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 al veolar P-CO2 in women than men, and in luteal (LUT) compared with follicula r (FOL) menstrual cycle phase would persist during normal oral contraceptiv e use and during early altitude exposure. Ventilation and blood gases were measured at baseline (636 mmHg approximate to 5400 ft, 1650 m) and during s imulated altitude at 426 mmHg ( approximate to 16 000 ft, 4880 m), after 1 h (Al) 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 signific antly 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 an d 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 equ ivalent increase in alveolar ventilation. The mean arterial-end tidal P-CO2 difference was newer > 2 mmHg in the groups, indicating no <(V)over dot>A/ <(Q)over dot> mismatch related to gender, PRO levels or altitude. All women had higher breathing frequency than men, resulting in greater deadspace ve ntilation. At altitude, the mean Pa-O2 was = 44 mmHg (Sa(O2) approximate to 79%) for all, indicating equivalent oxygenation, but alveolar-arterial PO2 differences were greater in women than men and higher when PRO was elevate d. These results show that, relative to men, women have a compensated respi ratory alkalosis, accentuated with elevated PRO. However, the ventilation r esponse to acute altitude is the same in women and men. (C) 2001 Elsevier S cience B.V. All rights reserved.