Heat Acclimation Improves Heat Tolerance Test Specificity in a Criteria-dependent Manner.

A major ongoing challenge in sports medicine and the military is the determination of when an athlete or Military Service Member is ready to return-to-activity (RTA) after suffering an exertional heat stroke. A Heat Tolerance Test (HTT) is one method used for RTA assessment, but its test specificity is unknown and may be affected by heat acclimation (HA) status. PURPOSE To characterize HTT specificity and determine any effect of HA on the outcome. METHODS Thirteen unacclimatized, healthy men (VO2peak:43.0±4.8 ml/kg/min) with no prior history of heat illness completed 8 days of HA using the HTT protocol (40°C/40% RH; 120 minutes; 5 km/hour and 2% grade). Heart rate (HR) and core temperature (Tcore) recorded every 5 minutes during exercise and at the end of 120 minutes (terminal value) were compared between days 1 and 8. Test specificity (given no prior history of heat illness, the probability of being heat tolerant) was calculated on days 1 and 8. RESULTS There was a significant reduction in HR and Tcore between days 1 and 8, indicating successful HA. All volunteers successfully completed 120 minutes of walking on all days. HTT specificity ranged between 54 and 85% on day 1 and 77 and 92% on day 8, depending on the HTT criteria used. CONCLUSION Young healthy men without any prior heat illness experienced a 15 to 46% false positive fail rate for the HTT without HA. After HA, the false positive fail rate decreased to between 8 and 13%. Outcomes of the HTT are significantly affected by the criteria used and by HA status. Use of the HTT for RTA decisions should be done with recognition of these effects.