Inert tracer gas washout from mixed venous blood: the sloping alveolar plateau.

The aim of this model study was to investigate the mechanisms underlying the sloping alveolar plateau for inert tracer gases supplied to the lung by mixed venous blood. Transpulmonary gas exchange was simulated in an asymmetric lung model for conditions at rest and in exercise. For highly soluble gases, the calculations show that the varying amount of tracer gas dissolved in superficial parenchymal tissue and capillary blood causes a sustained stratification in the acinus during expiration and that this is mainly responsible for the slope. For this type of tracer gas, the slope is almost independent of variations in the molecular diffusion coefficient (D) of the gases. In contrast, for poorly soluble gases, the contributions of local parallel inhomogeneities of gas concentrations in the acinus and the continued gas exchange across the alveolo-capillary membrane are mainly responsible for the slope. The first factor, which depends on the asymmetric branching pattern of intra-acinar airways, increases with decreasing D values. The contribution of continued gas exchange to the slope is most pronounced under exercise conditions. This contribution is almost independent of the blood/gas partition coefficient, lambda, for lambda values less than 4.0.