R. Berland, C. Briscoe, M. Schleh, B.C. Ruby FACSM, C.L. Dumke FACSM

The University of Montana, Missoula, MT

With the prevalence of occupations occurring in hot environments, it is crucial to develop a protocol to protect these individuals from heat related illness. We hypothesize that heat acclimation with mitigated fluid delivery will improve cardiovascular and perceptual strain associated with heat stress, and hydration maintenance. PURPOSE: To determine the impact of hydration status during heat acclimation on hydration retention and cardiovascular and perceptual drift. METHODS: Eight aerobically fit male subjects (23 ± 1.00 yrs, 82.1±2.14 kg, 53.18 ± 1.63 mL•kg-1•min-1 VO2 max, 13.8% ± 1.31% BF) completed a heat stress test (HST) two days prior to and following three acclimation bouts, in alternating days to allow for rehydration. HST and acclimation bouts consisted of 90 min treadmill walking at 50% VO2 max in hot conditions (40℃, 30% RH). Fluid delivery during acclimation bouts was administered in a randomized crossover design, where fluids were administered to promote a dehydrated (DEH = 0.5 mL/kg) or a euhydrated (EUH = 2.0 mL/kg) state. A five week washout period separated DEH from EUH acclimation periods. Participants wore standard wildland firefighter (WLFF) NoMex clothing with a cotton shirt. Peak heart rate (HR), peak rate of perceived exertion (RPE), percent dehydration (%DEH), and ΔBW were analyzed by a 2(trt) X 2(trial) ANOVA. RESULTS: HR decreased from PRE to POST (p<0.001) (178.9±13.3, 165.1±16.0, 182.3±11.8, 162.9±16.0, for DEH vs EUH pre-post respectively) but there was not an effect of treatments. RPE decreased between trials (p<0.001)(17.8±2.3, 14.5±2.6, 16.9±2.5, 13.6±2.3 for DEH vs EUH pre-post respectively) but no significance between treatments. ΔBW demonstrated a time and trtXtime interaction (p=0.030) with greater ΔBW in the DEH trial following acclimation (1.15±0.27, 1.83±0.39, 1.35±0.35, 1.57±0.38 for DEH vs EUH pre-post respectively). %DEH demonstrated a time and trtXtime interaction (p=0.033) with greater %DEH in the DEH trial following acclimation (1.48±0.38, 2.15±0.48, 1.72±0.49, 1.99±0.52 for DEH vs EUH pre-post respectively). CONCLUSION: Our data suggests that heat acclimation reduces cardiovascular and perceptual strain, however mitigated fluid delivery during acclimation did not alter this adaptation. Mitigated fluid delivery increases ΔBW and %DEH suggesting greater sweat capacity following the acclimation trials. This suggests that controlled dehydration may result in greater adaptations during short term heat acclimation.

Funding provided by the US Forest Service.

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