Airway research: the current status and future ... - Wiley Online Library

Anaesthesia, 2011, 66 (Suppl. 2), pages 3–10 doi:10.1111/j.1365-2044.2011.06928.x .....................................................................................................................................................................................................................

Airway research: the current status and future directions S. Isono,1 R. Greif2 and T. C. Mort3 1 Associate Professor, Department of Anesthesiology (B1), Graduate School of Medicine, Chiba University, Chiba, Japan 2 Professor, Department of Anesthesiology and Pain Therapy, University Hospital Bern and University of Bern, Bern, Switzerland 3 Senior Associate, Anesthesiology, Associate Director, Surgical ICU, Hartford Hospital, University of Connecticut School of Medicine, Connecticut, USA

Summary

We highlight the areas we think important for future development of the subspeciality. The ultimate goal is to improve patient care and safety and to do this, we need to identify how and where episodes of harm arise. Simply continuing with current practice does not represent the best path towards our ultimate goal; objective evidence is needed to inform changes in practice. . ......................................................................................................

Correspondence to: S. Isono Email: [email protected] Accepted: 12 Sept 2011

The upper airway serves various physiological functions such as respiration, speech, deglutition and airway protection. Anaesthesia and surgical procedures impair all of these. Difficult airway research has mainly focused upon tracheal intubation, mask ventilation and use of specific airway devices. This strategy has successfully identified risk factors for difficult airway management during induction of anaesthesia [1–3], it has driven invention of dedicated devices [4, 5], and it has led to development of difficult airway management algorithms [6, 7]. This has all resulted in reduced critical respiratory complications and mortality during induction [8]. Because there continues to be a substantial number of unreported airway-related deaths this strategy needs to be continued. Recent surveys and robust outcome studies [8–10] strongly indicate that a key driver is the need to refine the available difficult airway management algorithms. In this context, large clinical studies examining the efficacy of various strategies or of educational approaches will be needed. Airway protection during anaesthesia

Anaesthesia and neuromuscular blockade during induction impair airway protective functions. This has implications for patients with full stomachs and others at risk of regurgitation. ‘Rapid sequence induction - intubation 2011 The Authors Anaesthesia 2011 The Association of Anaesthetists of Great Britain and Ireland

with the Sellick manoeuvre (cricoid pressure) - is designed to minimise risk [11]. However, the efficacy of this established technique has been questioned because: (1) in 50% of patients there is lateral displacement of the oesophagus away from the cricoid cartilage [12]; (2) lower oesophageal sphincter tone is reduced [13]; (3) cricoid pressure may cause upper airway obstruction even in adults, particularly when an excessive force is applied [14]; and (4) it interferes with placement and performance of the laryngeal mask airway, should this be needed in a ‘failed intubation’ scenario [15]. Interestingly, the head and neck positions in Sellick’s original study differ greatly from those in current clinical practice, and pulmonary aspiration after induction has been reported [16]. Rather than continue to practise a technique that may not work, these uncertainties now warrant assessing the utility of the Sellick manoeuvre in a large clinical study. Sellick’s group originally advocated applying some concomitant positive pressure ventilation (with the Sellick manoeuvre preventing gastric inflation), but this is now virtually routinely prohibited as part of a rapid sequence induction except when intubation fails [17]. This alternative ‘apnoeic strategy’ was proposed by Stept and Safar because they considered inadequate paralysis could induce straining and regurgitation [18]. Their technique has become our standard for rapid 3

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S. Isono et al. Future directions of research Anaesthesia, 2011, 66 (Suppl. 2), pages 3–10 . ....................................................................................................................................................................................................................

