Oxygen Deficit Is a Sensitive Measure of Mild Gas Exchange Impairment at Inspired O2 between 12.5%–21%

Abstract

The oxygen deficit (OD) is the difference between the end‐tidal alveolar PO2 and the estimated PO2 of arterial blood derived from the SpO2 that has the potential to be used clinically as a proxy for the alveolar‐arterial PO2 difference. Previous work has shown that a recently developed technology, the Alveolar Gas Meter, is accurate in measuring the efficiency of pulmonary gas exchange, via the OD, in patients with a history of lung disease as well as in normal subjects breathing a hypoxic mixture of 12.5% O2. This study sought to determine how the OD varied as the percent of inspired O2 changed from 21% to 12.5% in 36 healthy subjects split by age (young 22–31, n=23; old 42–90, n=13) to assess the possibility of use in a clinical setting. The PO2 and PCO2 were continually measured as the subject breathed through a mouthpiece using rapidly responding electrochemical cells, and the end‐tidal PO2 and PCO2 were calculated and averaged over 5 steady‐state breaths, while a fingertip pulse oximeter measured the SpO2. After correcting for any change in P50 resulting from the PCO2 differing from 40 mmHg, the arterial PO2 was calculated from the SpO2 by inverting the Hill‐equation. Subjects breathed 12.5, 15, 17.5 and 21% oxygen mixtures (balanced order) and their OD was measured in triplicate. Across all inspired O2 levels, the OD was higher in the older cohort than the young (p <0.0001, repeated measures ANOVA, Table 1). The older cohort had a larger OD at all four oxygen levels (all p<0.0001), with that difference and its variance being greater at higher inspired O2 values (age × O2 interaction p=0.002). The OD at an inspired O2 of 12.5% was significantly less than at 21% for both age cohorts. This is consistent with the known increase in ventilation‐perfusion mismatching that occurs in healthy aging. The decrease in OD magnitude as the inspired oxygen was reduced in both cohorts is consistent with the increased accuracy of the estimated arterial PO2 when based on saturation values on the steep part of the O2‐Hb dissociation curve. Further, as arterial saturation decreases there is an expected decrease in the alveolar‐arterial PO2 difference. The persisting higher OD seen in older subjects, irrespective of inspired O2, suggests that the measurement of oxygen deficit remains sensitive to mild gas exchange impairment, even when breathing air (21% O2).Support or Funding InformationFinancial support from UCSD Measured Oxygen Deficit by Age Young Old p‐value All Inspired O2 (mean±SE) −0.4±0.6 10.6±1.0 <0.0001 Inspired O2 by Age Interaction 0.002 12.5% −1.4±0.7 5.7±0.6 ‐ 15% −2.7±1.0 5.2±1.8 15% vs 12.5%: NS 17.5% −2.7±1.0 9.4±1.8 17.5% vs 12.5%: NS 21% 5.1±1.3 21.9±2.1 21% vs 12.5%: <0.0001