National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses

Abstract

Objective: To present recommendations for the prevention, recognition, and treatment of exertional heat illnesses and to describe the relevant physiology of thermoregulation.

Background: Certified athletic trainers evaluate and treat heat-related injuries during athletic activity in “safe” and high-risk environments. While the recognition of heat illness has improved, the subtle signs and symptoms associated with heat illness are often overlooked, resulting in more serious problems for affected athletes. The recommendations presented here provide athletic trainers and allied health providers with an integrated scientific and practical approach to the prevention, recognition, and treatment of heat illnesses. These recommendations can be modified based on the environmental conditions of the site, the specific sport, and individual considerations to maximize safety and performance.

Recommendations: Certified athletic trainers and other allied health providers should use these recommendations to establish on-site emergency plans for their venues and athletes. The primary goal of athlete safety is addressed through the prevention and recognition of heat-related illnesses and a well-developed plan to evaluate and treat affected athletes. Even with a heat-illness prevention plan that includes medical screening, acclimatization, conditioning, environmental monitoring, and suitable practice adjustments, heat illness can and does occur. Athletic trainers and other allied health providers must be prepared to respond in an expedient manner to alleviate symptoms and minimize morbidity and mortality.

Heat illness is inherent to physical activity and its incidence increases with rising ambient temperature and relative humidity. Athletes who begin training in the late summer (eg, football, soccer, and cross-country athletes) experience exertional heat-related illness more often than athletes who begin training during the winter and spring.^1–5 Although the hot conditions associated with late summer provide a simple explanation for this difference, we need to understand what makes certain athletes more susceptible and how these illnesses can be prevented.

PURPOSE

This position statement provides recommendations that will enable certified athletic trainers (ATCs) and other allied health providers to (1) identify and implement preventive strategies that can reduce heat-related illnesses in sports, (2) characterize factors associated with the early detection of heat illness, (3) provide on-site first aid and emergency management of athletes with heat illnesses, (4) determine appropriate return-to-play procedures, (5) understand thermoregulation and physiologic responses to heat, and (6) recognize groups with special concerns related to heat exposure.

ORGANIZATION

This position statement is organized as follows:

1. Definitions of exertional heat illnesses, including exercise-associated muscle (heat) cramps, heat syncope, exercise (heat) exhaustion, exertional heat stroke, and exertional hyponatremia;

2. Recommendations for the prevention, recognition, and treatment of exertional heat illnesses;

3. Background and literature review of the diagnosis of exertional heat illnesses; risk factors; predisposing medical conditions; environmental risk factors; thermoregulation, heat acclimatization, cumulative dehydration, and cooling therapies;

4. Special concerns regarding exertional heat illnesses in prepubescent athletes, older athletes, and athletes with spinal-cord injuries;

5. Hospitalization and recovery from exertional heat stroke and resumption of activity after heat-related collapse; and

6. Conclusions.

DEFINITIONS OF EXERTIONAL HEAT ILLNESSES

The traditional classification of heat illness defines 3 categories: heat cramps, heat exhaustion, and heat stroke.^6–8 However, this classification scheme omits several other heat- and activity-related illnesses, including heat syncope and exertional hyponatremia. The signs and symptoms of the exertional heat illnesses are listed in Table 1.

Table 1. Signs and Symptoms of Exertional Heat Illnesses

Heat illness is more likely in hot, humid weather but can occur in the absence of hot and humid conditions.

Exercise-Associated Muscle (Heat) Cramps

Exercise-associated muscle (heat) cramps represent a condition that presents during or after intense exercise sessions as an acute, painful, involuntary muscle contraction. Proposed causes include fluid deficiencies (dehydration), electrolyte imbalances, neuromuscular fatigue, or any combination of these factors.^6,9–11,19

Heat Syncope

Heat syncope, or orthostatic dizziness, can occur when a person is exposed to high environmental temperatures.¹⁹ This condition is attributed to peripheral vasodilation, postural pooling of blood, diminished venous return, dehydration, reduction in cardiac output, and cerebral ischemia.^10,19 Heat syncope usually occurs during the first 5 days of acclimatization, before the blood volume expands,¹² or in persons with heart disease or those taking diuretics.¹⁰ It often occurs after standing for long periods of time, immediately after cessation of activity, or after rapid assumption of upright posture after resting or being seated.

