Monitoring heat strain in physically challenging environments
2012-07-27T12:23:37Z (GMT) by
Individuals working in physically demanding occupations can experience high levels of heat strain due to the physical demands of the job, harsh environmental conditions and/or the wearing of personnel protective equipment (PPE). The ability to monitor body core temperature (Tc) could reduce the risk of heat illness. The main purpose of this thesis was to examine non-invasive methods of Tc measurement. Studies 1, 2 and 3 investigated the validity of insulated skin temperature (Tis) as a non-invasive measure of Tc, in emergency service (ES) and military personnel. In Study 1, a model including Tis and micro-climate temperature (Tmc) was developed to predict rectal temperature (Tre) for ES personnel wearing PPE. The resulting standard error of the estimate (SEE=0.20 °C) was within the a-priori pre-defined SEE limit (0.20 °C), providing encouragement for the further investigation of Tis as a surrogate measure of Tc. Studies 2 and 3 sought to determine the validity of Tis in predicting Tre in military personnel, wearing PPE in temperate conditions (Study 2) and in a desert environment (Study 3). Although the SEE was outside the acceptable SEE in Study 2 (0.22 °C), the sensitivity (97 %) for predicting Tre values over 38.5 °C provided scope for a military application for Tis to predict Tc. In Study 3, Tis could not predict Tre in a desert environment, with simulated solar radiation directly affecting Tis and invalidating the prediction (SEE = 0.29 °C). Study 4 involved a series of experiments performed under six conditions in a thermal chamber with two clothing types, to determine whether the addition of other physiological and/or environmental factors might improve the prediction of Tre. A model including Tis and Tmc resulted in an SEE of 0.26 °C; with heart rate (HR) and work significantly reducing the SEE (0.23 °C) (p<0.05). Although the SEE achieved in the validation (0.27 °C) was larger than in Studies 1 and 2, these results provide novel information regarding the measures that explain the variance when predicting Tre in a wide range of heat stress conditions. To our knowledge this is the most detailed analysis of Tc prediction based on non-invasive sensors, with the inclusion of all the parameters that are likely to be relevant. The main conclusion from the work thus far was that it is unlikely a reliable prediction of Tc can be achieved, using Tis, for validity under different types of heat stress conditions. However, predictions of Tre for more specific conditions using Tis are achievable. Having collected a vast amount of data on participants demonstrating high levels of heat strain, it was considered valuable to analyse the drop-outs in more detail. More specifically the goal of Study 5 was to see whether measurements of individual heat strain (Tc, HR) and the combination of these in the Physiological Strain Index (PSI) had predictive power for individual drop-out. There were no differences in PSI between individuals who stopped from heat exhaustion (HE) (7.9 ± 0.8) and those who completed the trial (C) (8.3 ± 0.9). The only differences between these two groups were rate of rise of Tre, (C 0.03 ± 0.01 °C min-1 and; HE 0.04 ± 0.01 °C min-1), chest temperature (Tchest) (C 38.1 ± 1.0 °C and; HE 39.0 ± 0.6 °C) and the temperature gradient between Tchest and Tre (C 1.04 ± 1.07 °C, and; HE -0.05 ± 0.59 °C) (p<0.05); It was therefore concluded that PSI did not provide a good personal heat strain measure that would predict tolerance of the individual. In conclusion, Tis (with Tmc) is promising as a non-invasive measure of Tre, in ES and military personnel wearing fully-encapsulated PPE, based on the resulting SEE and sensitivity and specificity. With the current methodology, it is not valid in conditions with a solar load. The addition of HR and work together improve the prediction of Tre. The PSI does not enable identification of individuals who are approaching heat exhaustion, requiring the inclusion of other physiological responses which determine tolerance.