Repeated bouts of exercise in a heated environment are known to decrease resting body core temperature (Tc) and resting heart rate (HR). This process, known as exercise heat acclimation, leads to a variety of health benefits including improved cardiovascular health. Additionally, exercise training is associated with reduced mean arterial pressure (MAP). However, it is unknown whether passive chronic heat exposure induces the same changes in HR, MAP and resting Tc. PURPOSE: To examine the effects of chronic passive chronic heat stress on Tc, MAP, and HR. METHODS: Five healthy, sedentary subjects (4 females and 1 male; aged 21±1 yrs) were assigned to an 8-week hot water immersion program that involved 4-5 sessions per week. During each session, subjects were submerged to the clavicle in 40.5°C water until a rectal temperature (Tre) of 38.5°C was reached. Subjects remained partially immersed for an additional hour to maintain a Tre between 38.5-39.0°C. HR was measured using a HR monitor and Tre was measured with a rectal thermistor. HR and Tre were recorded both before and at 5-minute intervals during the heat stress. On a separate day, MAP was measured after ≥20 minutes of supine rest using brachial auscultation. RESULTS: Both resting MAP (81±1 vs. 76±2 mmHg, p=0.02) and resting Tre (37.4±0.5 vs. 36.8±0.4°C, p=0.03) decreased after 8 weeks of passive heat exposure with no change in resting HR (63±5 vs. 63±6 beats/min, p=0.26). During heat stress, HR rose linearly with Tre until Tre reached 38.5°C, after which both HR and Tre remained constant for the remaining hour of heating. Despite a lower resting Tre but unchanged HR after 8wks, there was no consistent difference in HR for a given Tre between 0 and 8wks once Tre was above a threshold of 37.5°C (HR at a Tre of 38.5°C: 107±12 bpm at 0wks and 109±15 bpm at 8wks, p=0.47). CONCLUSION: Chronic passive heat exposure reduces MAP and resting Tre similarly to what is observed with exercise heat acclimation. Interestingly, HR followed a similar pattern as a function of Tre before and after passive heat acclimation, suggesting that HR could be loosely used as an estimate for Tre in settings where rectal probes or other accurate body core temperature measures are not readily available, such as with patients partaking in hot water immersion programs outside of a laboratory environment.

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