Attempt to demonstrate the distribution component of the alveolar-arterial oxygen pressure difference

¹ Department of Physiology, The University of Buffalo School of Medicine, Buffalo, New York

Under normal conditions, the alveolar-arterial oxygen pressure difference (A-a difference) is believed to be due primarily to two causes, venous admixture (shunt component) and variation in ventilation-perfusion ratios (distribution component). In order to assess quantitatively the role played by the latter factor, the difference in O₂ tension between end-tidal air and arterial blood was measured in anesthetized dogs under two conditions: a) at normal ambient pressure (Pb = 747 mm Hg), breathing air; b) at low ambient pressure (Pb = 192 mm Hg), breathing 100% O₂. The essential feature of this procedure is that, while the inspired O₂ tension is identical in both conditions, there is no nitrogen in the gas inspired at low ambient pressure. Theoretical analysis indicates that, if all or part of the A-a difference measured under condition a is due to a variation in ventilation-perfusion ratios, then this difference must be significantly reduced under condition b. Such a decrease in the A-a difference should provide an index of the magnitude of the distribution component. However, in our experiment, the A-a difference was found to be the same in both conditions. Therefore, our results do not support the hypothesis that there exists a large distribution component of alveolar-arterial difference.