Alveolar-capillary diffusion of oxygen in dogs exercising in hypoxia

Abstract

To detect and quantify diffusion limitation in alveolar-capillary O 2 transfer, measurements of pulmonary gas exchange were performed in 6 awake, chronically tracheostomized dogs (mean body weight 28.3 kg) breathing low O 2 (arterial P O 2 35–39 Torr), with or without CO 2 added to inspired gas. From rest to exercise, with O 2 uptake averaging 23 ml/(min · kg), the ideal-alveolar-to-arterial P O 2 difference (P ai O 2 −Pa O 2 ) increased from 2.1 ± 0.2 Torr (mean ± SE) to 2.9 ± 0.2 in hypoxia, and from 1.9 ± 0.7 Torr to 3.0 ± 0.5 in hypercapnic hypoxia. The apparent pulmonary O 2 diffusing capacity (D l O 2 ), calculated from O 2 uptake, mixed-venous P O 2 , arterial P O 2 , and ideal-alveolar, was found to be increased by hypercapnia and exercise. During exercise D l O 2 averaged 74 ml/(min · Torr) in hypoxia and 76 ml/(min · Torr) in hypercapnic hypoxia. Because of the influence of inhomogeneity effects, these values should be considered as minimum values for the true pulmonary O 2 diffusing capactity. When compared to the pulmonary CO diffusing capacity (D l CO) previously determined by C 18O rebreathing in the same dogs in similar conditions, the D l O 2 /D l CO ratio averaged 1.2, thus being in accordance with the value predicted from the corresponding Krogh diffusion constant ratio. It is concluded that the D l O 2 and D l CO values determined during exercise in hypoxia may be considered to represent acceptable measures for alveolar-capillary diffusion conductance of lungs.