The ventilatory response to moderate hypocapnic exercise in human beings.

We examined the exercise hyperpnoea during hypocapnia which, by necessity, was brought about by voluntary hyperventilation (VHV). At cessation of VHV, ventilation (VE) decays gradually exhibiting a phenomenon known as the afterdischarge (AD; Eldridge, Med. Sci. Sports Exerc. 26:319--327, 1994). A new technique was employed to overcome the potentially contaminating influence of the AD upon the exercise hyperpnoea. Ventilatory parameters were measured in eight subjects during hyperoxic cycle ergometry at 3 intensities. The pure VE response to 'hypocapnic' exercise was obtained by subtracting the resting recovery following VHV from the response during 'hypocapnic exercise' (VEsub). VEsub increased abruptly at exercise onset at all work rates (phase 1) and remained at this level until PETCO2 achieved a threshold; thereafter, it rose further (phase 2) towards a second plateau. The phase 1 increase in VEsub is consistent with neurally mediated control. The apparent link between PETCO2 and phase 2 suggests the involvement of a second, humorally mediated mechanism.