Heatstroke During Endurance Exercise: Is There Evidence for Excessive Endothermy?

Dale E. Rae; Gideon J. Knobel; Theresa Mann; Jeroen Swart; Ross Tucker; Timothy D. Noakes

Disclosures

Med Sci Sports Exerc. 2008;40(7):1193-1204. 

In This Article

Summary

We describe three cases of exercise-induced heatstroke and one of cardiac arrest, to which hyperthermia may have contributed. Although the prevailing environmental conditions experienced by the cyclists (cases 1, 2, and 3) were warm, calculations suggest that these athletes were not exercising either with sufficient vigor or for sufficiently long period to explain their development of heatstroke. Similarly, the runner (case 4) ran slowly in mild conditions and should not have experienced progressive heat accumulation leading to heatstroke because his calculated rate of heat production was 48% of the maximal capacity for heat loss in the prevailing environmental conditions. Furthermore, a calculated sweat rate of only 542 mL·h-1 should have maintained his core temperature at 37.5°C (Fig. 5). We propose either that the heat-losing mechanisms in these athletes were inadequate or that, and perhaps more likely we think, excessive endogenous thermogenesis, initiated by unknown factors, may have produced the heatstroke.[22,36] We speculate that excessive endogenous thermogenesis was present in case 4, because this would explain why his cooling rate when immersed in an ice water bath was only 20% of the rate measured in other subjects with hyperthermia or heatstroke. In only 1 of 14 other cases of heatstroke described in the literature ( Table 5 ) could an imbalance between the calculated normal rate of heat production and expected potential rate of heat loss explain the development of heatstroke. The application of these equations to other similar cases in which heatstroke occurred (i) in only a few of many competitors, (ii) when the exercise intensity is modest, and (iii) when the environmental conditions are not extreme may identify other cases in which excessive endogenous thermogenesis may occur in patients diagnosed with exercise-induced heatstroke. This may allow the identification of currently unknown causes of such excessive thermogenesis and improve the understanding of this complex condition.[4,5] Of great practical importance was our observation that the failure to recognize heatstroke at the time of collapse in the cyclists with the immediate initiation of cooling probably contributed to the fatal outcomes.

This research was funded by the Harry Crossley and Neily Atkinson Staff Research Funds of the University of Cape Town, the Medical Research Council of South Africa and Discovery Health. The authors acknowledge the assistance of Dr Basil Bonner, the medical officer of the 2002 Argus Cycle Tour, in allowing access to the medical information pertaining to that race, and Ray Kriel of the Division of Anatomical Pathology, Faculty of Health Sciences, University of Cape Town, who assisted in the production of the photomicrographs from the autopsy of case 1. Finally, the authors acknowledge Ms. Michelle Perrins, Chief Medical Technologist, Division of Forensic Medicine, Faculty of Health Sciences, University of Cape Town, for cutting, preparing, and staining sections for microscopic examination.

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