Adaptation of exercise ventilation during an actively-induced hyperthermia following passive heat acclimation

Beaudin AE, Clegg ME, Walsh ML, White MD. Adaptation of exercise ventilation during an actively-induced hyperthermia following passive heat acclimation. Am J Physiol Regul Integr Comp Physiol 297: R605-R614, 2009. First published June 3, 2009; doi: 10.1152/ajpregu. 90672.2008.-Hyperthermia-induced hyperventilation has been proposed to be a human thermolytic thermoregulatory response and to contribute to the disproportionate increase in exercise ventilation ((V) over dot(E)) relative to metabolic needs during highintensity exercise. In this study it was hypothesized that (V) over dot(E) would adapt similar to human eccrine sweating ((E) over dot(SW)) following a passive heat acclimation (HA). All participants performed an incremental exercise test on a cycle ergometer from rest to exhaustion before and after a 10-day passive exposure for 2 h/day to either 50 degrees C and 20% relative humidity (RH) (n = 8, Acclimation group) or 24 degrees C and 32% RH (n = 4, Control group). Attainment of HA was confirmed by a significant decrease (P = 0.025) of the esophageal temperature (T-es) threshold for the onset of (V) over dot(SW) and a significantly elevated (E) over dot(SW) (P <= 0.040) during the post-HA exercise tests. HA also gave a significant decrease in resting T-es (P = 0.006) and a significant increase in plasma volume (P = 0.005). Ventilatory adaptations during exercise tests following HA included significantly decreased T-es thresholds (P <= 0.005) for the onset of increases in the ventilatory equivalents for O-2 ((V) over dot(E)/(V) over dot(O2)) and CO2 ((V) over dot(E)/(V) over dot(CO2)) and a significantly increased (V) over dot(E) (P <= 0.017) at all levels of T-es. Elevated (V) over dot(E) was a function of a significantly greater tidal volume (P = 0.003) at lower T-es and of breathing frequency (P <= 0.005) at higher T-es. Following HA, the ventilatory threshold was uninfluenced and the relationships between (V) over dot(O2) and either (V) over dot(E)/(V) over dot(O2) or (V) over dot(E)/(V) over dot(CO2) did not explain the resulting hyperventilation. In conclusion, the results support that exercise (V) over dot(E) following passive HA responds similarly to (V) over dot(SW), and the mechanism accounting for this adaptation is independent of changes of the ventilatory threshold or relationships between (V) over dot(O2) with each of (V) over dot(E)/(V) over dot(O2) and (V) over dot(E)/(V) over dot(CO2).