Because of the ease with which carbon dioxide diffuses across the alveolar membranes, the PaCO2 is a highly reliable indicator of alveolar ventilation. In this postoperative patient with respiratory acidosis and hypoxemia, the hypercarbia is diagnostic of alveolar hypoventilation. Acute hypoxemia can occur with pulmonary embolism, pulmonary edema, and significant atelectasis, but in all those situations the CO2 partial pressures should be normal or reduced as the patient hyperventilates to improve oxygenation. The absorption of gas from the peritoneal cavity may affect transiently the PaCO2 , but should have no effect on oxygenation.