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. Submitted on October 21, 1959
Prognostic Role of Alveolar-Arterial Oxygen Pressure Difference in Acute Pulmonary Embolism
