Acute physiological and psychophysical responses to different modes of heat stress

Abstract

Heat stress is common and arises endogenously and exogenously. It can be acutely hazardous while also increasingly advocated to drive health and performance-related adaptations. Yet, the nature of strain (deviation in regulated variables) imposed by different heating modes is not well established, despite the potential for important differences. We, therefore, compared three modes of heat stress for thermal, cardiovascular and perceptual strain profiles during exposure and recovery when experienced as a novel stimulus and an accustomed stimulus. In a crossover design, 13 physically active participants (five females) underwent 5 days of 60-min exposures to hot water immersion (40 • C), sauna (55 • C, 54% relative humidity) and exercise in the heat (40 • C, 52% relative humidity), and a thermoneutral water immersion control (36.5 • C), each separated by ≥4 weeks. Physiological (thermal, cardiovascular, haemodynamic) and psychophysical strain responses were assessed on days 1 and 5.Sauna evoked the warmest skin (40 • C; P < 0.001) but exercise in the heat caused the largest increase in core temperature, sweat rate, heart rate (post hoc comparisons all P < 0.001) and systolic blood pressure (P ≤ 0.002), and possibly decrease in diastolic blood pressures (P ≤ 0.130), regardless of day. Thermal sensation and feeling state were more favourable on day 5 than on day 1 (P ≤ 0.021), with all modes of heat being equivalently uncomfortable (P ≥ 0.215). Plasma volume expanded the largest extent during immersions (P < 0.001). The current data highlight that exercising in the heat generates a more complex strain profile, while passive heat stress in humid heat has lower tolerance and more cardiovascular strain than hot water immersion.

A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation

“…During HWI, participants were immersed up to the nipple for the first 30 min on days 1 and 5, whilst on days 2-4 participants were immersed to the neck until T re reached +1.5 • C, after which they were moved to a shallower depth to maintain T re for the remainder of the session. (Campbell et al, 2022). For purposes of measuring fluid regulatory adaptations, days 1, 2 and 5 involved an additional baseline blood sample being obtained via venepuncture, without stasis, before heat exposure and 45 min after completion.…”

“…This paper is published with a companion paper (Campbell et al ., 2022) that addresses how different modes of heat stress acutely impact physiological and psychophysical strain. That companion paper contains further detail on the general methodology, including of the participants, the different conditions, standardisations, measurements and analyses.…”

“…The slight stagnation shown in Sauna may be due to the slightly shorter exposure time typically seen in the Sauna condition (due to participant withdrawal; Campbell et al, 2022) this reduced the thermal impulse and may have slowed the rate of adaptation versus HWI and ExH.…”

“…One limitation of the current study was the inability of participants to last 60 min in the Sauna condition, which was not due to hyperthermia per se or hypotension either but likely thermal comfort (see: Campbell et al, 2022. This resulted in a lower thermal strain impulse (via area under the curve of T re ; Campbell et al, 2022) compared to HWI or ExH and likely influenced adaptive responses of Sauna.…”

“…During recovery this may have caused mild hypovolaemia and associated hypotension, thereby stimulating fluid retaining mechanisms after exposure. Given the complex interplay of factors governing extracellular fluid volumes, the magnitude of PVE may be partly explained by the strain elicited by the session itself (see companion paper for further discussion; Campbell et al, 2022), but also by the external factors during recovery.…”