Effects of cardiac output on ¹⁸O₂ lung diffusion in normal resting man

Single-breath resting diffusing capacities for CO (Dl_CO) and ¹⁸O₂ (Dl_O2 ) were measured simultaneously with pulmonary capillary blood flow (Q̇c) in nine normal males at hypoxic alveolar O₂ tensions (P_O2 42 torr). Dl_CO (54 ± 5 ml stpd . min^-1 . torr^-1) was significantly higher than Dl_O2 (33 ± 11 ml stpd ± min^-1 . torr^-1). Extrapolation of the linear regressions between Dl_CO and Dl_O2 and cardiac output (range 4860 to 12,270 ml . min^-1) suggest that Dl_O2 approximates Dl_CO at cardiac outputs of 18 L . min^-1 in our subjects. No significant differences between the previously measured diffusing capacity for ¹⁸O¹⁶O (Hyde et al., 1966) and the present value for ¹⁸O¹⁸O was found. The discrepancy between the measured Dl_O2 and the predicted Dl_O2 calculated indirectly from the Dl_CO is confirmed. Inhomogeneity of pulmonary capillary diffusing surface area to capillary blood flow (Dl/Q̇c) probably accounts for this discrepancy. DL/Q̇c inhomogeneity, along with right-to-left shunts and V̇a/Qc inhomogeneities, must be taken into account when arterial-alveolar O₂ gradients are evaluated.