Impact of Multiple Exertional Heat Strokes on Thermoregulation and Performance

Abstract

Heat stress can result in beneficial acclimation and/or harmful injury depending on the severity of exposure. The impact of multiple heat exposures on thermoregulatory and physical performance has not been well‐studied, but is an important consideration for warfighters, athletes, and others who routinely encounter hot environments. The aim of this study was to examine the effects of multiple exertional heat strokes (EHS) on thermoregulatory and performance variables. C57BL/6J male mice were implanted with an intra‐abdominal radiotelemetry device to measure core temperature (Tc; ±0.1°C). Four exercise training sessions were conducted at thermoneutral conditions (25°C) prior to heat exposure to familiarize mice with the forced running protocol. For the EHS protocol, mice rested inside a running wheel for approximately 35 minutes until the environmental chamber reached 37.5°C and then began running at 2.5m/min, accelerating 0.3m/min every 10 minutes until reaching their max speed when Tc reached 41.0°C; mice were removed from the heat when they collapsed. After recovering for either one day (1D), three days (3D), or seven days (7D), mice repeated the EHS protocol. Regardless of the number of recovery days, mice spent more time in the heat prior to collapse during the second heat exposure compared to the first (1D: 335±9 minutes vs 275±8 minutes, p<0.001; 3D: 331±9 minutes vs 275±7 minutes, p<0.001; 7D: 320±9 minutes vs 274±7 minutes, p<0.001). Maximum Tc (Tcmax) was higher during the second heat exposure after both 1D and 3D recovery compared to the first heat exposure (1D: 42.50±0.03°C vs 42.15±0.04°C, p<0.001; 3D: 42.40±0.04°C vs 42.13±0.04°C, p<0.001; 7D: 42.17±0.03°C vs 42.12±0.03°C, not significant) and during the second heat exposure Tcmax was higher in 1D compared to 3D (p<0.05) and 7D (p<0.001) and higher in 3D than 7D (p<0.001). Hypothermia depth, a measure of injury severity, was less pronounced after the second heat exposure compared to the first heat (1D: 34.57±0.13°C vs 31.31±0.34°C, p < 0.001; 3D: 33.33±0.19°C vs 31.86±0.22°C, p < 0.001; 7D: 33.59±0.30°C vs 31.77±0.46°C, p < 0.001). Hypothermia depth after the second heat was also less severe in 1D mice compared to 3D mice (p < 0.05) and trended less severe compared to 7D mice (p = 0.07). Taken together, these results demonstrate that a single exertional heat exposure can induce acclimation‐like measures that enhance performance and may protect against injury during subsequent heat exposures. Author views are not official US Army or DoD policy.