The contribution of the reflex hypoxic drive to the hyperpnoea of exercise.

Oxygen breath tests have been applied to six normal subjects at rest and during steady state exercise on a bicycle ergometer (200, 400 and 600 kpm) to estimate the contribution of the reflex hypoxic drive to total ventilation. The subjects were changed without their knowledge from air to 100% oxygen for 4--5 breaths, the fall in ventilation was recorded and expressed as a percentage of total ventilation (hypoxic drive). The average reflex hypoxic drive was 16.2% (SE +/- 2.6; n = 35) this remianed unaltered during all levels of steady state exercise. The maximum fall in ventilation occurred earlier in exercise (P less than 0.0005). At rest the maximum fall occurred on average 31.2 s (SE +/- 2.6) from the start of the first breath of oxygen. During the first level of steady state exercise the maximum fall occurred 19.4 s (SE +/- 0.5) from the start of the first breath of oxygen. It is concluded that the contribution of the reflex hypoxic drive to the total ventilation is unaltered by exercise although a substantial part of the hyperpnoea can be accounted for by the presence of this drive.