THE VENTILATORY EFFECTS OF SUSTAINED ISOCAPNIC HYPOXIA DURING EXERCISE IN HUMANS

To investigate how the ventilatory response to isocapnic hypoxia is modified by steady-state exercise, five subjects were studied at rest and performing 70 W bicycle exercise. At rest, isocapnic hypoxia (end-tidal P(O2) 50 Torr) for 25 min resulted in a biphasic response: an initial increase in ventilation was followed by a subsequent decline (HVD). During exercise, an end-tidal P(O2) of 55-60 Torr was used. The magnitude of the initial ventilatory response to isocapnic hypoxia was increased from a mean +/- SE of 1.43 +/- 0.323 L/min per % arterial desaturation at rest to 2.41 +/- 0.424 L/min per % during exercise (P < 0.05), but the magnitude of the HVD was reduced from 0.851 +/- 0.149 L/min per % at rest to 0.497 +/- 0.082 L/min per during exercise (P < 0.05). The ratio of HVD to the acute hypoxic response was reduced from 0.696 +/- 0.124 at rest to 0.202 +/- 0.029 during exercise (P < 0.01). We conclude that while exercise augments the ventilatory sensitivity to hypoxia, it also has a direct effect on the mechanism by which sustained hypoxia depresses peripheral chemosensitivity.