Thermal Hyperpnea in Humans Changes Following Passive Heat Acclimation

Abstract

IntroductionExercise ventilation was shown to increase after a 10‐day, 2 h·day−1 passive heat acclimation (HA) to 50°C and 20 % RH and this study assessed if thermal hyperpnoea for those at rest show a similar adaptation.PurposeIt was hypothesized that similar to that seen for eccrine sweating and cutaneous blood flow, the core temperature thresholds for human pulmonary ventilation would decrease after heat acclimation. It was also hypothesized that pulmonary ventilation would increase following heat acclimation as it does for an active heat acclimation.MethodsSix male participants performed two 40°C immersions separated by a 10‐day passive heat acclimation. Heat acclimation included 10 consecutive days with participants’ rectal temperatures maintained between 38.5–39.0°C. The heat acclimation was performed in a climatic chamber at 50°C and 20 % RH where participants remained seated at rest for 2 hr·day−1. A repeated measures ANOVA was employed with factors of Acclimation State (Pre & Post) and delta TES (0 to 1.4°C from rest). The p value was set at 0.05. An ANOVA with oxygen consumption as the covariate was employed to assess for heat acclimation‐induced changes in pulmonary ventilation. This study was approved by the SFU Office of Research Ethics.ResultsAcclimation was confirmed by a decrease in resting esophageal temperature (TES) from 37.70 ± 0.19 (mean ± SD) to 37.31 ± 0.11°C (p=0.001), as well as by significant decreases in the TES thresholds for temple cutaneous blood velocity from 37.30 ± 0.12 to 37.07 ± 0.13°C (p=0.014) and for forehead eccrine sweating from 37.60 ± 0.25 to 37.25 ± 0.22°C (p=0.03). The pulmonary ventilation thresholds were 38.28 ± 0.28 before and 37.84 ± 0.44°C after heat acclimation (p=0.13). After accounting for the covariate of oxygen consumption, a significant main effect of Acclimation State (F=10.05, p=0.025) plus an acclimation state x delta TES interaction for pulmonary ventilation (F=3.6, p=0.008) were explained by a significantly greater pulmonary ventilation following acclimation at delta TES of 0.6, 0.8, 1.0°C (0.018 <p < 0.05).ConclusionFollowing a passive heat acclimation the resting TES and TES thresholds for human thermoregulatory responses were significantly lower and pulmonary ventilation was significantly increased during passive heating as indicative of an increased thermal hyperpnea.Support or Funding Information Funding SourceThis project was supported by Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.