Alveolar-arterial O 2 and CO 2 pressure differences in the anesthetized, artificially ventilated dog

Abstract

The alveolar-arterial pressure differences of O 2 and CO 2, (A-a)Do 2 and (a-A)Dco 2, were determined in the artificially ventilated dog in pentobarbital anesthesia over a wide range of oxyganation. The alveolar P CO 2 , was between about 30 and 38 nun Hg. The (A-a)Do 2 was 21.4±9.1 mm Hg (± = standard deviation) at normoxia, 10.0±3.8 nun Hg at deep hypoxia, and 54.8+-46.4 or 70.7+-43.8 mm Hg at 60 or 100% O 2, resp., the difference between the two levels ofhyperoxia being highly significant. The (a-A)Dco 2 showed a tendency to increase from 1.0±1.1 mm Hg at deep hypoxia to 3.8 ±2.5 mm Hg at normoxia and to 5.6 ±2.9 mm Hg at hyperoxia, but the differences between hypoxia and normoxia or between normoxia and hyperoxia were not significant. The shunts computed from the (A-a)Do 2, at the two levels of hyperoxia were 3.2±3.1 % and 4.5-2.9%, resp., the difference being significant. There seems to exist an alveolar dead space component over the whole range of oxygenation as demonstrated by the (a-A)Dco,. Since an appreciable contribution of atelectasis to the shunt measured at hyperoxia could be ruled out on the basis of systematic investigations reported in a companion paper, the difference between the two levels of hyperoxia probably reflects a change in anatomical extrapulmonary veno-arterial shunting. The (A-a)Do 2 at normoxia could largely be accounted for by the alveolar dead space component and by the extrapulmonary anatomical venoarterial shunting. The (A-a)Do 2 showed a gradual decrease with deeper hypoxia but became constant in the range of 40 to 20 mm Hg Pao 2.