Individual variability in achievement of short-term heat acclimation during a fixed intensity protocol.

INTRODUCTION: Heat acclimation (HA) occurs with ∼8-10d of repeated heat exposure; however, adaptations can occur earlier (<7d; termed short-term heat acclimation; STHA). PURPOSE: To test the hypothesis that some, but not all, young healthy men would achieve STHA after 5d of a standard HA protocol. METHODS: We conducted a novel, post-hoc analysis of data from 13 healthy men (21 ± 3 yrs; 173 ± 8 cm; 75.1 ± 12.2 kg) who participated in an 8d HA protocol (120 min treadmill walking: 5 km h-1, 2% grade; 40 °C, 40% relative humidity). Core temperature (Tc), heart rate (HR), and whole body sweating rate (SR) were compared across 1d, 5d, and 8d of HA. Criteria for HA were delta (Δ)Tc ≤ 0.9 °C and ΔHR≤33 beats·min-1. RESULTS: Group averages on 1d, 5d, and 8d of HA for end exercise Tc (38.1 ± 0.34; 37.9 ± 0.3; 37.8 ± 0.3 °C) and HR (134 ± 17; 122 ± 13; 121 ± 13 beats·min-1) were decreased by 5d (p < 0.05) and did not decrease further by 8d. ΔTc (1.1 ± 0.36; 0.95 ± 0.45; 0.95 ± 0.33 °C) and ΔHR (36 ± 18; 33 ± 17; 33 ± 15 beats·min-1) showed similar patterns (p > 0.05). At 5d, 31% (4/13) of the volunteers achieved HA; the remaining 9 volunteers had further adaptations between 5d and 8d. SR was not different across 1d, 5d, and 8d (860 ± 148; 908 ± 210; 873 ± 203 mL h-1 respectively; p > 0.05). CONCLUSION: Our results suggest that during a fixed-intensity HA protocol in young men, using criteria of changes in Tc and HR, ∼1/3 achieve STHA. Future research should evaluate potential mechanisms, and whether similar inter-individual variability occurs in women.

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

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.