Dynamic Relationship Between Heart Rate And Core Temperature During Exercise-heat Stress And Iced Sheet Cooling

Athletes, military personnel, and others who perform physical work in the heat are at risk for exertional heat illness due to the combined presence of high ambient temperature and elevated metabolic heat production. Cooling with iced sheets is an immediate treatment used by the military and others to rapidly decrease body temperature in a field setting. Physiologically, increases in core temperature (Tc) and heart rate (HR) contribute to the cardiovascular strain associated with exercising in the heat. This increased HR is linked to the combination of metabolic (exercise) and thermal (environmental) requirements for systemic blood flow (i.e. cardiac output). Mechanisms include both autonomic neural and local influences of temperature on the heart. PURPOSE: To quantify the dynamic response of HR to changes in Tc during exercise in the heat and subsequent iced sheet cooling. We hypothesized that for a given Tc, HR would be higher during increases in Tc compared to during decreases in Tc. METHODS: Nine healthy (n = 8 males) volunteers (age 25 ± 6 y, ht 176.9 ± 9.7 cm, wt 86.6 ± 15.9 kg, body surface area 2.03 ± 0.2 m2) participated in a exercise-heat stress protocol consisting of up to 180 minutes of treadmill walking (3.5 mi·hr-1, 5% grade, in 40 °C, 30% RH). Volunteers then lay supine and underwent iced-sheet cooling (ISC) (bed sheets placed in 5-gallon ice buckets with 2/3 ice, 1/3 cool water; rotated out with pre-soaked sheets every 3 min) until Tc decreased to ≤38.0 °C. Individual changes in HR during both exercise and cooling were compared at Tc = 38.2 and 38.6 °C using 2-way analysis of variance. RESULTS: HR was significantly higher for a given Tc when Tc was increasing during exercise compared to when it was decreasing during cooling. For example, during exercise, HR was 150 ± 13 bpm at Tc = 38.2 °C and 157 ± 9 bpm at Tc = 38.6 °C, while during cooling, HR was 96 ± 10 bpm at Tc = 38.2 °C and 104 ± 14 bpm at Tc = 38.6 °C (both p < 0.01). CONCLUSION: These data confirm the efficacy of iced-sheet cooling for decreasing cardiovascular strain associated with heat-related events. The fact that cooling resulted in lower HR for a given Tc suggests that exercise associated autonomic reflexes predominated over direct effects of temperature on sinus node control of HR. Supported by USAMRDC; author views not official US Army or DoD policy.