The Effectiveness of a Standardized Ice-Sheet Cooling Method Following Exertional Hyperthermia

Abstract

Introduction: Exertional heat illnesses remain a major threat to military service members in the United States and around the world. Exertional heat stroke (EHS) is the most severe heat illness, characterized by core hyperthermia and central nervous system dysfunction. Per current Army regulations, iced-sheet cooling (ISC) is the recommended immediate treatment for heat casualties in the field, but concerns have been raised regarding the efficacy of this approach. Thus, the purpose of this study was to quantify the cooling rate of ISC following exertional hyperthermia.

Materials and methods: We utilized a randomized crossover design with 2 experimental trials. In both trials, exertional hyperthermia was induced by walking (3.5 mph at 5% grade) on a treadmill in an environmental chamber (40 °C, 30% RH) for up to 3 hours or until core body temperature reached 39.2 °C. After the walking portion, individuals either received ISC (experimental trial) or cooling and rested supine in the same environmental conditions for 30 minutes with no ISC (control trial). For ISC, bed sheets soaked in ice water were applied (per Army guidance) at the neck, chest, and groin with another sheet covering the body. Sheets were rotated and resoaked every 3 minutes until core temperature decreased to <38.0 °C.

Results: By design, participants finished exercise with increased core temperature (38.8 ± 0.39 °C vs. 38.90 ± 0.34 °C, ISC and control trials, P = 1.00). The ISC trial provided significantly (P = .023) greater cooling rates, 0.068 °C/min 95% confidence interval [CI; 0.053, 0.086], compared to the control trial, 0.047 °C/min 95% CI [0.038, 0.056]. Additionally, the time to decrease to less than 38.0 °C was significantly (P = .018) faster in the ISC trial (median = 9.3 minutes) compared to the control trial (median = 26.6 minutes).

Conclusion: ISC increases the cooling rate of those recovering from exertional hyperthermia. With the observed cooling rate, we can extrapolate that ISC would reduce core temperature by ∼2 °C within 30 minutes during a case of EHS. We conclude that ISC provides a safe and effective alternative for the field where cold water immersion resources may not be readily available.