Arterial blood gases were analyzed in 69 patients during breathing of room air and oxygen on the day before anesthesia and about two hours after anesthesia. Changes in uniformity of distribution of the inspired gas were also measured before and after anesthesia in 20 patients, using on open-circuit nitrogen-washout method. Significant negative correlations between PaO2 and age were found during breathing of room air and 100 per cent oxygen both pre- and postoperatively. During breathing of room air, the slope of the regression line became significantly steeper after anesthesia; with oxygen, there was no significant change after anesthesia. The nitrogen clearance delay was significantly greater after anesthesia, and this increase was most significant in the elderly. There was a significant negative correlation between age and the difference between preand postoperative FRC's (per cent). These results indicate that postoperative hypoxemia can be attributed not only to intrapulmonary shunting but also to the mismatching of ventilation to perfusion owing to increasing abnormality of distribution of ventilation with advancing age.
(C) 1972 American Society of Anesthesiologists, Inc.