Heat stress and susceptibility to exertion-associated illnesses in military personnel

Background: In populations working under heat stress, Exertional Heat Illness (EHI) may prove fatal. High core body temperature (Tc) and circulatory insufficiency are implicated. A surrogate for heat strain (e.g. elevated Tc, heart rate) could improve diagnosis, or reflect susceptibility. The glycopeptide copeptin is released during regulation of blood osmolality and volume; 'cut-off' levels (e.g. >20 pmol.L-1) may aid diagnosis and prognosis in a variety of medical disorders. Aims: (1) Characterise EHI risk in UK military personnel (2) Evaluate plasma copeptin responses to heat acclimatisation (3) Assess whether copeptin reflects exertional heat strain. Studies: (1) Epidemiology. i. Analysis of military notifications identified incomplete EHI surveillance. ii. Questionnaire respondents in a hot training environment appeared at low individual risk of EHI, but reported inadequate acclimatisation time. iii. Case reporting review highlighted the absence of recognised risk factors in a significant proportion of EHI casualties and increased hospitalisation risk in supposedly 'low risk' states. (2) Laboratory. Serial heat-exercise tests were conducted in the UK and during acclimatisation to a hot environment (Cyprus). In the UK, post-exercise copeptin >20 pmol.L-1 was associated with higher Tc and adrenocortical responses versus <20 pmol.L-1. With acclimatisation, post-exercise copeptin >20 was associated with higher heart rate and adrenocortical responses. (3) Field. During initial heat acclimatisation under moderate heat stress (Kenya), higher copeptin reflected greater Tc responses. In acclimatised personnel subject to more severe heat stress (Cyprus), higher copeptin associated with increased heart rate. Conclusions: In military personnel, heat illness is under-reported and the time required for heat adaptation may be under-appreciated. Occult susceptibility is common and has potential to undermine preventive practices. Copeptin did not reflect group changes in Tc with acclimatisation, but copeptin >20 pmol.L-1 associated with increased heat strain in all settings. The diagnostic/prognostic value of copeptin should be investigated further in suspected EHI cases.