Adult respiratory distress syndrome has been called "shock lung" or "traumatic wet lung" and occurs under a variety of circumstances. The diagnosis can be made based on bilateral pulmonary infiltrates on chest x-ray, a PaO2 /FiO2 ratio of less than 200, and pulmonary wedge pressures of less than 18 mm Hg (low filling pressures exclude the diagnosis of pulmonary edema). Three major physiologic alterations include (1) hypoxemia usually unresponsive to elevations of inspired O2 concentration; (2) decreased pulmonary compliance, as the lungs become progressively stiffer and harder to ventilate; and (3) decreased functional residual capacity. Progressive alveolar collapse occurs owing to leakage of protein-rich fluid into the interstitium and the alveolar spaces with the subsequent radiologic picture of diffuse fluffy infiltrates bilaterally. Ventilatory abnormalities develop that result in shunt formation, decreased resting lung volume, and increased dead-space ventilation.