sequence induction, based only on their preliminary observations in just 80 patients with a full stomach (and without a control group). Notably, we also lack clear evidence against applying positive pressure ventilation during a rapid sequence induction. Yet, using a modern-day pressure-controlled ventilator, we could easily and accurately limit peak inspiratory pressure to a level lower than the oesophageal barrier pressure, and so maintain oxygenation (with or without cricoid force), in a way that does not insufflate the stomach [19]. It is surprising that > 50% of cases of primary pulmonary aspiration leading to an adverse outcome occur during maintenance of anaesthesia [9], and mortality rates of this complication appear to be higher than might be presumed from previous reports [16]. In the majority of these reported cases, first generation supraglottic airway devices (SADs) were used. We must now ask ourselves if this class of device is being inappropriately or over-used, especially in conjunction with positive pressure ventilation, neuromuscular blockade and in patients with a propensity to ‘full stomachs’, and perhaps whether some of the newer second generation SADs offer benefit in these circumstances. Postoperative upper airway obstruction

Whereas the incidence of death or severe brain injury during induction of anaesthesia appears to have declined after the introduction of formal airway management algorithms, the incidence after extubation does not seem to have changed [8, 20]. In the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society (NAP4) [9], one third of cases occurred after the end of anaesthesia, during emergence or in the recovery area. All were due to airway obstruction. Specific guidelines are awaited and there are few investigations on upper airway functions during this period of recovery from anaesthesia. One question concerns the relative merits of ‘awake’ tracheal extubation vs ‘deep’ extubation (i.e. before return of full consciousness) [6]. The proper answer relies upon knowledge of upper airway patency during this recovery period [21]. Some of the relevant physiology is discussed elsewhere in this issue [22], but it is clear that the upper airway at the end of surgery is not the same as that before anaesthesia induction. Residual muscle paralysis is significantly associated with a high incidence of hypoxaemia and of upper airway obstruction in the post-anaesthesia care unit [23]. However, we still do 4

not know how other potential risk factors for airway obstruction influence outcomes. Airway patency is determined by complex interactions between neuromuscular control mechanisms and inherent structural properties [24]. A compromised neuromuscular function due to residual paralysis is not the sole cause of obstruction immediately after extubation, and judicious use of neuromuscular monitoring or sugammadex may not completely solve the problem. Tissue swelling caused by repeated airway interventions themselves or by surgical or other direct trauma (particularly by upper airway surgery or even excessive fluid administration) may increase resistance to airflow and adversely affect mucosal airway receptors [25, 26]. Reduction of the functional residual capacity of the lung, which is a major mechanical change in the respiratory system during anaesthesia, may significantly increase upper airway collapsibility, particularly in the obese [27]. Sub-narcotic levels of anaesthetics may enhance the effect of residual muscle paralysis [28]. Furthermore, opioids for postoperative pain relief and comorbidities such as obesity and obstructive sleep apnoea may cause airway obstruction [24]. Immediately after tracheal extubation, we lose highly reliable continuous respiratory monitoring such as capnography and tidal volume measurements and therefore, upper airway obstruction may remain undetected until development of severe desaturation, even in the post-anaesthesia care unit or intensive care unit (ICU). There is a need for effective respiratory monitoring suitable for non-tracheally intubated patients and part of the solution may involve increased use of already available technology such as capnography in recovery [29]. Current interventions to alleviate airway obstruction such as insertion of a nasal airway, nasal continuous positive airway pressure or oxygen administration by facemask are not always well tolerated by postoperative patients. Systematic evaluation of new airway devices

Every year new airway devices enter the market, designed to help secure the airway or facilitate tracheal intubation [30, 31]. Yet in contrast to introduction of drugs, there is no requirement to produce evidence of clinical efficacy before marketing [32–34]. High-powered clinical studies carry substantial cost and this might be one further reason why there is a deficiency of good clinical trials in the field. Furthermore, the required end points are varied. With SADs, for example, we need to know the insertion success and speed, laryngeal 2011 The Authors Anaesthesia 2011 The Association of Anaesthetists of Great Britain and Ireland

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Anaesthesia, 2011, 66 (Suppl. 2), pages 3–10 S. Isono et al. Future directions of research . ....................................................................................................................................................................................................................