Exercise (Heat) Exhaustion

Exercise (heat) exhaustion is the inability to continue exercise associated with any combination of heavy sweating, dehydration, sodium loss, and energy depletion. It occurs most frequently in hot, humid conditions. At its worst, it is difficult to distinguish from exertional heat stroke without measuring rectal temperature. Other signs and symptoms include pallor, persistent muscular cramps, urge to defecate, weakness, fainting, dizziness, headache, hyperventilation, nausea, anorexia, diarrhea, decreased urine output, and a body-core temperature that generally ranges between 36°C (97°F) and 40°C (104°F).^6,9,10,13,19

Exertional Heat Stroke

Exertional heat stroke is an elevated core temperature (usually >40°C [104°F]) associated with signs of organ system failure due to hyperthermia. The central nervous system neurologic changes are often the first marker of exertional heat stroke. Exertional heat stroke occurs when the temperature regulation system is overwhelmed due to excessive endogenous heat production or inhibited heat loss in challenging environmental conditions²⁰ and can progress to complete thermoregulatory system failure.^19,21 This condition is life threatening and can be fatal unless promptly recognized and treated. Signs and symptoms include tachycardia, hypotension, sweating (although skin may be wet or dry at the time of collapse), hyperventilation, altered mental status, vomiting, diarrhea, seizures, and coma.^6,10,14 The risk of morbidity and mortality is greater the longer an athlete's body temperature remains above 41°C (106°F) and is significantly reduced if body temperature is lowered rapidly.^22–24

Unlike classic heat stroke, which typically involves prolonged heat exposure in infants, elderly persons, or unhealthy, sedentary adults in whom body heat-regulation mechanisms are inefficient,^25–27 exertional heat stroke occurs during physical activity.²⁸ The pathophysiology of exertional heat stroke is due to the overheating of organ tissues that may induce malfunction of the temperature-control center in the brain, circulatory failure, or endotoxemia (or a combination of these).^29,30 Severe lactic acidosis (accumulation of lactic acid in the blood), hyperkalemia (excessive potassium in the blood), acute renal failure, rhabdomyolysis (destruction of skeletal muscle that may be associated with strenuous exercise), and disseminated intravascular coagulation (a bleeding disorder characterized by diffuse blood coagulation), among other medical conditions, may result from exertional heat stroke and often cause death.²⁵

Exertional Hyponatremia

Exertional hyponatremia is a relatively rare condition defined as a serum-sodium level less than 130 mmol/L. Low serum-sodium levels usually occur when activity exceeds 4 hours.¹⁹ Two, often-additive mechanisms are proposed: an athlete ingests water or low-solute beverages well beyond sweat losses (also known as water intoxication), or an athlete's sweat sodium losses are not adequately replaced.^15–18 The low blood-sodium levels are the result of a combination of excessive fluid intake and inappropriate body water retention in the water-intoxication model and insufficient fluid intake and inadequate sodium replacement in the latter. Ultimately, the intravascular and extracellular fluid has a lower solute load than the intracellular fluids, and water flows into the cells, producing intracellular swelling that causes potentially fatal neurologic and physiologic dysfunction. Affected athletes present with a combination of disorientation, altered mental status, headache, vomiting, lethargy, and swelling of the extremities (hands and feet), pulmonary edema, cerebral edema, and seizures. Exertional hyponatremia can result in death if not treated properly. This condition can be prevented by matching fluid intake with sweat and urine losses and by rehydrating with fluids that contain sufficient sodium.^31,32

RECOMMENDATIONS

The National Athletic Trainers' Association (NATA) advocates the following prevention, recognition, and treatment strategies for exertional heat illnesses. These recommendations are presented to help ATCs and other allied health providers maximize health, safety, and sport performance as they relate to these illnesses. Athletes' individual responses to physiologic stimuli and environmental conditions vary widely. These recommendations do not guarantee full protection from heat-related illness but should decrease the risk during athletic participation. These recommendations should be considered by ATCs and allied health providers who work with athletes at risk for exertional heat illnesses to improve prevention strategies and ensure proper treatment.

Prevention

1. Ensure that appropriate medical care is available and that rescue personnel are familiar with exertional heat illness prevention, recognition, and treatment. Table 2 provides general guidelines that should be considered.⁷ Ensure that ATCs and other health care providers attending practices or events are allowed to evaluate and examine any athlete who displays signs or symptoms of heat illness^33,34 and have the authority to restrict the athlete from participating if heat illness is present.

