Acute respiratory distress syndrome: evaluation and management

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COPYRIGHT© 2012 EDIZIONI MINERVA MEDICA

REVIEW

Acute respiratory distress syndrome: evaluation and management I. CORTÉS, O. PEÑUELAS, A. ESTEBAN CIBER de Enfermedades Respiratorias, Intensive Care Department, Hospital Universitario de Getafe, Madrid, Spain

ABSTRACT Acute respiratory distress syndrome (ARDS) is a life-threatening condition that affects patients admitted in the Intensive Care Units (ICUs) under mechanical ventilation. ARDS is a process of non-hydrostatic pulmonary edema and hypoxemia associated with a variety of conditions, resulting in a direct (e.g., pneumonia) or indirect (e.g., sepsis) lung injury and is associated with a significant morbidity and mortality. A large body of clinical and basic research has focused in ventilatory strategies and novel pharmacological therapies but, nowadays, treatment is mainly supportive. Mechanical ventilation is the hallmark of the management of these patients. In the last decades, the recognition that mechanical ventilation can contribute to harming the lung has changed the goals of this therapy and has driven research to focus in ventilatory strategies that mitigate lung injury. This review emphasizes clinical aspects in the evaluation and management of ARDS in the ICUs and updates the latest advances in these therapies. (Minerva Anestesiol 2012;78:343-57) Key words: Respiratory distress syndrome, adult - Respiration, artificial - Intensive Care Units.

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urrently, acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) are life-threatening diseases with significant morbidity and mortality. They are common causes of acute respiratory failure among critical care patients and can complicate many different illnesses.1 Since it was first described in 1967 2 a large body of research has been ongoing in order to understand ALI/ARDS and to improve the clinical outcomes of this entity. ALI/ARDS is a clinical syndrome, characterized by the acute onset of severe hypoxemia with diffuse bilateral infiltrates in the chest radiograph and without evidence of left atrial hypertension. The difference between both entities is the degree of hypoxemia. In ALI, the ratio of arterial oxygen tension (PaO2) to the fraction of inspired oxygen (FiO2) is ≤300, while in ARDS it is ≤200. Many medical and surgical conditions can lead to this common pattern of lung injury (Figure 1). Traditionally, these disorders have been

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classified into direct (pulmonary) or indirect (extrapulmonary) lung injuries. According to recent epidemiological studies, direct/pulmonary injuries are the most common causes of ARDS,3-6 accounting for 55-75% of the cases. All of these disorders lead to an inflammatory injury to the lung, affecting both the vascular endothelium and the alveolar epithelium.7, 8 In the early phases of ALI/ARDS, alveolar macrophages release pro-inflammatory cytokines such as tumor necrosis factor (TNF- ), interleukin (IL)-1, IL-6, IL-8. These cytokines attract neutrophils to the lungs, where they are activated and they release a wide variety of injurious substances, such as reactive oxygen species and proteolytic enzymes (e.g., neutrophil elastase).9 These substances play an important role in the damage of the alveolar epithelium and the endothelium, favoring an increased permeability, which leads to a proteinaceous edema fluid filling alveolar space. Moreover, the ability of clear-

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ACUTE RESPIRATORY DISTRESS SYNDROME

Figure 1.—Pulmonary and extrapulmonary factors associated with acute respiratory distress.

ing of this fluid is impaired in ARDS.10 Surfactant production is also decreased, due to the injury to the alveolar epithelial type II cells. Besides, the remaining surfactant is dysfunctional and can lead to alveolar collapse.7, 11 Activation of the coagulation cascade also takes place in ARDS,12 favoring a procoagulant state that matches up with an inhibition of fibrinolytic processes. Epidemiology Estimating accurately the incidence of ALI/ ARDS is hindered by the variability of data and the differences between their sources. The variable application of the definition and the disparity in the population included in different studies limits the accuracy of the incidence data of this clinical entity. Data concerning incidence of ALI/ARDS arise from large prospective cohort studies.3-6, 13-18 Estimated incidence raised by these studies range from 5 to 80 per 100,000 person-years. In the ICU setting, 7-10% of ad-

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mitted patients and 5-8% of the mechanically ventilated ones meet criteria for ALI/ARDS.19 Both ALI and ARDS are associated with considerable mortality. Reported mortality rates ranged from 25% to nearly 75%. This substantial variability depends on multiple aspects: the type of study (observational versus randomized controlled trials), the definition used, and the population included, etc. Usually, the mortality rate in observational studies is significantly higher than the one reported in randomized controlled trials (RCTs). This is mainly the result of the way the enrollment takes place in RCTs, where only a selected group of patients is typically included in these studies. Some of these trials excluded more than 75% of patients assessed for eligibility.20 Therefore, the mortality rates reported by RCTs should be analyzed separately from those of observational cohort studies, which may represent a more realistic scenario. Although some recent studies and a meta-analysis 21 have suggested that mortality from ARDS may be decreasing, this feeling

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CORTÉS

Table I.—Definition for the Acute Respiratory Distress Syndrome according with the American-European Consensus Conference (AECC). Acute onset of severe hypoxemia Bilateral pulmonary infiltrates No evidence of left atrial hypertension

PaO2/FiO2 ratio≤300 for ALI and ≤200 in ARDS PAWP (*)