The cumulative heat strain index--a novel approach to assess the physiological strain induced by exercise-heat stress.

The cumulative heat strain index (CHSI) is a new approach for assessing the total physiological strain experienced by subjects exposed to an exercise-heat stress. The index is based on inherent physiological logic that combines the thermoregulatory strain, which is described by the area under the hyperthermic curve, and the circulatory strain, which is characterized by heart-beat count. According to this model, the index reflects the dynamics of changes in the thermoregulatory and cardiovascular components and accounts for the complementary nature of the interaction between them. Mathematically, the index is calculated as follows: CHSI =[ sigma(0-t) hb-fc(0) x t] x l0(-3) x [ integral (0-t) Tre x dt-Tre(0) x t] (units) Where: hb=heart beats, fc(0)=initial lowest heart rate (bpm), Tre = rectal temperature (Tre(0) = baseline Tre) (degrees C) and t = time (min) from the onset of measurements. Four sets of data, from various former studies, have been used to demonstrate the index's applicability and its sensitivity to differentiate between levels of stain under various stressful conditions (e.g. clothing insulation, acclimation to heat and levels of tolerance to heat). In all cases, the index was found to be a sensitive tool for assessing the level of strain. Furthermore, the CHSI can be used to predict potential strain. The index's high sensitivity arises from its nature, which reflects miniature differences in the pattern of changes in the dynamics of physiological responses and therefore is a powerful and practical tool for evaluating even minor changes in strain.