A 43-year-old trauma patient develops acute respiratory distress syndrome (ARDS) and has difficulty oxygenating despite increased concentrations of inspired O2 . After the positive endexpiratory pressure (PEEP) is increased, the patient's oxygenation improves. What is the mechanism by which this occurs?

PEEP improves oxygenation by increasing functional residual capacity by keeping the alveoli open at the end of expiration. Extravascular lung water is shifted from the alveolar to the interstitial space. The overall result is to increase surface area for diffusive exchange of gases. Potential negative effects of increased PEEP include alveolar overdistention resulting in barotraumas (pneumothoraces), decreased venous return and decreased cardiac output, and increased minute ventilation requirements due to increased dead-space ventilation.