure trigger (triggering) To start the inspiratory time. See machine triggering, patient triggering trigger variable The variable (usually pressure, volume, flow, or time) that is used to start the inspiratory time. trigger window The period comprised of the entire expiratory time minus a short “refractory” period required to reduce the risk of triggering a breath before exhalation is

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complete. If a signal from the patient (indicating an inspiratory effort) occurs within this trigger window, inspiration starts and is defined as a patient triggered event. See intermittent mandatory ventilation, machine triggering, patient triggering, synchronization window ventilatory pattern A sequence of breaths (CMV, IMV, or CSV) with a designated control variable (volume or pressure) for the mandatory breaths (or the spontaneous breaths for CSV). ventilatory period The time from the start of inspiratory flow of one breath to the start of inspiratory flow of the next breath; inspiratory time plus expiratory time; the reciprocal of ventilatory frequency. Also called total cycle time or total breath cycle. volume control A general category of ventilator modes for which both inspiratory flow and tidal volume are predetermined by a targeting scheme to have particular waveforms independent of respiratory system mechanics. Usually, flow and tidal volume may be set directly by the operator. Alternatively, the ventilator may determine tidal volume based on operator preset values for frequency and minute ventilation or the ventilator may determine inspiratory flow based on operator set tidal volume and inspiratory time. When inspiratory volume and flow are preset, we further specify that inspiration must start out with the preset flow to avoid confusion with dual targeting that may switch from a preset pressure to a preset flow and volume (eg. Volume Assured Pressure Support). See dual targeting scheme. Note that setting tidal volume is a necessary but not sufficient criterion for volume control. The reason is that some ventilators use pressure control with adaptive targeting and allow the operator to set a tidal volume but not an inspiratory flow. In this case, the tidal volume setting refers to the between-breath tidal volume target, not a within-breath target. See adaptive targeting scheme. Likewise, setting inspiratory flow is also a necessary but not sufficient criterion for volume control. For example, the Bird Mark 7 ventilator requires an inspiratory flow setting but has no tidal volume setting. Instead the operator sets the inspiratory pressure, which is also the cycle variable. Hence, breaths are pressure controlled, and changing lung mechanics change the rate of pressure rise, the inspiratory time, and hence the delivered tidal volume as in other examples of pressure control. According to the equation of motion, volume control means that both volume and flow are predetermined as the independent variables and pressure is thus the dependent variable. See pressure control and equation of motion. volume cycling Inspiratory time ends when inspiratory volume reaches a preset threshold (ie, tidal volume). VC-CMV Volume controlled continuous mandatory ventilation. VC-IMV Volume controlled intermittent mandatory ventilation. volume target A preset value for tidal volume that the ventilator is set to attain either within a breath or as an average over multiple breaths.

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volume triggering The starting of inspiratory flow due to a patient inspiratory effort that generates an inspiratory volume signal larger than a preset threshold (ie, the trigger sensitivity setting). work of breathing The general definition of work is the integral of pressure with respect to volume during an assisted inspiration. There are two general components of work related to mechanical ventilation. One kind is the work performed by the ventilator on the patient, which is reflected by a positive change in airway pressure above baseline during inspiration. The second component is the work the patient does on the ventilator to (eg, to trigger inspiration), which is reflected by a negative change in airway pressure below baseline during inspiration.

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