The effect of metabolic acid-base changes on the ventilatory changes at the end of heavy exercise

Summary

The purpose of this study was to determine the effect of altered metabolic acid-base status on the changes in ventilation in the transition from heavy exercise above anaerobic threshold to rest. Seven subjects ingested, in a randomized and blind manner, either NaHCO₃ or CaCO₃ (placebo) at a dose of 300 mg · kg⁻¹ body mass and ran on a treadmill for five minutes (90%$\dot V_{{\text{O}}_{{\text{2max}}} }$ and above anaerobic threshold) on ten different occasions. Changes in minute ventilation in the exercise transitions were studied by starting and stopping the treadmill abruptly with a remote switch. The fast increase in ventilation at the start of exercise was not accompanied by a corresponding fast drop in ventilation at the end of exercise (P≪0.001) and the effects of chemicals on these changes were not significantly different (P<0.05). A single-component exponential model, without a time delay, was used to determine the time constants of off-transitional decay in ventilation for the two chemicals in each subject. Parametric and non-parametric statistical tests revealed that the time constants were not as significantly different as the venous pH measurements which were significantly higher with NaHCO₃ (P≪0.001). The results indicate that the absence of fast change in ventilation at the end of heavy exercise is not due to lactic acidosis and the consequent slow ventilatory changes in the off-transition of heavy exercise are at least partly mediated by non-humoral factors such as a central neural reverberatory mechanism.