Ventilation is greater in women than men, but the increase during acute altitude hypoxia is the same.

We wished to determine whether the previously reported lower arterial or alveolar P(CO2) in women than men, and in luteal (LUT) compared with follicular (FOL) menstrual cycle phase would persist during normal oral contraceptive use and during early altitude exposure. Ventilation and blood gases were measured at baseline (636 mmHg approximately 5400 ft, 1650 m) and during simulated altitude at 426 mmHg ( approximately 16000 ft, 4880 m), after 1 h (A1) and during the 12th h (A12), in 18 men (once) and in 19 women twice, during LUT and FOL and in 20 women twice while on placebo (PLA) or highest progestin dose (PIL) oral contraceptives. At baseline, Pa(CO2) was significantly higher in men than all women by 3.3 mmHg. When progesterone-progestin (PRO) was elevated in women, Pa(CO2) was significantly lower than in FOL and PLA, but the latter were still significantly lower than men. At altitude the P(CO2) differences between men and women and PRO levels persisted, with PA(CO2) falling by 3.6 and 7.3 mmHg at A1 and A12 in all, indicating an equivalent increase in alveolar ventilation. The mean arterial-end tidal P(CO2) difference was never >2 mmHg in the groups, indicating no VA/Q mismatch related to gender, PRO levels or altitude. All women had higher breathing frequency than men, resulting in greater deadspace ventilation. At altitude, the mean Pa(O2) was approximately 44 mmHg (Sa(O2) approximately 79%) for all, indicating equivalent oxygenation, but alveolar-arterial P(O2) differences were greater in women than men and higher when PRO was elevated. These results show that, relative to men, women have a compensated respiratory alkalosis, accentuated with elevated PRO. However, the ventilation response to acute altitude is the same in women and men.