After exertional heat stroke, is cooling to <40°C sufficient? Revisiting an established protocol in a mouse model

Abstract

Exertional Heat Stroke (EHS) is a life‐threatening illness that can lead to multisystem tissue damage and increased mortality. To attenuate these consequences, immediate cooling to <40°C is the current recommendation. It is believed that the rate of cooling is a key factor in preventing tissue injury; however, much is unknown. In this study, we have used a model which allows us to test the effect of different cooling rates on recovery from EHS.METHODSThirteen female mice were randomly assigned to one of two groups, a slow‐cooling group (SC) and the rapid‐cooling group (RC). Both groups underwent emitter implantation surgery which allowed us to monitor core temperature (Tc) in real time. After two weeks of recovery, mice were introduced to free running wheels inside their cages for another two weeks. After that, mice received forced running wheel training for one week. On the day of EHS exposure, mice were placed inside a chamber, kept at 37.5°C/40% humidity. Mice ran at closely controlled speeds until they reached symptom limitation (i.e. loss of consciousness at ~42.3°C). The RC group was immediately taken outside the chamber and allowed to recover at room temperature ~23°C. However, the SC group was kept inside the running wheel enclosure until consciousness (~9 min) and then allowed to recover for another 2 hours inside their home cage at 37.5°C.RESULTSThe total thermal area averaged higher in the SC group, but did not reach statistical significance (P=0.1291). The time mice spent >39.5°C to the maximum Tc (Tc,max), and from Tc,max to 39.5°C was not statistically different. However, the SC group exhibited greater mortality than the RC (P=0.012). In the SC group, the time spent to cool from Tc,max to 37.5°C was statistically significant (P<0.0025). Also, the time from Tc,max to the minimum core temperature was statistically significant (P<0.0001).CONCLUSIONSIn conclusion, this model helped us to distinguish the effect of different cooling rates on mortality. The data suggests that quickly cooling to only 40°C may be insufficient to avoid heat injury or death, but cooling to 37.5°C may improve outcomes.Support or Funding InformationSupported by the BK and Betty Stevens Endowment and a Fellowship from King Saud University, Saudi Arabia