The effects of D/Q and D/VA inequalities on pulmonary oxygen diffusing capacity estimates

Bohr-integrated estimates of steady-state pulmonary diffusing capacity for oxygen (DLO2) were calculated using lung compartments with different VA/Q ratios. Compartmental diffusing capacity was distributed in proportion to either ventilation or blood flow. This method was applied to 115 gas exchange data sets previously collected from healthy human subjects during normoxic (PIO2 = 148 Torr) or acute hypoxic (PIO2 = 80 Torr) exercise. Ventilation-weighted estimates of oxygen diffusing capacity exceeded blood flow-weighted estimates by 10 and 78% during hypoxia and normoxia, respectively. While blood flow-weighted DLO2 estimates during heavy exercise were greater in hypoxia than in normoxia (mean +/- SEM of 87.1 +/- 4.4 vs 53.0 +/- 3.1 ml.min-1.Torr-1), ventilation-weighted estimates were not significantly different (93.2 +/- 5.5 vs 96.3 +/- 9.4 ml.min-1.Torr-1). These results show that the effect of D/Q inequality on DLO2 estimates is greater at higher levels of inspired oxygen. Furthermore, a D/Q mismatch of similar magnitude as the VA/Q mismatch that is observed during heavy exercise can account for a previously observed discrepancy between normoxic and hypoxic oxygen diffusing capacity estimates.