Effect of acid-base status on the kinetics of the ventilatory response to moderate exercise

To determine the influence of altered carotid body drive on exercise ventilatory kinetics, five subjects performed four repetitions of constant-load cycle ergometer exercise during air and O2 breathing under each of the following conditions: 1) metabolic acidosis, (NH4Cl, 0.3 g . kg-1 . day-1); 2) metabolic alkalosis (NaHCO3, 0.7 g . kg-1 . day-1); and 3) control (CaCO3, 0.1 g . kg-1 . day-1). Ventilatory and gas exchange variables were computed, breath-by-breath, and the time constant of the ventilatory response in each condition was determined by a least-squares technique. While breathing air, metabolic acidosis caused the magnitude of the ventilatory response to increase and the time constant of the ventilatory kinetics to decrease. With metabolic alkalosis the increase in ventilation caused by exercise tended to be smaller and time constant larger although these changes were not statistically significant. Hyperoxia slowed the ventilatory response in the three acid-base conditions to a similar value. Thus hyperoxia slowed the ventilatory kinetics to a greater degree during acidosis than during control or alkalosis. We conclude that ventilatory dynamics during moderate exercise can be appreciably influenced by the acid-base status with acidosis significantly speeding the response dynamics. And, as these effects are abolished by hyperoxia, they appear to be mediated via the carotid bodies, in the human.