Heat strain measurement using heart rate : cardiac thermal extra-pulses (CTEP)

The results of working conditions surveys show that nearly one worker in five says exposed to heat. Conventional evaluation of thermal strain methods were designed for long exposures, continuous, in some thermal underwear but are particularly unsuited and source of risk if exposures are in tight clothing and / or when they are brief and intense. These circumstances are increasingly common in the workplace. A simple method to evaluate these situations is available from the collection of heart rate (HR, in beats per minute or bpm). Indeed, many laboratory studies have shown that the thermal component of the increase in HR measured during recovery after exposure, called cardiac thermal extra pulses (CTEP) is an accurate reflection of the increase in core temperature because of the thermal load. In a series of field studies of work in the heat the CTEP and oral temperature (tor in °C) were measured in 98 employees holding 18 different jobs. The relationship between the change in tor between the beginning and end of an exposure (dtor) and CTEP is of the form:
dtor = 0,03 CTEP r ² = 0.03 = 0.70, p <0.0001, n = 132
The diversity of thermal conditions, individual characteristics, professional activities and clothing encountered during this study as well as its consistency with published data collected in laboratory situations, allow to assert the validity of this relationship and the security of its application in a wide range of work situations. In addition, the large number of measurements made it possible to calculate averages and take into account the interindividual variability. Thus, if a rise in tbu 1 ° C corresponds to an average of 31 bpm CTEP, to keep dtor within 1 ° C recommended by international standards, less than 20 bpm CTEP help ensure that 95 % of an exposed population is below this limit. If exposure to heat lasts more than an hour, the sweat losses should also be measured in order to control both the risk of dehydration and hyperthermia.