Alveolar-capillary diffusion and ventilation-perfusion inhomogeneity: a mathematical model

In analysis of ventilation-perfusion inhomogeneity with inert gases, it is commonly assumed that the effects of alveolar-capillary diffusion can be ignored. This approximation may not be valid during exercise or in the presence of oedema or emphysema. A mathematical model is presented of inert gas exchange that allows for a diffusion limitation as well as ventilation-perfusion inhomogeneity. These effects can be distinguished by examining how partial pressure ratios of various inert gases change with solubility and alveolar-capillary diffusivity. Furthermore, the model suggests that the input of inert gases by inhalation rather than venous infusion is feasible and that sampling of mixed-venous blood is unnecessary. Hence, the inert gas study of abnormal pulmonary gas exchange may yield more information and may be more readily applied to clinical studies than currently recognised.