Oxygen deficit is a sensitive measure of mild gas exchange impairment at inspired O between 12.5% and 21.

The oxygen deficit (OD) is the difference between the end-tidal alveolar Po and the calculated Po of arterial blood based on measured oxygen saturation that acts as a proxy for the alveolar-arterial Po difference. Previous work has shown that the alveolar gas meter (AGM100) can measure pulmonary gas exchange, via the OD, in patients with a history of lung disease and in normal subjects breathing 12.5% O. The present study measured how the OD varied at different values of inspired O. Healthy subjects were split by age (young 22-31; = 23; older 42-90; = 13). Across all inspired O levels (12.5, 15, 17.5, and 21%), the OD was higher in the older cohort 10.6 ± 1.0 mmHg compared with the young -0.4 ± 0.6 mmHg ( < 0.0001, using repeated measures ANOVA), the difference being significant at all O levels (all < 0.0001). The OD difference between age groups and its variance was greater at higher O values (age × O interaction; = 0.002). The decrease in OD with lower values of inspired O in both cohorts is consistent with the increased accuracy of the calculated arterial Po based on the O-Hb dissociation curve and with the expected decrease in the alveolar-arterial Po difference due to a lower arterial saturation. The persisting higher OD seen in older subjects, irrespective of the inspired O, shows that the measurement of OD remains sensitive to mild gas exchange impairment, even when breathing 21% O.