Predicting diffusive alveolar oxygen transfer from carbon monoxide-diffusing capacity in exercising foxhounds

Although lung diffusing capacity for carbon monoxide (DL(CO)) is a widely used test of diffusive O2 transfer, few studies have directly related DL(CO) to O2-diffusing capacity (DL(O2)); none has used the components of Dl(CO), i.e., conductance of alveolar membrane and capillary blood, to predict DL(O2) from rest to exercise. To understand the relationship between DL(CO) and DL(O2) at matched levels of cardiac output, we analyzed cumulative data from rest to heavy exercise in 43 adult dogs, with normal lungs or reduced lung capacity following lung resection, that were studied by two techniques. 1) A rebreathing (RB) technique was used to measure Dl(CO) and pulmonary blood flow at two O2 tensions, independent of O2 exchange. DL(CO) was partitioned into CO-diffusing capacity of alveolar membrane and pulmonary capillary blood volume using the Roughton-Forster equation and converted into an equivalent DL(O2), [DL(O2)(RB)]. 2) A multiple inert-gas elimination technique (MIGET) was used to measure ventilation-perfusion distributions, O2 and CO2 exchange under hypoxia, to derive DL(O2) [DL(O2)(MIGET)] by the Lilienthal-Riley technique and Bohr integration. For direct comparisons, DL(O2)(RB) was interpolated to the cardiac output measured by the Fick principle corresponding to DL(O2)(MIGET). The DL(O2)-to-DL(CO) ratio averaged 1.61. Correlation between DL(O2)(RB) and DL(O2)(MIGET) was similar in normal and post-resection groups. Overall, DL(O2)(MIGET) = 0.975 DL(O2)(RB); mean difference between the two techniques was under 5% for both animal groups. We conclude that, despite various uncertainties inherent in these two disparate methods, the Roughton-Forster equation adequately predicts diffusive O2 transfer from rest to heavy exercise in canines with normal, as well as reduced, lung capacities.