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

Laboratory for Exercise and Environmental Physiology, Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada

Submitted 4 August 2008 ; accepted in final form 2 June 2009

Hyperthermia-induced hyperventilation has been proposed to be a human thermolytic thermoregulatory response and to contribute to the disproportionate increase in exercise ventilation (E) relative to metabolic needs during high-intensity exercise. In this study it was hypothesized that E would adapt similar to human eccrine sweating (_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°C and 20% relative humidity (RH) (n = 8, Acclimation group) or 24°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 _SW and a significantly elevated _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 O₂ (E/O₂) and CO₂ (E/CO₂) and a significantly increased E (P 0.017) at all levels of T_es. Elevated 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 O₂ and either E/O₂ or E/CO₂ did not explain the resulting hyperventilation. In conclusion, the results support that exercise E following passive HA responds similarly to _SW, and the mechanism accounting for this adaptation is independent of changes of the ventilatory threshold or relationships between O₂ with each of E/O₂ and E/CO₂.

eccrine sweating; skin temperatures; plasma volume; ventilatory equivalents