2. Conduct a thorough, physician-supervised, preparticipation medical screening before the season starts to identify athletes predisposed to heat illness on the basis of risk factors^34–36 and those who have a history of exertional heat illness.

3. Adapt athletes to exercise in the heat (acclimatization) gradually over 10 to 14 days. Progressively increase the intensity and duration of work in the heat with a combination of strenuous interval training and continuous exercise.^{6,9,14,33,37–44} Well-acclimatized athletes should train for 1 to 2 hours under the same heat conditions that will be present for their event.^6,45,46 In a cooler environment, an athlete can wear additional clothing during training to induce or maintain heat acclimatization. Athletes should maintain proper hydration during the heat-acclimatization process.⁴⁷

4. Educate athletes and coaches regarding the prevention, recognition, and treatment of heat illnesses^{9,33,38,39,42,48–51} and the risks associated with exercising in hot, humid environmental conditions.

5. Educate athletes to match fluid intake with sweat and urine losses to maintain adequate hydration.^* (See the “National Athletic Trainers' Association Position Statement: Fluid Replacement in Athletes.”⁵²) Instruct athletes to drink sodium-containing fluids to keep their urine clear to light yellow to improve hydration^{33,34,52–55} and to replace fluids between practices on the same day and on successive days to maintain less than 2% body-weight change. These strategies will lessen the risk of acute and chronic dehydration and decrease the risk of heat-related events.

6. Encourage athletes to sleep at least 6 to 8 hours at night in a cool environment,^41,35,50 eat a well-balanced diet that follows the Food Guide Pyramid and United States Dietary Guidelines,^56–58 and maintain proper hydration status. Athletes exercising in hot conditions (especially during twice-a-day practices) require extra sodium from the diet or rehydration beverages or both.

7. Develop event and practice guidelines for hot, humid weather that anticipate potential problems encountered based on the wet-bulb globe temperature (WBGT) (Table 3) or heat and humidity as measured by a sling psychrometer (Figure 1), the number of participants, the nature of the activity, and other predisposing risk factors.^14,51 If the WBGT is greater than 28°C (82°F, or “very high” as indicated in Table 3, Figure 1), an athletic event should be delayed, rescheduled, or moved into an air-conditioned space, if possible.^69–74 It is important to note that these measures are based on the risk of environmental stress for athletes wearing shorts and a T-shirt; if an athlete is wearing additional clothing (ie, football uniform, wetsuit, helmet), a lower WBGT value could result in comparable risk of environmental heat stress (Figure 2).^75,76 If the event or practice is conducted in hot, humid conditions, then use extreme caution in monitoring the athletes and be proactive in taking preventive steps. In addition, be sure that emergency supplies and equipment are easily accessible and in good working order. The most important factors are to limit intensity and duration of activity, limit the amount of clothing and equipment worn, increase the number and length of rest breaks, and encourage proper hydration.

   Modify activity under high-risk conditions to prevent exertional heat illnesses.^19,21 Identify individuals who are susceptible to heat illnesses. In some athletes, the prodromal signs and symptoms of heat illnesses are not evident before collapse, but in many cases, adept medical supervision will allow early intervention.

8. Check the environmental conditions before and during the activity, and adjust the practice schedule accordingly.^{29,38,41,42,60} Schedule training sessions to avoid the hottest part of the day (10 AM to 5 PM) and to avoid radiant heating from direct sunlight, especially in the acclimatization during the first few days of practice sessions.^{9,29,33,34,38,40,50,60}

9. Plan rest breaks to match the environmental conditions and the intensity of the activity.^33,34 Exercise intensity and environmental conditions should be the major determinants in deciding the length and frequency of rest breaks. If possible, cancel or postpone the activity or move it indoors (if air conditioned) if the conditions are “extreme or hazardous” (see Table 3) or “very high” (see Figure 1) or to the right of the circled line (see Figure 2). General guidelines during intense exercise would include a work:rest ratio of 1:1, 2:1, 3:1, and 4:1 for “extreme or hazardous” (see Table 3) or “very high” (see Figure 1), “high,” “moderate,” or “low” environmental risk, respectively.^41,77 For activities such as football in which equipment must be considered, please refer to Figure 2 for equipment modifications and appropriate work:rest ratios for various environmental conditions. Rest breaks should occur in the shade if possible, and hydration during rest breaks should be encouraged.

