Pulmonary Ventilation-Perfusion Relations after Heart Valve Replacement or Repair in Man*

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Abstract
Pre and postoperative studies were made in 18 patients selected at random from those undergoing aortic or mitral surgery. Arbitrarily the postoperative studies were made 20-24 hours after surgery. At this time when air was breathed, physiologic shunts of up to 35% of the cardiac output caused hypoxemia. When 100% oxygen was given the average physiologic shunt was 13% of cardiac output. It was concluded that the most important single cause of this shunting was diffuse atelectasis, usually invisable to x-ray. Extracorporeal circulation had no apparent effect in increasing physiologic shunting. Post-operatively the increase in shunting of air, as compared with 100% oxygen, was significant in the group of patients who had mitral valve replacement or open mitral valvulopasty (PL .01). This increase in physiologic shunting after changing to air was caused by maldistribution of ventilation in relation to perfusion and was equal to an average increase in right to left shunt of 12.5% of the cardiac output. The average postoperative right to left shunt on air of these patients who had mitral valve replacement was 26% of cardiac output. These large shunts of air, the result of atelectasis, the normal anatomical shunt and maldistribution caused the mean arterial oxygen of these patients when breathing air to be only 53 mm Hg.