Hyperthermia and Exertional Heatstroke During Running, Cycling, Open Water Swimming, and Triathlon Events

Abstract

Few previous epidemiological studies, sports medicine position statements, and expert panel consensus reports have evaluated the similarities and differences of hyperthermia and exertional heatstroke (EHS) during endurance running, cycling, open water swimming, and triathlon competitions. Accordingly, we conducted manual online searches of the PubMed and Google Scholar databases using pre-defined inclusion criteria. The initial manual screenings of 1192 article titles and abstracts, and subsequent reviews of full-length pdf versions identified 80 articles that were acceptable for inclusion. These articles indicated that event medical teams recognized hyperthermia and EHS in the majority of running and triathlon field studies (range, 58.8 to 85.7%), whereas few reports of hyperthermia and EHS appeared in cycling and open water swimming field studies (range, 0 to 20%). Sports medicine position statements and consensus reports also exhibited these event-specific differences. Thus, we proposed mechanisms that involved physiological effector responses (sweating, increased skin blood flow) and biophysical heat transfer to the environment (evaporation, convection, radiation, and conduction). We anticipate that the above information will help race directors to distribute pre-race safety advice to athletes and will assist medical directors to better allocate medical resources (eg, staff number and skill sets, medical equipment) and optimize the management of hyperthermia and EHS.

Keywords: athlete; epidemiology; heat illness; pathophysiology; thermoregulation.

Figure 1

The effect of 4 air velocities on the internal heat storage of 9 men who exercised for 2 h on a stationary cycle ergometer in a controlled hot environment (33.0°C, 59%rh), as reported by Saunders et al. During these 4 repeated experiments, subjects consumed fluids which replaced 58–61% of sweat losses. Redrawn with the permission of the publisher John Wiley and Sons, © 2005 Scandinavian Physiological Society from Saunders AG, Dugas JP, Tucker R, Lambert MI, Noakes TD. The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment. Acta Physiol Scand. 2005;183(3):241–255. aSignificantly different from all other data points (P < 0.05 to 0.005); ^bSignificantly different from 50 km·h⁻¹ (P < 0.05).

Figure 2

The effect of water temperature on internal body temperature during 20–80 min of swimming performed during 6 controlled research studies. Each study is identified in this graph by its reference citation number. These experiments involved a range of exercise intensities (see inset box) and various exercise modes (ie, swimming flume, swimming pool, or tethered to a swim ergometer). The horizontal dashed line represents the border between net heat loss and net heat gain during exercise. The vertical dashed line depicts the maximum water temperature (31°C, 87.8°F) for open water swimming competitions that is allowed by the international governing body World Aquatics.,