Abstract
Sedentary individuals can not tolerate the same increase in rectal temperature (Tre) during uncompensable heat stress (UHS) compared with endurance trained subjects, partially due to intestinal ischemia and endotoxin leakage that occurs with the redistribution of splanchnic blood flow. Heat acclimation (HA) may be an adaptive strategy for these individuals to increase plasma volume and delay endotoxin leakage, thus increasing thermotolerance during UHS. PURPOSE: The purpose was to examine the relationship between plasma volume (PV) changes and thermotolerance during UHS following 9 days of HA in sedentary males. METHODS: Seven sedentary males (mean±SD: 28±7 y, 75.3±8.0 kg, VO2peak= 42±6 mL·kg-1·min-1) walked to exhaustion on a treadmill (4.5 km·h-1, 2% grade) in 40°C and 30% R.H. wearing combat clothing and a protective hooded overgarment on 9 days (over 2 consecutive weeks). PV was determined prior to beginning HA using Indocyanine Green™ and resting hematocrit and hemoglobin values measured on subsequent days were used to calculate the change in PV. RESULTS: PV increased significantly over the 9 days from 49.6±9.0 to 53.3±10.3 mL·kg-1. The change in Tre from beginning to end of the UHS also increased significantly over the 9 days from 1.94±0.38 to 2.37±0.3°C owing to both a significant reduction in resting Tre from 36.83±0.32 to 36.59±0.26°C and a non-significant increase (p<0.08) in Tre at exhaustion (38.76±0.4 to 38.95±0.31°C). Total heat storage increased significantly from 6.7±1.3 to 8.0±1.2 kJ·kg-1. However, PV changes were not related to changes in heat storage or the changes in initial or final core temperatures during UHS. CONCLUSION: Although 9 days of HA induced an expansion of PV in these sedentary males, these changes were not directly related to the increase in heat storage that occurred. Funded by a Defence R&D Canada Technology Investment Fund and the US Navy Office of Naval Research.
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