The thermal-circulatory ratio (TCR): An index to evaluate the tolerance to heat

Introduction The common practice in the Israel Defense Force is that all exertional heat related injuries victims undergo a heat tolerance test (HTT) as a part of the "return to duty" process. The purpose of this study was to develop a quantitative, supportive physiological index for the assessment of the HTT based on the understanding that heat strain level should combine the thermal and cardiovascular strains. Materials and methods The HTT results of 104 individuals with a history of heat injuries were retrospectively analyzed after randomly divided into two groups (an analysis group and a validation group). Rectal temperature and heart rate were monitored continuously during the test. Using the ratio between those two variables we constructed the TCR (Thermal-Circulatory Ratio) index and defined thresholds for determining heat tolerance based on the HTT. Results Using a TCR value of 0.279 [°C/bpm] or less after completing the 120 min HTT can be used as a significant measure to distinguish between heat tolerance and heat intolerance individuals with sensitivity and specificity of 100% of 89%, respectively. In addition, a TCR value of 0.320 [°C/bpm] or less calculated after 60 min was found as a significant measure to determine heat tolerance with 100% sensitivity and 69% specificity. The latter threshold may assist in significantly shortening the HTT for those individuals whose TCR value matches this criterion. Discussion and conclusion A new index (TCR) that combines the thermal and cardiovascular responses to exercise-heat stress was found to be a valid measure, with high sensitivity and specificity, to support the distinguishing between heat tolerance and heat intolerance individuals following a HTT. Furthermore, the suggested index may enable to shorten the HTT, which will make the test more efficient.