Physiologic Strain during Treadmill Electrocardiography in the Medical Evaluation of Candidates for Hazardous Materials Duty, with and without Added Heat Stress

Purpose: To compare the Physiologic Strain Index (PSI) induced by maximal treadmill exercise with and without added heat stress as part of medical evaluation for hazardous materials (hazmat) work requiring protective ensembles which block evaporative and convective heat transfer. Also, to extend comparison of PSI versus published values from hazmat simulations. Methods: Candidates for hazmat duty (N=203) underwent maximal, symptom-limited Bruce protocol treadmill electrocardiography (standard Bruce test wearing gym clothes, SBT) during which changes in tympanic temperature (TT) and heart rate (HR) enabled calculation of PSI. A subgroup of candidates (N=39) later performed a second Bruce test, wearing novel, inexpensive apparel chosen to impede dissipation of metabolic heat (Hot Bruce test, HBT). Thermal discomfort was gauged using the Young index (4, neutral; 8, maximal discomfort). Results: SBT duration was 12.2 ± 2.6 SD minutes and the rise in TT and PSI averaged 0.5 ± 0.4 ̊C and 5.9 ± 1.1, respectively. The rate of rise of TT was 0.038 ̊C per min. of treadmill exertion. In the subgroup of 39 candidates, HBT duration was 13.7 ± 3.3 min. (p>0.05). TT rose more after HBT than SBT: 1.3 ± 0.7 ̊C vs. 0.5 ± 0.4 ̊C (p<0.001). The rate of rise of TT during HBT was 0.10 ̊C per min. and was associated with greater physiological strain (PSI=7.4 vs. 6.2, p<0.001). The Young index was 6.2 ± 0.8 for SBT vs. 7.3 ± 0.6 (p<0.001) for HBT. Maximal heart rate was 181 bpm during both SBT and HBT.