CHAPTER 52

#### KEY POINTS

• Acute lung injury (ALI) and its more severe form, the acute respiratory distress syndrome (ARDS), are common causes of acute hypoxemic respiratory failure (AHRF). The 2012 Berlin Definition eliminated the term ALI; however, this term remains common to older literature.

• Both ALI and ARDS are characterized by hypoxemia that is resistant to oxygen therapy; this is due to widespread alveolar filling or collapse.

• Initial therapy for all patients with ALI and ARDS should be supplemental oxygen; failure to achieve 95% arterial saturation or greater confirms the presence of a large right-to-left shunt.

• Most patients with ALI and ARDS require ventilatory support because their AHRF is typically severe and may be prolonged.

• If a patient with severe hypoxemia as indicated by arterial blood gas analysis has a clear chest radiograph, consider a possible error (eg, incorrect fractional inspired oxygen [$FiO2$] or arterial oxygen tension [$PaO2$]); in such situations, also consider the possibility of other types of right-to-left shunts (eg, intracardiac shunts or pulmonary arteriovenous malformations) or the continued perfusion of an unventilated or poorly ventilated lung (eg, due to acute mucous plugging of one main bronchus).

• The acute phase of ALI and ARDS is characterized by an exudative alveolar flooding due to pulmonary capillary leak and by extensive alveolar collapse due to loss of normal surfactant activity; while interventions directed at modulating inflammatory or other pathways of lung injury, optimizing alveolar fluid clearance, or restoring surfactant function hold theoretical promise, at present no specific pharmacologic therapy has been shown to improve outcomes; currently one should provide lung-protective mechanical ventilation and other supportive care while identifying and treating the precipitating causes of ALI or ARDS.

• Lung-protective ventilation of patients with ALI and ARDS should use a strategy with low tidal volumes and limits to end-inspiratory pressure (ie, plateau pressures [Pplat]), to reduce the risk of ventilator-induced lung injury (VILI); such a strategy gives higher priority to the goal of decreasing the risk of VILI by limiting end-inspiratory lung volume and pressure than the traditional goal of keeping arterial carbon dioxide tension ($PCO2$) and pH in the normal range.

• The target for oxygenation should be a $PaO2$ between 55 and 80 mm Hg (88%-95% saturation); one should achieve this by adjusting $FiO2$ and positive end-expiratory pressure (PEEP) with the goal of decreasing $FiO2$ to 0.5 to 0.6 (or less), concentrations that are less concerning for pulmonary oxygen toxicity. However, despite this recommendation, it should be noted that the safety of a permissive hypoxemia approach on nonpulmonary organ function has not been demonstrated.

• In general, the ventilatory strategy should start with a tidal volume of 6 mL/kg of predicted body weight (PBW) and with a Pplat target that does not exceed 30 cm H2O; if Pplat exceeds 30 cm H2O with a 6 mL/kg PBW tidal volume, the latter should be decreased to ...

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