Emphysema as possible explanation for the alteration of expiratory PO2 and PCO2 curves.

The present explanation of the shape of deformed expiratory partial pressure curves of O2, CO2, N2 and foreign gases as being caused by sequential convective emptying of compartments with decreasing VA/Q is put into question, since the same deformation can be observed in healthy subjects during simultaneous breathing of helium and SF6, where the SF6 curve is deformed and the helium curve has a normal shape. The high molecular weight of SF6 causes a delay of its gaseous diffusion from the peripheral air spaces of the lung. A similar delay can be caused by emphysematous changes such as a longer distance for diffusion, an increased alveolar volume and a reduction of the number of small airways, where the gas has to pass through. Different data from cross-sectional and longitudinal studies, from induced changes of bronchomotor tone, from comparison of patients with relatively pure emphysema and patients with airway obstruction and from autopsy suggest that stratification and serial inhomogeneities, caused by emphysema, are at least a reasonable, if not a better explanation for the deformation than the VA/Q concept with sequential emptying of parallel units.