Pulmonary O2 diffusing capacity estimates from assumed log-normal VA/Q distributions

Steady-state pulmonary gas exchange has been measured in hypoxia in 33 mongrel dogs with the aim of comparing DLO2 estimates obtained with three procedures differing by the models assumed for functional inhomogeneity. In the first procedure the lung was assumed to be homogeneous and the corresponding DLO2 estimate was 15 mumol . min-1. Torr-1 . kg-1. In the second procedure, which is the one commonly used in respiratory physiology, alveolar dead space was considered as the unique form of functional inhomogeneity and the corresponding DLO2 estimate was 31 mumol . min-1. Torr-1 . kg-1. In the third procedure, which has been specially worked out for this study, functional inhomogeneity was represented by a log-normal distribution of the VA/Q ratios and the corresponding DLO2 estimate was 50 mumol . min-1 . Torr-1 . kg-1. The relation between the DLO2 estimates by the second and by the third procedures was found to depend upon the blood pH. This could be explained on the basis of the effects of acidosis on the blood capacitances for O2 and for CO2. Analysis suggests that in hypoxia where normally the O2 capacitance is about half the CO2 one, the third procedure yields DLO2 estimates about twice as large as those obtained by the second one.