10. Implement rest periods at mealtime by allowing 2 to 3 hours for food, fluids, nutrients, and electrolytes (sodium and potassium) to move into the small intestine and bloodstream before the next practice.^34,50,77

11. Provide an adequate supply of proper fluids (water or sports drinks) to maintain hydration^{9,34,38,40,50,60} and institute a hydration protocol that allows the maintenance of hydration status.^34,49 Fluids should be readily available and served in containers that allow adequate volumes to be ingested with ease and with minimal interruption of exercise.^49,52 The goal should be to lose no more than 2% to 3% of body weight during the practice session (due to sweat and urine losses).^78–82 (See the “National Athletic Trainers' Association Position Statement: Fluid Replacement in Athletes.”⁵²)

12. Weigh high-risk athletes (in high-risk conditions, weigh all athletes) before and after practice to estimate the amount of body water lost during practice and to ensure a return to prepractice weight before the next practice. Following exercise athletes should consume approximately 1–1.25 L (30–34 oz) of fluid for each kilogram of body water lost during exercise.^†

13. Minimize the amount of equipment and clothing worn by the athlete in hot or humid (or both) conditions. For example, a full football uniform prevents sweat evaporation from more than 60% of the body.^{29,33,40,51,77} Consult Figure 2 for possible equipment and clothing recommendations. When athletes exercise in the heat, they should wear loose-fitting, absorbent, and light-colored clothing; mesh clothing and new-generation cloth blends have been specially designed to allow more effective cooling. ^‡

14. Minimize warm-up time when feasible, and conduct warm-up sessions in the shade when possible to minimize the radiant heat load in “high” or “very high” or “extreme or hazardous” (see Table 3, Figure 1) conditions.⁷⁷

15. Allow athletes to practice in shaded areas and use electric or cooling fans to circulate air whenever feasible.⁶⁶

16. Include the following supplies on the field, in the locker room, and at various other stations:

    • A supply of cool water or sports drinks or both to meet the participants' needs (see the “National Athletic Trainers' Association Position Statement: Fluid Replacement in Athletes”⁵² for recommendations regarding the appropriate composition of rehydration beverages based on the length and intensity of the activity)^29,34,38
    • Ice for active cooling (ice bags, tub cooling) and to keep beverages cool during exercise^29,38
    • Rectal thermometer to assess body-core temperature^{39,74,75,87,88}
    • Telephone or 2-way radio to communicate with medical personnel and to summon emergency medical transportation^38,39,48
    • Tub, wading pool, kiddy pool, or whirlpool to cool the trunk and extremities for immersion cooling therapy^35,65

17. Notify local hospital and emergency personnel before mass participation events to inform them of the event and the increased possibility of heat-related illnesses.^41,89

18. Mandate a check of hydration status at weigh-in to ensure athletes in sports requiring weight classes (eg, wrestling, judo, rowing) are not dehydrated. Any procedures used to induce dramatic dehydration (eg, diuretics, rubber suits, exercising in a sauna) are strictly prohibited.⁵² Dehydrated athletes exercising at the same intensity as euhydrated athletes are at increased risk for thermoregulatory strain (see the “National Athletic Trainers' Association Position Statement: Fluid Replacement in Athletes”⁵²).

    Recognition and Treatment

19. Exercise-associated muscle (heat) cramps:

    • An athlete showing signs or symptoms including dehydration, thirst, sweating, transient muscle cramps, and fatigue is likely experiencing exercise-associated muscle (heat) cramps.
    • To relieve muscle spasms, the athlete should stop activity, replace lost fluids with sodium-containing fluids, and begin mild stretching with massage of the muscle spasm.
    • Fluid absorption is enhanced with sports drinks that contain sodium.^52,60,87 A high-sodium sports product may be added to the rehydration beverage to prevent or relieve cramping in athletes who lose large amounts of sodium in their sweat.¹⁹ A simple salted fluid consists of two 10-grain salt tablets dissolved in 1 L (34 oz) of water. Intravenous fluids may be required if nausea or vomiting limits oral fluid intake; these must be ordered by a physician.^6,7,52,90,91
    • A recumbent position may allow more rapid redistribution of blood flow to cramping leg muscles.

