Respiratory mechanics influence VO2max in acute hypoxia in females
Extreme Physiology & Medicine volume 4, Article number: A64 (2015)
The magnitude of decrease in maximal oxygen uptake (VO2 max) in hypoxia (H) is more pronounced in male endurance athletes. In these subjects, high pulmonary ventilation (VE) could be beneficial in maintaining VO2 max in H . Because females have smaller chest walls and narrower airways than males, VE during intensive exercise is mechanically limited . Thus, it is hypothesised that in females, respiratory response and mechanics influenced the magnitude of decrease in VO2 max in H relative to males, despite lower VO2 max in females than in males. To test this hypothesis, we studied 22 healthy males and females as they performed an exhaustive cycling test in H and normoxia (N) conditions.
Twenty-two healthy males (n = 12; VO2 max: 51 (7) ml.kg-1.min-1, age; 21 (2) yr, stature; 172 (3) cm, mass; 66 (6) kg) and females (n = 10; VO2 max = 44 (6) ml.kg-1.min-1, age; 21 (1) yr, stature; 159 (4) cm, mass; 53 (8) kg) performed the incremental cycle exercise test until exhaustion under N (20.9 % O2) and H (15.0 % O2) conditions. During the exercise test, we measured VO2 max and VE max. To mechanically assess the respiratory work, we measured transpulmonary pressure by subtracting mouth pressure from esophageal pressure and calculated work of breathing (WOB) as the integrated area of the Ptp-volume loop.
The percentage decrease in VO2 max in H (% dVO2 max) tended to be larger in females than in males (−16% in males and −21% in females, p < 0.06). VE/VO2 was significantly (p < 0.05) higher in females than males, and it was significantly (p < 0.01) higher in H than in N in both genders. In females only, the % dVO2 max in H was significantly correlated to the extent of change in VE max (r = 0.79, p < 0.05). In comparison with N, WOB/VE in H tended to be lower in males (−13.1%) whereas it was 14.6% higher in females (not significant). Furthermore, in females, the % dVO2 max in H was significantly correlated to WOB/VE in H (r = −0.76, p < 0.05).
These results suggest that females have lower ventilatory mechanical efficiency than males, and VE is one of the factors causing this decrease in VO2 max in H. Further, there is a possibility that the oxygen demand at the respiratory muscles greatly increases against the increase in VE in H. Thus, high respiratory muscle work compromises blood flow to the active muscles , thereby limiting their peak work rate and VO2 max in H.
Our findings demonstrated that in females, the respiratory muscle work efficiency affected the decrease in VO2 max in H, a decrease which tended to be larger in females than in males, despite the lower VO2 max in females compared with males.
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Hanamaru, T., Yoshikawa, T., Nishiyasu, T. et al. Respiratory mechanics influence VO2max in acute hypoxia in females. Extrem Physiol Med 4 (Suppl 1), A64 (2015). https://doi.org/10.1186/2046-7648-4-S1-A64