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

• Heat intolerant individuals are at increased risk for heat injuries and heat stroke.
• The heat tolerance test is a standardized heat stress-exercise challenge that distinguishes between heat tolerant and intolerant individuals.
• Heat intolerance criteria: body core temperature elevates above 38.5 °C; heart rate elevates above 150 bpm; neither tends to plateau.
• Tendency to plateau of Tc is a rise of less than 0.45 °C during the last hour of the HTT.

BackgroundThe 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 methodsThe 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 (t120–0, t60–0, t120–60, and t120–100) 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.ResultsA 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.ConclusionDuring 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 (Tc120–60) can be used as a supporting measure to distinguish between heat tolerant and heat intolerant individuals.