20. Heat syncope:

    • If an athlete experiences a brief episode of fainting associated with dizziness, tunnel vision, pale or sweaty skin, and a decreased pulse rate but has a normal rectal temperature (for exercise, 36°C to 40°C [97°F to 104°F]), then heat syncope is most likely the cause.¹⁹
    • Move the athlete to a shaded area, monitor vital signs, elevate the legs above the level of the head, and rehydrate.

21. Exercise (heat) exhaustion:

    • Cognitive changes are usually minimal, but assess central nervous system function for bizarre behavior, hallucinations, altered mental status, confusion, disorientation, or coma (see Table 1) to rule out more serious conditions.
    • If feasible, measure body-core temperature (rectal temperature) and assess cognitive function (see Table 1) and vital signs.¹⁹ Rectal temperature is the most accurate method possible in the field to monitor body-core temperature.^{34,74,75,87,88} The ATC should not rely on the oral, tympanic, or axillary temperature for athletes because these are inaccurate and ineffective measures of body-core temperature during and after exercise.^75,89,92
    • If the athlete's temperature is elevated, remove his or her excess clothing to increase the evaporative surface and to facilitate cooling.^6,93
    • Cool the athlete with fans,⁹⁴ ice towels,^29,38 or ice bags because these may help the athlete with a temperature of more than 38.8°C (102°F) to feel better faster.
    • Remove the athlete to a cool or shaded environment if possible.
    • Start fluid replacement.^6,52,93,95
    • Transfer care to a physician if intravenous fluids are needed^{6,52,90,91,96} or if recovery is not rapid and uneventful.

22. Exertional heat stroke:

    • Measure the rectal temperature if feasible to differentiate between heat exhaustion and heat stroke. With heat stroke, rectal temperature is elevated (generally higher than 40°C [104°F]).¹⁹
    • Assess cognitive function, which is markedly altered in exertional heat stroke (see Table 1).
    • Lower the body-core temperature as quickly as possible.^34,70,77 The fastest way to decrease body temperature is to remove clothes and equipment and immerse the body (trunk and extremities) into a pool or tub of cold water (approximately 1°C to 15°C [35°F to 59°F]).^{32,91,92,97–99} Aggressive cooling is the most critical factor in the treatment of exertional heat stroke. Circulation of the tub water may enhance cooling.
    • Monitor the temperature during the cooling therapy and recovery (every 5 to 10 minutes).^39,87 Once the athlete's rectal temperature reaches approximately 38.3°C to 38.9°C (101°F to 102°F), he or she should be removed from the pool or tub to avoid overcooling.^40,100
    • If a physician is present to manage the athlete's medical care on site, then initial transportation to a medical facility may not be necessary so immersion can continue uninterrupted. If a physician is not present, aggressive first-aid cooling should be initiated on site and continued during emergency medical system transport and at the hospital until the athlete is normothermic.
    • Activate the emergency medical system.
    • Monitor the athlete's vital signs and other signs and symptoms of heat stroke (see Table 1).^34,95
    • During transport and when immersion is not feasible, other methods can be used to reduce body temperature: removing the clothing; sponging down the athlete with cool water and applying cold towels; applying ice bags to as much of the body as possible, especially the major vessels in the armpit, groin, and neck; providing shade; and fanning the body with air.^39,95
    • In addition to cooling therapies, first-aid emergency procedures for heat stroke may include airway management. Also a physician may decide to begin intravenous fluid replacement.⁸⁷
    • Monitor for organ-system complications for at least 24 hours.

23. Exertional hyponatremia:

    • Attempt to differentiate between hyponatremia and heat exhaustion. Hyponatremia is characterized by increasing headache, significant mental compromise, altered consciousness, seizures, lethargy, and swelling in the extremities. The athlete may be dehydrated, normally hydrated, or overhydrated.¹⁹
    • Attempt to differentiate between hyponatremia and heat stroke. In hyponatremia, hyperthermia is likely to be less (rectal temperature less than 40°C [104°F]).¹⁹ The plasma-sodium level is less than 130 mEq/L and can be measured with a sodium analyzer on site if the device is available.
    • If hyponatremia is suspected, immediate transfer to an emergency medical center via the emergency medical system is indicated. An intravenous line should be placed to administer medication as needed to increase sodium levels, induce diuresis, and control seizures.
    • An athlete with suspected hyponatremia should not be administered fluids until a physician is consulted.

