Abstract
Background: Maintaining appropriate brain blood flow is vital to supplying oxygen and energy substrates, and removing metabolic by-products (carbon dioxide and heat). However, high body temperature results in hyperventilation, which lowers blood carbon dioxide levels thereby constricting cerebral blood vessels and reducing flow. This exacerbates brain temperature as metabolism (thus heat production) continues at the same rate. High core (especially brain) temperature is considered the main cause of heat tolerance. Accordingly, initial core temperature is known to influence heat tolerance. Core temperature increases by ~0.5 °C across the day and the menstrual cycle but whether these regulated variations affect heat tolerance, cerebrovascular or respiratory control in uncompensable exercise heat stress remains unknown. Furthermore, research into exercise heat tolerance typically excludes the opportunity for behavioural thermoregulation, which is the most effective means of controlling core temperature. Behavioural thermoregulation is driven by thermal discomfort, which may be a key confounding factor (e.g., affect hyperventilation or motivation). Therefore, the mechanisms that drive heat tolerance remain unknown; leading us to question the effect behavioural thermoregulation has on heat tolerance.
Aim: To assess (i) how diurnal changes in core temperature affect heat tolerance, cerebrovascular and respiratory control; (ii) what mechanisms (physiological, psychological or both) drive heat tolerance; and (iii) whether these responses show a sex effect. It was hypothesised that (i) thermal perceptions, ventilatory and cerebrovascular responses to exercise heat stress would be worse in the afternoon; (ii) improved perception of thermal discomfort would alleviate ventilatory and cerebrovascular responses; and (iii) physiological and perceptual responses would be more severe in females.
Methods: Thirteen healthy, aerobically fit volunteers (seven females) undertook semi-recumbent cycling (25% work rate max) wearing a water-perfused suit (~48 °C) until volitional exhaustion on three occasions; once in the morning (AM) and twice in the afternoon. One afternoon was without (PM) and with a misting fan (PM+Fan), under the participant’s control, provided to improve thermal comfort. Core (rectal; Tc) and skin temperature (Tsk), heart rate, blood pressure, ventilation and relevant perceptions were measured each 0.25 °C rise in Tc, with additional measurements of non-invasive intracranial pressure (nICP), internal carotid artery blood flow (ICA), and respiratory function at larger intervals.
Results: Baseline Tc was 0.35 °C lower in the AM vs PM (95% Cl: -0.55, -0.16; p=0.0001) but tolerance Tc was not different (39.10 ± 0.71, 39.11 ± 0.50; p=0.947), and thus the rise in Tc was greater in the AM (95% Cl: 0.0001, 0.80; p=0.047). Tolerance Tc was not different in the PM+Fan (vs PM, 95% Cl: -0.44, 0.24; p=0.764). Females’ baseline and tolerance Tc were not different to the males across the three conditions (baseline difference <0.17 °C, tolerance difference <0.54; p>0.35). The ICA (mL/min) was not different between the AM and PM (95% Cl: -89, 62; p=0.904), decreased 5% more so in the PM+Fan (vs PM, 95% Cl: 0.4, 148.6; p=0.049). The ICA flow was not different between males and females (p=0.476). The change in nICP from baseline to tolerance was not different across conditions (time p=0.545; interaction p=0.887) or between males and females (p=0.499). The ventilation response to increasing Tc was not different across conditions (9.8-12.4 L/min/°C; p>0.148). Thermal discomfort (TD) responses to increasing Tc were not different between AM (4 ± 3 a.u/°C) and PM (4 ± 2 a.u/°C; p=0.537) or with the PM+Fan (4 ± 1 a.u/°C; p= 0.098) but the modelled Tc threshold for onset of TD was 0.23 °C higher in the PM than the AM. Sex showed no difference to physiological measures but thermal sensation (TS), feeling scale (FS) and rate of perceived exertion (RPE) slopes were less severe in females (p<0.032) in the PM+Fan compared to males.
Conclusions: Maximal core temperature attained, rather than the relative change in temperature, dictated tolerance during uncompensable exercise heat stress, i.e., thermal reserve was greater in the morning. In contrast to hypothesis (i), thermal perceptions, ventilatory and cerebrovascular responses were not different in the afternoon. Hypothesis (ii), that the addition of behavioural thermoregulation would improve perception of comfort, was not supported. However, behavioural thermoregulation may alleviate the severity of the heat-induced respiratory responses in a minority of individuals. In contrast to hypothesis (iii), sex had no effect on cardiorespiratory or cerebrovascular variables but perceptual responses (i.e. TS, FS and RPE) in females were less severe than males in the presence of behavioural thermoregulation.