Key Points

• Hydration sufficient to replace fluid lost in sweat is essential to prevent heat stroke.

• Athletes exercising in the heat who exhibit mental status changes must be immediately removed from competition and cooled.

• Ice-water immersion produces the most rapid decrease in core body temperature.

Heat stroke is the third leading cause of death in high school athletes (Lee-Chiong and Stitt, 1995). This is tragic because these deaths should be largely avoidable.

The exercising human is an engine operating at about 25% efficiency, resulting in 3 W of heat production for every watt of work, and requires a biologic radiator to avoid overheating. Humans dissipate heat through convection, conduction, radiation, and evaporation, with evaporative sweat loss being the most significant. Higher temperatures limit heat dissipation from convection and conduction, warm sunny days elevate body temperature through radiant heating, and higher humidity decreases evaporative cooling. Thus, the combination of high ambient temperature, radiant heat from the sun, and high humidity works synergistically to create dangerous playing conditions that promote the development of heat illness.

The wet bulb globe temperature (WBGT) index incorporates ambient temperature, relative humidity, and the amount of radiant heat coming from the sun to provide a measure of the risk of overheating. The American College of Sports Medicine Inter-Association Task Force on Exertional Heat Illnesses Consensus Statement (2006) recommends that WBGT readings from 18° to 23° C (64.4°-73.4° F) result in moderate risk, 23° to 28° C (73.4°-82.4° F) in high risk, and more than 28°C (82.4° F) in extreme risk. The cumulative effect of successive days of exercise in the heat must also be considered. In a U.S. Marine Corps, investigators demonstrated that the risk of exertional heat illness was best predicted by considering the current and the previous day's WBGT index (Wallace et al., 2005).

Because evaporative cooling is the primary mechanism for heat dissipation, adequate hydration is essential to keep the biologic radiator functioning. Losses of 2% to 3% of body weight are common with high-intensity exercise in the heat (Galloway, 1999). Below 5% fluid losses, performance and thermoregulation are impaired, and thirst is an inconsistent stimulus to rehydrate, so regular, planned fluid consumption is essential. Fluid recommendations vary, but experts have suggested about 500 mL of fluid intake 2 hours or less before exercise and then about 250 mL every 20 minutes during exercise (Convertino et al., 1996). Because of differences in sweat rate, acclimatization, intensity of exercise, clothing, protective equipment, and environmental factors, individual fluid requirements vary. Thus, recording an athlete's nude weight in the morning and evening is an effective method for determining adequate rehydration. If athletes are losing more than 2% to 3% of their body weight with training, they need to consume more fluids during training. If they cannot regain the lost weight before the next morning's training, they need to consume additional fluids after training and during recovery time. For every kilogram of body weight lost, 1 L of fluid should be consumed. Cooler, flavored fluids are recommended to increase palatability and absorption.

Heat illness is classified as heat edema, heat cramps, heat syncope, heat exhaustion, and heat stroke (Table 29-1) (Binkley et al., 2002; Eichner, 1998). Heat stroke is of the greatest concern, with hallmark features of an elevated core temperature higher than 40.5° C (105° F) and associated mental status changes. Any athlete exhibiting mental status changes and participating in an environment conducive to heat illness requires immediate removal from participation and active cooling. An ice-water tub should be prepared in advance if rapid cooling may be necessary in high-risk events, and an affected athlete should be fully submerged, with only the head above water (Smith, 2005). Other methods of cooling, such as applying ice bags to the neck, axilla, and groin, or using cold-water spray combined with fanning, can be effective, but the rate of core body temperature loss is slower than in ice-water immersion. Close monitoring of mental status and vital signs (e.g., core temperature) is indicated, and athletes should be transported to the hospital if they do not exhibit improving mental status with normalization of vital signs. The National Athletic Trainers' Association Exertional Heat Illness Position Statement is an excellent reference regarding proper preparedness for heat illness (Binkley et al., 2002).

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