Heart Rate as an Index of Thermal Stress

Heat stress is difficult to quantify in actual work situations. The purpose of this study is to show that Cardiac Thermal Extra Pulse (CTEP) which is the mean heart rate (HR) of the 3rd, 4th and 5th minute of recovery minus rest HR before the exposure is a simple index of thermal strain. CTEP were recorded on 98 workers during 132 actual work periods. The relationship between the oral temperature increase from the beginning to the end of the heat exposure (dtor) and the corresponding CTEP is dtor = 0.05 + 0.29 CTEP (p<0.001; see = 0.18 °C). From these results, it can be concluded that as long as the CTEP remains below 20 beats per minute the increase of oral temperature is lower than 1 °C and thus the thermal strain is physiologically acceptable if the exposure time is less than one hour. The easy use of new HR recorders could make CTEP a reference parameter for simple thermal strain quantification.