24. Return to activity

Table 2. Prevention Checklist for the Certified Athletic Trainer*

Table 3. Wet-Bulb Globe Temperature Risk Chart^62–67*

Figure 1.

Risk of heat exhaustion or heat stroke while racing in hot environments. However, Figure 2 may be better suited for estimating heat-stroke risk when equipment is worn. Reprinted with permission from Convertino VA, Armstrong LE, Coyle EF, et al. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc. 1996;28:i–vii.³¹

Figure 2.

Heat stress risk temperature and humidity graph. Heat-stroke risk rises with increasing heat and relative humidity. Fluid breaks should be scheduled for all practices and scheduled more frequently as the heat stress rises. Add 5° to temperature between 10 AM and 4 PM from mid May to mid September on bright, sunny days. Practices should be modified for the safety of the athletes to reflect the heat-stress conditions. Regular practices with full practice gear can be conducted for conditions that plot to the left of the triangles. Cancel all practices when the temperature and relative humidity plot is to the right of the circles; practices may be moved into air-conditioned spaces or held as walk-through sessions with no conditioning activities.

Conditions that plot between squares and circles: increase rest-to-work ratio with 5- to 10-minute rest and fluid breaks every 15 to 20 minutes; practice should be in shorts only with all protective equipment removed.

Conditions that plot between triangles and squares: increase rest-to-work ratio with 5- to 10-minute rest and fluid breaks every 20 to 30 minutes; practice should be in shorts with helmets and shoulder pads (not full equipment).

Adapted with permission from Kulka J, Kenney WL. Heat balance limits in football uniforms: how different uniform ensembles alter the equation. Physician Sportsmed. 2002;30(7):29–39.⁶⁸

In cases of exercise-associated muscle (heat) cramps or heat syncope, the ATC should discuss the athlete's case with the supervising physician. The cases of athletes with heat exhaustion who were not transferred to the physician's care should also be discussed with the physician. After exertional heat stroke or exertional hyponatremia, the athlete must be cleared by a physician before returning to athletic participation.⁹² The return to full activity should be gradual and monitored.^8,87

BACKGROUND AND LITERATURE REVIEW

Diagnosis

To differentiate heat illnesses in athletes, ATCs and other on-site health care providers must be familiar with the signs and symptoms of each condition (see Table 1). Other medical conditions (eg, asthma, status epilepticus, drug toxicities) may also present with similar signs and symptoms. It is important to realize, however, that an athlete with a heat illness will not exhibit all the signs and symptoms of a specific condition, increasing the need for diligent observation during athletic activity.

Nonenvironmental Risk Factors

Athletic trainers and other health care providers should be sensitive to the following nonenvironmental risk factors, which could place athletes at risk for heat illness.

Dehydration Sweating, inadequate fluid intake, vomiting, diarrhea, certain medications,^89,101–103 and alcohol^104,105 or caffeine¹⁰⁶ use can lead to fluid deficit. Body-weight change is the preferred method to monitor for dehydration in the field, but a clinical refractometer is another accurate method (specific gravity should be no more than 1.020).^{34,49,107–110} Dehydration can also be identified by monitoring urine color or body-weight changes before, during, and after a practice or an event and across successive days.^53,54

The signs and symptoms of dehydration are thirst, general discomfort, flushed skin, weariness, cramps, apathy, dizziness, headache, vomiting, nausea, heat sensations on the head or neck, chills, decreased performance, and dyspnea.⁵² Water loss that is not regained by the next practice increases the risk for heat illness.¹¹⁰

Barriers to Evaporation Athletic equipment and rubber or plastic suits used for “weight loss” do not allow water vapor to pass through and inhibit evaporative, convective, and radiant heat loss.^111,112 Participants who wear equipment that does not allow for heat dissipation are at an increased risk for heat illness.¹¹³ Helmets are also limiting because a significant amount of heat is dissipated through the head.

