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

Abstract and Introduction

Purpose: Five of 28,753 cyclists participating in an annual 109-km bicycle race died, four within 24 h of the race and the fifth 17 d later. All five deaths were reported to be the consequence of exertional heatstroke. One runner of 6874 participating in an annual 56-km ultramarathon developed heatstroke and required active cooling for 10 h to achieve normothermia. The purpose of this article was to postulate (i) why only 6 of 35,627 athletes were hospitalized for heatstroke in these races, (ii) if exercise alone could have elevated their body temperatures sufficiently to cause heatstroke, and (iii) why the runner required such prolonged cooling.
Methods: Clinical and autopsy data are presented for three of the cyclists and the runner for whom access to this information was granted. Calculations were made to predict the work rates necessary to produce their measured rectal temperatures.
Results: The rectal temperatures of two of the cyclists were 42.0 and 41.2°C on hospitalization, and that of the runner was 41.8°C on collapse. Standard calculations showed that in the prevailing environmental conditions and with their exercise speeds, none should have developed exertional heatstroke. The third cyclist experienced a cardiac arrest to which his elevated (rectal) temperature may have contributed.
Conclusion: The hyperthermic states experienced by the cases presented may have resulted from failure of their heat-losing mechanisms. Alternatively, they might have resulted from excessive endothermy, triggered by physical exertion and other unknown initiating factors. Excessive endothermy should be considered in cases of heatstroke that occur in mild to moderate environmental conditions. Furthermore, prompt initiation of cooling is crucial in all cases of suspected heatstroke.

Sudden death during exercise is an unusual phenomenon in young- to middle-aged athletes. For example, during both the annual Marine Corps (1976 to 1994) and Twin Cities (1982 to 1994) marathons in the United States, there have been only 4 deaths of 215,413 runners.[28] All these deaths were cardiac deaths without heatstroke or hyperthermia. In the London marathon, there have been 8 cardiac deaths and 2 fatal cerebrovascular accidents in 650,000 completed marathon runs.[47]

In view of the apparent rarity of the occurrence, we therefore report that five deaths occurred during a single endurance event, the 2002 Argus Cycle Tour-an annual 109-km bicycle race in Cape Town, South Africa. Unfortunately, we were unable to obtain permission to access the information of two cyclists whose hospital admission records indicate that they were treated for heatstroke and that they died of multiple organ failure typical of this condition. Therefore, our report includes details of only three of these cases. Family members of these three individuals gave their consent for us to access their medical records and clinical data to present their cases. The cyclists were on the road between 8 h 53 min and 15 h 00 min, during which time the average temperature was 28.7°C (range = 21.9-31.4°C), the sky was partly cloudy until 10 h 00 min where after it was clear, the relative humidity was approximately 62%, and the average wind speed was 3.1 m·s-1. Table 1 shows the hourly weather conditions during the race for each of the three cyclists clearly indicating that when the athletes collapsed during the latter part of the race, the ambient temperatures were at the highest. These weather conditions, although hot, were not extreme in comparison to previous and subsequent Argus Cycle Tour races ( Table 2 ). One cyclist collapsed during and another immediately after the race, both without evidence for an obvious predisposing cause, and both died within 24 h. The third cyclist experienced a cardiac arrest during the ride and died of septicemia 17 d later. On the basis of clinical and autopsy evidence, all three deaths were reported to be the consequence of exertional heatstroke. We also report a further case of heatstroke in a competitor in a 56-km footrace, the 2006 Two Oceans Marathon. Consent was obtained from the runner to access his medical records and clinical data to present his case. The runner began his race at 7 h 00 min and completed it shortly before 14 h 00 min, at an average running speed of 8.2 km·h-1. During this period, the average ambient temperature was 18.1°C (range = 7.0-25.0°C), the sky was clear, the relative humidity was 61.4%, and the average wind speed was 1.9 m·s-1 ( Table 3 ). The runner collapsed at the end of the race at which time the environmental temperature was 24.3°C and relative humidity was 36%. His rectal temperature was 41.8°C, and he required almost 10 h of external cooling to normalize his body temperature. In contrast to the cyclists, the runner survived. We report these four cases to pose and partially to answer four questions: (i) Why did only 5 of 28,753 cyclists and 1 of 6874 runners in their respective races develop heatstroke? (ii) What is the probability that exercise alone could have elevated the body temperatures of only six athletes to levels causing exertional heatstroke? (iii) Why did the surviving athlete require 10 h of cooling before his body temperature normalized? and (iv) Why did the athletes develop this condition only in these races and not during other bouts of equivalent exercise?

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