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Ventilatory responses to carbon dioxide at low and high levels of oxygen are elevated after episodic hypoxia in men compared with women

¹John D. Dingell Veterans Administration Medical Center, ²Department of Internal Medicine, Wayne State University School of Medicine, ³Department of Biomedical Engineering, Wayne State University, and ⁴Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan 48201

Submitted 25 May 2004 ; accepted in final form 16 July 2004

We hypothesized that the acute ventilatory response to carbon dioxide in the presence of low and high levels of oxygen would increase to a greater extent in men compared with women after exposure to episodic hypoxia. Eleven healthy men and women of similar race, age, and body mass index completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the end-tidal partial pressure of carbon dioxide (PET_CO2) below 25 Torr. Subjects then rebreathed from a bag containing a normocapnic (42 Torr), low (50 Torr), or high oxygen gas mixture (150 Torr). During the trials, PET_CO2 increased while the selected level of oxygen was maintained. The point at which minute ventilation began to rise in a linear fashion as PET_CO2 increased was considered to be the carbon dioxide set point. The ventilatory response below and above this point was determined. The results showed that the ventilatory response to carbon dioxide above the set point was increased in men compared with women before exposure to episodic hypoxia, independent of the oxygen level that was maintained during the rebreathing trials (50 Torr: men, 5.19 ± 0.82 vs. women, 4.70 ± 0.77 l·min^–1·Torr^–1; 150 Torr: men, 4.33 ± 1.15 vs. women, 3.21 ± 0.58 l·min^–1·Torr^–1). Moreover, relative to baseline measures, the ventilatory response to carbon dioxide in the presence of low and high oxygen levels increased to a greater extent in men compared with women after exposure to episodic hypoxia (50 Torr: men, 9.52 ± 1.40 vs. women, 5.97 ± 0.71 l·min^–1·Torr^–1; 150 Torr: men, 5.73 ± 0.81 vs. women, 3.83 ± 0.56 l·min^–1·Torr^–1). Thus we conclude that enhancement of the acute ventilatory response to carbon dioxide after episodic hypoxia is sex dependent.

long-term facilitation; carbon dioxide set point; chemoreflex sensitivity; sex

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