Kinetics of the ventilatory and metabolic responses to moderate-intensity exercise in humans following prior exercise-induced metabolic acidaemia.

As the time constant of the phase 2 (ø2) ventilatory response (tauV'(E)) to moderate exercise (< lactate threshold, thetaL) is reduced by exogenous procedures that augment peripheral (carotid) chemosensitivity (hypoxia; chronic metabolic acidaemia), we examined whether an acute endogenous metabolic acidaemia had a similar effect. Six subjects completed two tests (A, B), each comprising two 6-min bouts separated by a 6-min "0" W recovery: A:- 90% thetaL, 90% thetaL; B:- supra-thetaL (50% between thetaL and peak V'O2), 90% thetaL. For Protocol A, the bout 2 sub-thetaL tauV'E was similar to bout 1. However, for Protocol B, where the initial supra-thetaL metabolic acidaemia was still evident at the end of the subsequent sub-thetaL bout, the sub-thetaL tauV'E was shorter; tauV'E/tauV'O2 and tauV'E/tauV'CO2 were reduced; and the transient end-tidal PO2undershoot was less marked. We conclude that an acute, endogenous metabolic acidaemia speeds ø2 V'(E) kinetics in moderate exercise, consistent with carotid chemoreception contributing to the tightness of arterial pH-CO2 regulation and the magnitude of the transient arterial hypoxaemia.