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, SFe) and (C2H2, Freon 22). The gases of each pair share almost the same blood-gas partition coefïicient 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 suffìcient 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 ma¡' be ascribed to the contribution of diffusionlimited 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 u'ashout), and the difference amounts to about 560/o and
760/o of the slope during mixed venous washout at rest and at the highest level of exercise, respectively. As a ñrst approximation, this difference may be considered to represent the contribution to the slope of the continued gas exchange across the alveolo-capillary membrane during expiration. ln theory. this contribution can be computed from the ratio of the alveolar ventilation and alveolar volume (VA/VA), and this is confirmed by our results.