Alveolar to Arterial Gas Exchange During Constant-load Exercise in Healthy Active Men and Women

Abstract

Exercise-induced arterial hypoxemia (EIAH) can result from inadequate hyperventilation and/or inefficient alveolar to arterial gas exchange. For these circumstances to occur demand typically exceeds capacity; as such, EIAH is more often reported in highly trained individuals (∼50%) during progressive maximal exercise. However, it has also been observed during lower levels of exercise in some individuals and is suggested to have a higher incidence in trained women. PURPOSE: To determine if the alveolar to arterial O2 difference (AaDO2) and ventilatory response during constant-load exercise in healthy active, but not highly trained, individuals suggests an innate sex difference that would make females more susceptible to EIAH. METHODS: Thirty-one healthy, active male (M) and 33 female (F) subjects (VO2MAX: 39±7 vs. 31±6mL/min/Kg, M vs. F) completed 18min of exercise on a cycle ergometer (40 and 75% peak W 9min/stage) with the final stage progressing towards maximal by end exercise (VO2: 90±7 vs. 92±9%VO2MAX, M vs. F). At rest and every 3min during exercise arterial blood gases were drawn and cardiac output (Q) was measured, while gas exchange was continuously assessed. Partial pressure of alveolar O2 (PAO2), AaDO2 and change from baseline for PaO2, PaCO2 and SaO2 were calculated. RESULTS: Both males and females demonstrated AaDO2 widening with each 3min increment of exercise (M: 8.6±4.4, 9.2±5.2, 10.3±5.6, 14.9±5.6, 16.5±6.6, 16.3±6.2 vs. F: 7.5±3.7, 8.3±5.0, 9.5±4.1, 14.4±6.9, 16.4±7.1, 19.1±6.0; for minutes 3, 6, 9 at 40% and 12, 15, 18 at 75%). There was no evidence of a difference in ventilatory response or a relative hypoventilation between sexes. No significant arterial hypoxemia was noted, but 6 M (19%) and 5 F (15%) had a drop in SaO2 ≥2% by the final minute of exercise. There was a negative relationship between ΔSaO2 and Q, AaDO2, and ΔPaCO2 during 75%VO2MAX exercise (Q: r= -0.31 p=0.01; AaDO2: r= -0.28 p=0.03; ΔPaCO2: r= -0.43 p=0.00). CONCLUSIONS: There was no evidence of pulmonary gas exchange inefficiencies during exercise that would increase EIAH susceptibility in females. The reported higher incidence of EIAH in highly trained females compared males, therefore appears to be a result of training induced changes rather than innate sex specific inefficiencies. Funding support: NIH HL71478 and AHA 56051Z