A Single Heat Exposure Improves Heat Tolerance in a Mouse Exertional Heat Stroke Model

Abstract

Exertional heat stroke (EHS) is an ongoing medical issue for military personnel. Currently there is poor insight into the etiology of organ damage sustained during EHS and what factors may increase susceptibility. Recent evidence suggests that prior heat injury can increase the likelihood of subsequent EHS. Individuals that have experienced prior EHS report being less tolerant to heat and are flagged in the military for extra surveillance when conducting training exercises. The goal of this study was to determine the impact of prior EHS on heat tolerance and running performance during a subsequent EHS event. Using C57BL/6J male mice, we hypothesized that prior EHS would decrease thermoregulatory performance in the heat, as measured by total heating time to collapse (min), total distance run (m), maximal speed achieved (m/min), and thermal load (°C·min) compared to those without prior EHS. Using a forced running EHS protocol (environmental temperature [Tenv]=37.5 ± 0.2°C), mice were allowed to recover for 24 hours (EHS 24h; n=31), 3 days (EHS 3D; n=22) or 7 days (EHS 7D; n=21) before being subjected to a secondary EHS. To our surprise, mice exposed to two EHS events exhibited a significant increase in time to collapse, distance ran, and thermal load compared to the single EHS group. Mice in the EHS 24h group performed best in three of four performance measures; heating time to collapse (344.55±8.89 min), distance ran (1908.21±92.30 m) and max speed (8.71±0.23m/min) during their second EHS. EHS 24h mice also displayed the highest maximum core temperature (42.50 ± 0.3°C, Tc Max) prior to loss of consciousness, which was significantly higher than the other groups (p<0.02). EHS 3D mice had the highest thermal load (1035±30.8°C·min) compared with the EHS 7D group only (930.75±24.65°C·min, p<0.01). Overall the EHS 24h mice had the best performance and showed the highest increase in performance from their initial EHS, suggesting an increase in heat tolerance. The EHS 3D and EHS 7D group performance measures showed a decay over time, regressing towards initial heat performance levels. Author views not official US Army or DoD policy.