Refining the distinction between heat tolerant and intolerant individuals during a Heat tolerance test

Background: The heat tolerance test (HTT) is a standardized physiological test that constitutes one of the considerations in the Israel Defence Forces (IDF) for return to duty after a heat injury. The HTT consists of a 2. h controlled exercise-heat stress with a threshold of maximal rectal temperature (Tc) and heart rate (HR) values above which subjects are referred to as heat intolerant; the dynamics of the HR and Tc during the test, which tend to plateau during the 2nd hour of the test, is also considered. Since "tendency to plateau" is a subjective measure, this study aimed to quantify the tendency to plateau during a HTT. Material and methods: The physiological results of 102 HTT subjects (83 normal and 19 heat-intolerant) served as the database for analysis. The first 28 subjects, who were considered heat tolerant (HT) by an experienced examiner, served to evaluate a "normal" dynamic of Tc and HR during a HTT. Then, we applied the results on seven heat tolerant and seven heat intolerant (HI) subjects in order to determine which of the two variables (dTc or dHR) in different time intervals (t_120-0, t_60-0, t_120-60, and t_120-100) may best distinguish heat tolerant from heat intolerant subjects. During the 3rd stage of the study 60 random test results (post factum: 48 heat tolerant and 12 heat intolerant subjects) were evaluated. Post-hoc results of heat tolerance determined by dTc were compared to the expert diagnosis of the test. Results: A rise of less than 0.45. °C in Tc during the 2nd hour of the HTT was found acceptable to define a tendency to plateau of the Tc with a 100% sensitivity and specificity. We did not find a numerical value of HR increase over time during the HTT acceptable to define a tendency of the HR to plateau. Conclusion: During the last hour of the heat tolerance test, there is a tendency to plateau in Tc in heat tolerant individuals. It is concluded that a rise in Tc of less than 0.45°C during this period (Tc_120-60) can be used as a supporting measure to distinguish between heat tolerant and heat intolerant individuals.