seal pressure, how easily accessible the oesophagus is for gastric drainage, and whether or not the SAD also serves as a conduit for tracheal intubation (either blind or under flexible fiberscoptic guidance). As with our risk assessment for new drugs, we want to know common side effects and rare complications of the device. At the very least, we wish to know if the new product is equivalent (i.e. not inferior) to any existing products. However, if we arbitrarily set the ‘clinically relevant range’ of performance by which to judge equivalence at the usual level of statistical significance, then where one of 20 attempts fails in an existing device, the new device could fail in two of 20 attempts and yet be regarded as non-inferior. This does not seem persuasive, so the anaesthesia community has to decide, for each new device (and perhaps based upon the intended use), a precise clinically relevant range for purposes of such ‘non-inferiority’. Because the incidence of a difficult airway or failed intubation is relatively rare, large numbers of patients need to be enrolled initially in any given trial. This can only be achieved by multicentre studies [35, 36]. This conclusion introduces a strategic, planning aspect to our subspeciality. Because manufacturers do not currently have any legal obligation to conduct the needed research, more subtle or indirect ways need to be explored. One potential driver is the sheer size of the market (in the UK alone, 3 million general anaesthetics are administered every year) [37] and one important factor favouring one device over the other will be the clinical evidence supporting it. This strategy has been recently employed by the UK’s Difficult Airway Society (DAS) in its ‘ADEPT’ program to encourage airway-related research [34]. Another potential driver that is an incentive to anaesthesia researchers is the apparent simplicity of some of the relevant questions. Should we administer neuromuscular blocking agents before confirmation of the ability to ventilate the patient’s lungs [38–42]? Is cricoid force useful or harmful during rapid sequence induction [43–51]? Does the improved glottic view conferred by videolaryngoscopes actually translate to better outcomes [52]? What are the proper limits to using SADs in terms of patients’ body weight, duration of surgical procedure, or patient position (e.g. lateral or prone) [53, 54]? Answers to these simple questions (and many others like them) will have enormous impact upon large areas of practice. Even where absolute impact is modest, the sheer number of patients receiving anaesthesia will mean there is a huge potential increase in patient safety. We need to make a 2011 The Authors Anaesthesia 2011 The Association of Anaesthetists of Great Britain and Ireland

switch from ‘eminence-based’ opinions (from the best knowledge of the authors) to more solid evidencebased conclusions. Training and research in training

Although high-quality investigations are being undertaken [55–59], one problem with our use of devices relates to how we are taught to practise and maintain competencies. There is some evidence that manikin studies can help in training and its assessment, in human factors skills and in maintaining competency [60–62]. However, we know little about how skills acquired in manikins are transferred into practice. Very little research has explored how changes in team behaviour can be transferred into the anaesthesia department culture. Perhaps parallels can be drawn with aviation, where pre-operative ⁄ pre-flight checklists are essential, akin to the recently introduced surgical safety campaign [63–67]. It appears that the quality of pre-operative airway assessment is poor and that there is little planning of airway management. Backup plans for failed primary strategies are rarely discussed, and specific concerns are rarely shared with the wider team such as nursing and surgical staff [9]. Another blind spot in educational research is specific training in the airway management of obese patients. Currently available manikins fail to simulate this group of patients, so that trainees learn directly on these patients for the first time. Work-based assessments using well established educational tools to provide structured feedback [68– 70] need to be adapted for specific airway management situations. Finally, it is relevant in this context to ask: ‘Who trains the trainers?’. How should we best train airway teachers so that they provide efficient teaching that enhances understanding and retention of airway knowledge, the skills to manage the airway and the attitudes to the team approach (including surgeons), with the aim of improving safety for patients? The practical challenge: evidence into practice

Not all problems can be solved by recourse to technology. Difficult airway management outside the operating room (e.g. a remote location including the ICU) may be one of the most challenging clinical situations to confront an anaesthetist. Characteristically, the patient is critically ill and the airway team can include many with no formal airway training at all. A videolaryngoscope is not a simple solution, in the 5

