From the Pulmonary and Critical Care and Sleep Medicine Divisions, Brigham and Women’s Hospital and Harvard Medical

A 55-year-old man who is 178 cm tall and weighs 95 kg is hospitalized with commu-nity-acquired pneumonia and progressively severe dyspnea. His arterial oxygen satu-ration while breathing 100 % oxygen through a face mask is 76%; a chest radiograph shows diffuse alveolar infiltrates with air bronchograms. He is intubated and receives mechanical ventilation; ventilator settings include a tidal volume of 1000 ml, a posi-tive end-expiratory pressure (PEEP) of 5 cm of water, and a fraction of inspired oxygen (FiO2) of 0.8. With these settings, peak airway pressure is 50 to 60 cm of water, pla-teau airway pressure is 38 cm of water, partial pressure of arterial oxygen is 120 mm Hg, partial pressure of carbon dioxide is 37 mm Hg, and arterial blood pH is 7.47. The diagnosis of the acute respiratory distress syndrome (ARDS) is made. An intensive care specialist evaluates the patient and recommends changing the current ventilator settings and implementing a low-tidal-volume ventilation strategy. The Cl inic a l Problem Acute lung injury is defined by the American–European Consensus Conference as the acute onset of impaired gas exchange (the ratio of the partial pressure of arterial oxyge