The sloping alveolar plateau of tracer gases washed out from mixed venous blood in man.

We have investigated the slope of the alveolar plateau for inert tracer gases that were washed out from mixed venous blood. Two pairs of tracer gases were used (He, SF6) and (C2H2, Freon 22). The gases of each pair share almost the same blood-gas partition coefficient but they have different diffusive properties in the gas phase. The experiments were performed in healthy subjects at rest and at three levels of exercise (75, 150, 225 W). Each experiment started with the alveolar washin of the tracer gases by adding these gases to inspired air. This washin was continued for several minutes in order to dissolve sufficient amounts of the tracer gases in the body tissues. Subsequently, the tracer gases were washed out. In this paper, the slopes of the alveolar plateaus are defined as the relative increase of the concentration per second. Steeper slopes were found for the heavier gases (SF6 and Freon 22) in comparison with those for the lighter gases of the two pairs (He and C2H2). This finding may be ascribed to the contribution of diffusion-limited gas mixing in the lung to the slope of the alveolar plateau. For each gas, the slope for the first expiration during washout (alveolar washout) was considerably smaller than that for the later part of washout (mixed venous washout), and the difference amounts to about 56% and 76% of the slope during mixed venous washout at rest and at the highest level of exercise, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)