Illness Athletes who are currently or were recently ill may be at an increased risk for heat illness because of fever or dehydration.^114–116

History of Heat Illness Some individuals with a history of heat illness are at greater risk for recurrent heat illness.^8,117

Increased Body Mass Index (Thick Fat Layer or Small Surface Area) Obese individuals are at an increased risk for heat illness because the fat layer decreases heat loss.¹¹⁸ Obese persons are less efficient and have a greater metabolic heat production during exercise. Conversely, muscle-bound individuals have increased metabolic heat production and a lower ratio of surface area to mass, contributing to a decreased ability to dissipate heat.^119–121

Wet-Bulb Globe Temperature on Previous Day and Night When the WBGT is high to extreme (see Table 3), the risk of heat-related problems is greater the next day; this appears to be one of the best predictors of heat illness.¹²¹ Athletes who sleep in cool or air-conditioned quarters are at less risk.

Poor Physical Condition Individuals who are untrained are more susceptible to heat illness than are trained athletes. As the O₂max of an individual improves, the ability to withstand heat stress improves independent of acclimatization and heat adaptation.¹²² High-intensity work can easily produce 1000 kcal/h and elevate the core temperature of at-risk individuals (those who are unfit, overweight, or unacclimatized) to dangerous levels within 20 to 30 minutes.¹²³

Excessive or Dark-Colored Clothing or Equipment Excessive clothing or equipment decreases the ability to thermoregulate, and dark-colored clothing or equipment may cause a greater absorption of heat from the environment. Both should be avoided.¹¹³

Overzealousness Overzealous athletes are at a higher risk for heat illness because they override the normal behavioral adaptations to heat and decrease the likelihood of subtle cues being recognized.

Lack of Acclimatization to Heat An athlete with no or minimal physiologic acclimatization to hot conditions is at an increased risk of heat-related illness.^8,37,83,124

Medications and Drugs Athletes who take certain medications or drugs, particularly medications with a dehydrating effect, are at an increased risk for a heat illness.^{101–106,125–136} Alcohol, caffeine, and theophylline at certain doses are mild diuretics.^106,137,138 Caffeine is found in coffee, tea, soft drinks, chocolate, and several over-the-counter and prescription medications.¹³⁹ Theophylline is found mostly in tea and anti-asthma medications.¹⁴⁰

Electrolyte Imbalance Electrolyte imbalances do not usually occur in trained, acclimatized individuals who engage in physical activity and eat a normal diet.¹⁴¹ Most sodium and chloride losses in athletes occur through the urine, but athletes who sweat heavily, are salty sweaters, or are not heat acclimatized can lose significant amounts of sodium during activity.¹⁴² Electrolyte imbalances often contribute to heat illness in older athletes who use diuretics.^143,144

Predisposing Medical Conditions

The following predisposing medical conditions add to the risk of heat illness.

Malignant Hyperthermia Malignant hyperthermia is caused by an autosomal dominant trait that causes muscle rigidity, resulting in elevation of body temperature due to the accelerated metabolic rate in the skeletal muscle.^145–147

Neuroleptic Malignant Syndrome Neuroleptic malignant syndrome is associated with the use of neuroleptic agents and antipsychotic drugs and an unexpected idiopathic increase in core temperature during exercise.^148–151

Arteriosclerotic Vascular Disease Arteriosclerotic vascular disease compromises cardiac output and blood flow through the vascular system by thickening the arterial walls.^115,152

Scleroderma Scleroderma is a skin disorder that decreases sweat production, thereby decreasing heat transfer.^149,153

Cystic Fibrosis Cystic fibrosis causes increased salt loss in sweat and can increase the risk for hyponatremia.^154,155

Sickle Cell Trait Sickle cell trait limits blood-flow distribution and decreases oxygen-carrying capacity. The condition is exacerbated by exercise at higher altitudes.^156,157

Environmental Risk Factors

When the environmental temperature is above skin temperature, athletes begin to absorb heat from the environment and depend entirely on evaporation for heat loss.^113,158,159 High relative humidity inhibits heat loss from the body through evaporation.⁶¹

The environmental factors that influence the risk of heat illness include the ambient air temperature, relative humidity (amount of water vapor in the air), air motion, and the amount of radiant heat from the sun or other sources.^2,9,41 The relative risk of heat illness can be calculated using the WBGT equation.^{2,43,50,69,77,160,161} Using the WBGT index to modify activity in high-risk settings has virtually eliminated heat-stroke deaths in United States Marine Corps recruits.¹⁵⁹ Wet-bulb globe temperature is calculated using the wet-bulb (wb), dry-bulb (db), and black-globe (bg) temperature with the following equation^{49,62,85,162,163}:

