Sex Differences In Respiratory And Circulatory Cost And Arterial Oxygen Saturation During Hypoxic Walking

Abstract

PURPOSE: Since women have smaller lungs and a decreased capacity for lung diffusion compared to men, these differences may increase the work required for women to maintain a given rate of pulmonary ventilation (VE), resulting in greater exercise-induced arterial hypoxemia (EIAH). Previous studies have ignored the energy expenditure (EE) on circulation (i.e., heart rate; HR) and ventilation during exercise. METHODS: We sought to investigate sex differences in EE, VE and HR in response to changes in SpO2. We hypothesized that women would experience greater EIAH, and that the contribution rate of EE, VE, and HR in response to changes in SpO2 would be different between the sexes. We measured EE during walking on a level gradient under normoxia (room air, 21% O2), and moderate hypoxia (13% O2). Ten healthy young men and ten healthy young women walked on a treadmill at seven speeds (0.67-1.67 m s-1). Each walking speed lasted for four minutes. EE was calculated using pulmonary oxygen uptake and carbon dioxide output. RESULTS: During walking, reductions in SpO2 trended slightly greater in women under hypoxia (71.5 ± 4.5 % for men and 67.7 ± 6.1 % for women at the fastest gait speed, P > 0.05). Hypoxia-induced elevation in EE, HR, and VE were calculated by the difference between values in hypoxia and normoxia. Using a multivariate model that combined EE, VE, and HR to predict ΔSpO2 (hypoxia-induced reduction), we obtained a very strong fit model both for men (r2 = 0.900, P < 0.001) and for women (r2 = 0.957, P < 0.001). We also tried to estimate the relative contributions of ΔEE, ΔVE, and ΔHR to predict ΔSpO2 by using standard partial regression coefficients. The contribution rate to predict ΔSpO2 was markedly different between men and women. In women, the effect of ΔEE and ΔVE were greater (EE: 28.1% in women vs. 15.8% in men; VE: 4.1% in women vs. 1.7% in men). Conversely, in men the contribution of ΔHR was greater (82.5 % in men and 67.9 % in women). Moreover, significant sex differences in breathing frequency and tidal volume were observed (P < 0.05, respectively). CONCLUSIONS: These findings suggested that high-altitude adaptation in response to hypoxemia has different underlying mechanisms between men and women. Our results can help to explain how men and women adapt high-altitude environments.