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RÉSUMÉ Afin de montrer que le recueil de la fréquence cardiaque (Fc) permet de définir des durées limites d’exposition (DLE) à la chaleur de façon simple et sûre, les extrapulsations cardiaques thermiques (EPCT) ont été mesurées chez 98 salariés au cours de 131 séquences de travail réel dans 18 situations de travail différentes. La variation de la température buccale (dtbu) entre le début et la fin d’une exposition à la chaleur est liée aux EPCT par la relation dtbu = 0,032 EPCT (p < .0001, ese = 0,18 oC). L’erreur standard de l’estimation (ese) permet de calculer la dtbu moyenne pour laquelle 95 % de la population aura une dtbu inférieure à 1 oC (1 – (1,65 . 0,18)) soit 0,7 oC. La relation permet ensuite de calculer la valeur des EPCT correspondant à cette dtbu moyenne de 0,7 oC ((0,7/0,032) soit 21 battements par minutes (bpm). Ainsi, tant que les EPCT sont égales ou inférieures à 20 bpm, l’astreinte thermostatique sera acceptable pour 95 % de la population. Ce résultat, calculé sur des mesures réalisées sur une centaine de salariés dans des conditions variées d’exposition, montre que la limite de 20 bpm d’EPCT peut être utilisée de façon sûre pour déterminer des DLE qui n’induisent pas une astreinte thermique excessive. La simplicité des appareillages de mesure de la Fc fait de ce paramètre une référence pour la quantification de l’astreinte thermique.

Evaluation of the heat strain by heart rate Thermal strain remains difficult to quantify in actual work conditions despite the standardization of several strain models. Indeed, occupational physicians must record numerous parameters in order to use these models, which makes them difficult to apply. In addition, these models are appropriate only in cases of long lasting exposure. In exposures shorter than one hour, which are the most frequent at the workplace, the time limit values (TLV) computed by these models appear unsafe. The purpose of this study was to show that the recording of heart rate (HR) allows a simple determination of safe TLV. In this study, rest HR increases due to thermal strain (EPCT) were recorded on 98 workers during 131 actual work periods representing 18 different work situations. The EPCT is the difference in beats per minute (bpm) between the mean HR of the 3rd, 4th and 5th minutes of rest after the end of the exposure and sitting at rest HR before the exposure. In this field study, oral temperature was considered as a good index of core temperature. Variation in oral temperature (dtbu) from the beginning to the end of the heat exposure is related to EPCT : dtbu = 0,032 EPCT. This relationship is significant (p < .0001) and the standard error of the estimation (see) is 0,18 oC. Because this study was conducted on a large sample of workers, the results allow to compute a safety limit of EPCT which protects 95 % of the population against an increase of core temperature greater than 1 oC. Thus, maximal mean dtbu increase is given by : mean + 1,65 see equals 1 oC. The mean dtbu increase must then be lower than 0,7 oC (1 – (1,65 . 0,18)) which corresponds to a maximal EPCT increase (0,7/ 0,032) of 21 beats per minute (bpm). From our results, it is concluded that as long as the EPCT value remains equal or below 20 bpm the increase in oral temperature is lower than 1 oC for 95 % of the exposed population, rendering the thermal strain acceptable. This result has been obtained from recordings out of almost one hundred workers in various working and exposure conditions. The EPCT limit of 20 bpm can be used safely to define the physiological acceptability of the heat exposure at a workplace and, if necessary, to determine TLV. Easy use of heart rate recorders means that heart rate can be a reference parameter for the thermal strain quantification as well as for the evaluation of physical workload. Indeed, in the conditions of the present field study, the physical work load reach frequently levels which induce high and even hazardous heart rates.