Pulmonary Ventilation-perfusion Relations after Heart Valve Replacement of Repair in Man.

Decreased oxygenation of arterial blood has been recognized as a postoperative complication of major surgery since the report in 1943 by Maier and Cournand (1). Subsequent studies confirmed their finding by demonstrating low arterial oxygen saturation or tension, particularly after thoracic and cardiac surgery (2-6). These findings suggest increased physiologic shunting (or intrapulmonary venous admixture to arterial blood) as a cause for postoperative hypoxemia. The use of extracorporeal circulation in cardiac surgery has led to such considerable problems of ventilation and oxygenation (5, 7-9) that specific lung damage is conjectured as a consequence of cardiopulmonary bypass (7, 8, 10). The physiologic (or total) shunt is that part of the right heart output which does not participate in blood-gas exchange, but is returned unoxygenated to the left heart. The four causes recognized as contributing to the total shunt are the anatomical shunt, a diffusing gradient, uneven distribution of ventilation in relation to perfusion, and atelectasis (11). The anatomical shunt through bronchial, pleural, and Thebesian veins constitutes approximately 2%o 1 of cardiac output (13). A diffusing gradi-