Lung Recruitability and Positive End-Expiratory Pressure Setting in ARDS Caused by COVID-19

In ARDS, mechanical ventilation represents the milestone treatment to restore adequate gas exchange but may itself aggravate lung damage by ventilator-induced lung injury because incorrect ventilator settings are applied. Established strategies to prevent ventilator-induced lung injury include limiting tidal volume, plateau and driving pressure, and the extensive use of prone position, which are all interventions capable of improving survival.1 Differently, the role of positive end-expiratory pressure (PEEP) is debated: although the use of low PEEP (5 to 8 cm H2O) in mild-to-moderate cases (Pao2/Fio2, > 200 mm Hg) seems wise, there is no conclusive evidence to support the setting of higher vs lower PEEP in patients with moderate-to-severe disease (Pao2/Fio2, ≤ 200 mm Hg). Five different randomized studies that compared higher vs lower PEEP, with high PEEP set according to respiratory system mechanics,2 to oxygenation impairment,3,4 to maximize respiratory system compliance,5 or to achieve different degrees of positive end-expiratory transpulmonary pressure6 failed to detect a significant clinical benefit. Physiologically, PEEP always generates some sort of hyperinflation in the aerated compartment (ie, the baby lung), although it may reduce risk of ventilator-induced lung injury solely when significant alveolar recruitment occurs because of reopening of collapsed tissue, finally increasing the size of the aerated lung available for tidal ventilation. Indeed, the potential for lung recruitment as response to PEEP has wide interindividual variability.7 Mechanistically, in early moderate-to-severe ARDS, PEEP setting should aim to a balance between its capability to recruit new alveoli and the unavoidable overinflation produced in already open tissue.8,9 High PEEP is beneficial only in patients who have greater potential for lung recruitment, in whom PEEP increases the size of the aerated lung available for tidal ventilation, yielding reduced dynamic strain (ratio of tidal volume to functional residual capacity10). Conversely, in patients who are not or are poorly recruitable, PEEP only enhances lung injury by increasing static stress and strain in the baby lung. Patients with COVID-19-induced acute respiratory failure are treated with relatively high PEEP (14 cm H2O on average),11 which may depend on the positive oxygenation response to PEEP that commonly is observed in these patients, which may happen to be a falsely reassuring clinical finding, however.12,13 Improved oxygenation does not necessarily imply alveolar recruitment and less injurious ventilation but may reflect a PEEP-induced reduction in cardiac output and a change in the distribution of alveolar perfusion and/or hypoxic vasoconstriction. On the other hand, PEEP-induced significant recruitment may be accompanied by worsen or unchanged oxygenation when intracardiac shunt is increased in the presence of patent foramen ovale.14, 15, 16, 17