Respiratory mechanics influence VO₂max in acute hypoxia in females

Introduction

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 (V_E) could be beneficial in maintaining VO₂ max in H [1]. Because females have smaller chest walls and narrower airways than males, V_E during intensive exercise is mechanically limited [2]. 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.

Methods

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 V_E 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.

Results

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). V_E/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 V_E max (r = 0.79, p < 0.05). In comparison with N, WOB/V_E 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/V_E in H (r = −0.76, p < 0.05).

Discussion

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 V_E in H. Thus, high respiratory muscle work compromises blood flow to the active muscles [3], thereby limiting their peak work rate and VO₂ max in H.

Conclusion

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.