Ventilatory response to sustained hypoxia during exercise.

The ventilatory stimulating effects of hypoxia occurring at carotid bodies are potentiated by exercise. However, hypoxia also has central ventilatory depressive effects; the potential interactions between this hypoxic depression and exercise have not been studied. We examined the ventilatory response to a 20 min period of isocapnic hypoxia (end-tidal O2, PETO2, of 50 mm Hg), preceded and followed by a 5 min period of isocapnic hyperoxia in seven normal adult males at rest and during moderate exercise (45-75 W). When hypoxia was introduced at rest (PETO2 = 42 mmHg), ventilation initially increased from 13.73 +/- 3.04 (mean +/- SD) to 23.69 +/- 5.48 1.min-1 and then slowly declined to 19.01 +/- 4.68 1 min-1. The increase was caused by increases in tidal volume and respiratory frequency, but the decline was solely in tidal volume. During a background of moderate exercise (PETCO2 = 46 mmHg), introduction of hypoxia caused ventilation to increase from 30.84 +/- 6.31 to 56.44 +/- 10.58 1.min-1. Ventilation subsequently did not decline; at the end of the hypoxic period, ventilation was 57.06 +/- 12.59 1.min-1. The increase was also associated with an increase in tidal volume and respiratory frequency, as seen as rest, but with much larger magnitudes. Despite the absence of ventilatory decline, there was still a decline in tidal volume, but it was compensated by an increase in respiratory frequency. We conclude that exercise potentiated the acute ventilatory response to hypoxia by modifying both tidal volume and respiratory frequency but that exercise abolished or greatly reduced hypoxic decline by increasing respiratory rate.