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absence of integrated teamwork. Yet, airway difficulties and catastrophes arise even in apparently nearoptimal conditions, such as the operating room itself [71]. If the outcome is adverse in this scenario, is it the patient, the equipment, the anaesthetist, poor judgment, some intangible pressure to avoid the correct solution (such as an awake tracheal intubation), failure to adhere to established algorithms, poor planning, substandard execution or just bad luck that is at fault [6, 8, 72–77]? These questions may need to be asked in settings of a formal inquiry if an outcome is tragic. Unfortunately, a perfect plan and its execution do not guarantee a good outcome; conversely, substandard management does not make catastrophe inevitable. Just because another anaesthetist has successfully managed an obese patient with a receding chin, limited mouth opening and sleep apnoea with a SAD, it does not make this the correct way to manage this patient. Or, as O’Sullivan et al. observe: ‘…If we cross a motorway once or even several times successfully with our eyes shut, it does not mean it is the proper way to cross a road.’[78]. We all desire more ‘evidence’ but those who are experts in airway management temper that evidence with their own extensive experience. Thus, the purpose of evidence is not to generate rigid management protocols, but to offer a rational basis for the use of equipment or interventions, that can be tailored by the expert to an individual patient. Complications of difficult airway management and their avoidance

Critical complications in airway management are plentiful and include: oesophageal intubation, airway trauma caused by multiple intubation attempts; aspiration; hypoxaemia; and haemodynamic instability such as hypotension, bradycardia, arrhythmias or frank cardiac arrest [9, 79–84]. Yet there is a great deal of denial that complications arise. A manuscript was recently submitted to a leading anaesthesia journal investigating the introduction of the ASA Difficult Airway Algorithm into a critical care setting, finding this to reduce the incidence of adverse events by 50–75% [84]. Reviewers comments included statements like: ‘what is so important?[in this manuscript]...[cardiac] arrest occurs commonly in this setting’ and :‘it is hard to believe that the rate of complications is possibly this high’. The overall complication rate is difficult to estimate as near-misses are often not reported; serious events such as neurological damage or death often become the 6

focus of legal rather than scientific inquiry, and ‘minor’ complications (e.g. dental damage) are commonly reported but regarded only as a nuisance . The NAP4 study is probably the best prospective estimate available of the mortality and morbidity of airway-related complications. It yielded a point estimate of one serious adverse event every 22 000 general anaesthetic cases. However, the study possibly missed three quarters of cases, which, if correct, would be consistent with an incidence of 1:5500. Furthermore, the data followed a Poisson distribution [85], meaning that the incidence could be as high (5% of the time) as 1:1375. Important near-misses such as severe hypoxia or failed intubation are clearly likely to be much higher than any of these estimates. The findings outside the operating room are even more notable, with a fatal airway event occurring in the emergency department up to 35 times more often than during anaesthesia and in the ICU 70 times more often [9, 71]. Remarkably, unanticipated difficult tracheal intubation is independently associated with a nine-fold increase in mortality [85, 86]. Complications are likely to depend on factors such as training, number of intubation attempts, a restricted laryngoscopic view and the patient’s location (i.e. ward vs operating theatre vs ICU) [9, 71, 86, 87]. Since we are concerned with preventing hypoxaemia, we must appreciate that pre-oxygenation methods may not work as well in those who are critically ill [88–91]. Methods for denitrogenation studied in the healthy may not apply in other patient groups and it remains to be seen if specific interventions work (such as elevating the head of the bed, non-invasive bi-level positive airway pressure, non-invasive ventilation and alveolar recruitment manoeuvres, pre-intubation insertion of a SAD or supplemental nasal oxygen administration) [92–97]. Conclusions

When we are asked by a trainee we are supervising: ‘why do you do it that way?’, our answers should now be couched in the terminology of evidence-based methodology. Although interventions not rigorously tested (or even formally reviewed) for several decades are perhaps entrenched in widespread practice, there is today much less room for such habits or dogma. Our interest in how to manage the airway now needs to be extended to research in airway protection, airway maintenance during and after recovery from anaesthesia and especially airway management in the

2011 The Authors Anaesthesia 2011 The Association of Anaesthetists of Great Britain and Ireland

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ICU. Within these themes, there are specific research questions that will encourage vibrant thinkers in our field to challenge some of our old habits. The results arising from these enquiries will improve our collective standing as ‘guardians of the airway’, promoting patient safety. The answer to the question ‘why do we do it this way’ will then have a clear and rational answer.

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Competing interests

No external funding or competing interests declared.

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References

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