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

Hyperthermia-induced hyperventilation has been proposed to be a human thermolytic thermoregulatory response and to contribute to the disproportionate increase in exercise ventilation (VE) relative to metabolic needs during high-intensity exercise. In this study it was hypothesized that VE would adapt similar to human eccrine sweating (Esw) 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.sub.es]) threshold for the onset of [E.sub.sw] and a significantly elevated [E.sub.sw] (P [less than or equal to] 0.040) during the post-HA exercise tests. HA also gave a significant decrease in resting [T.sub.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.sub.es] thresholds (P [less than or equal to] 0.005) for the onset of increases in the ventilatory equivalents for [O.sub.2] (VE/V[O.sub.2]) and C[O.sub.2] (VE/Vc[o.sub.2]) and a significantly increased VE (P [less than or equal to] 0.017) at all levels of [T.sub.es]. Elevated VE was a function of a significantly greater tidal volume (P = 0.003) at lower [T.sub.es] and of breathing frequency (P [less than or equal to] 0.005) at higher [T.sub.es]. Following HA, the ventilatory threshold was uninfluenced and the relationships between V[o.sub.2] and either VE/V[o.sub.2] or VE/Vc[o.sub.2] did not explain the resulting hyperventilation. In conclusion, the results support that exercise VE following passive HA responds similarly to [E.sub.sw], and the mechanism accounting for this adaptation is independent of changes of the ventilatory threshold or relationships between V[o.sub.2] with each of VE/V[o.sub.2] and VE/Vc[o.sub.2]. eccrine sweating; skin temperatures; plasma volume; ventilatory equivalents