When there is no radiant heat load, T_{db =} T_bg, and the equation is reduced⁶² to

This equation is used to estimate risk as outlined in Table 3.^{13,40,50,61,85} This index was determined for athletes wearing a T-shirt and light pants.¹⁵⁸ The WBGT calculation can be performed using information obtained from electronic devices⁴² or the local meteorologic service, but conversion tables for relative humidity and T_db are needed to calculate the wet-bulb temperature.^50,162 The predictive value from the meteorologic service is not as accurate as site-specific data for representing local heat load but will suffice in most situations. When WBGT measures are not possible, environmental heat stress can be estimated using a sling psychrometer (see Figures 1, 2).

Several recommendations have been published for distance running, but these can also be applied to other continuous activity sports. The Canadian Track and Field Association recommended that a distance race should be cancelled if the WBGT is greater than 26.7°C (80°F).³⁹ The American College of Sports Medicine guidelines from 1996 recommended that a race should be delayed or rescheduled when the WBGT is greater than 27.8°C (82°F).^31,72,73 In some instances, the event will go on regardless of the WBGT; ATCs should then have an increased level of suspicion for heat stroke and focus on hydration, emergency supplies, and detection of exertional heat illnesses.

Thermoregulation

Thermoregulation is a complex interaction among the central nervous system (CNS), the cardiovascular system, and the skin to maintain a body-core temperature of 37°C.^9,43,51,164 The CNS temperature-regulation center is located in the hypothalamus and is the site where the core temperature setpoint is determined.^{9,43,82,158,164–166} The hypothalamus receives information regarding body-core and shell temperatures from peripheral skin receptors and the circulating blood; body-core temperature is regulated through an open-ended feedback loop similar to that in a home thermostat system.^{158,165,167,168} Body responses for heat regulation include cutaneous vasodilation, increased sweating, increased heart rate, and increased respiratory rate.^{38,43,51,164,165}

Body-core temperature is determined by metabolic heat production and the transfer of body heat to and from the surrounding environment using the following heat-production and heat-storage equation^166,167:

where S is the amount of stored heat, M is the metabolic heat production, R is the heat gained or lost by radiation, K is the conductive heat lost or gained, Cv is the convective heat lost or gained, and E is the evaporative heat lost.

Basal metabolic heat production fasting and at absolute rest is approximately 60 to 70 kcal/h for an average adult, with 50% of the heat produced by the internal organs. Metabolic heat produced by intense exercise may approach 1000 kcal/h,^51,164 with greater than 90% of the heat resulting from muscle metabolism.^9,40,42,166

Heat is gained or lost from the body by one or more of the following mechanisms^9,85:

Radiation The energy is transferred to or from an object or body via electromagnetic radiation from higher to lower energy surfaces.^{9,43,51,85,166}

Conduction Heat transfers from warmer to cooler objects through direct physical contact.^{9,43,51,85,166} Ice packs and cold-water baths are examples of conductive heat exchange.

Convection Heat transfers to or from the body to surrounding moving fluid (including air).^{9,43,51,85,166} Moving air from a fan, cycling, or windy day produces convective heat exchange.

Evaporation Heat transfers via the vaporization of sweat ^§ and is the most efficient means of heat loss.^51,158,169 The evaporation of sweat from the skin depends on the water saturation of the air and the velocity of the moving air.^170–172 The effectiveness of this evaporation for heat loss from the body diminishes rapidly when the relative humidity is greater than 60%.^9,20,164

Cognitive performance and associated CNS functions deteriorate when brain temperature rises. Signs and symptoms include dizziness, confusion, behavior changes, coordination difficulties, decreased physical performance, and collapse due to hyperthermia.^168,173 The residual effects of elevated brain temperature depend on the duration of the hyperthermia. Heat stroke rarely leads to permanent neurologic deficits⁵¹; however, some sporadic symptoms of frontal headache and sleep disturbances have been noted for up to 4 months.^168,174,175 When permanent CNS damage occurs, it is associated with cerebellar changes, including ataxia, marked dysarthria, and dysmetria.¹⁷⁴

Heat Acclimatization

Heat acclimatization is the physiologic response produced by repeated exposures to hot environments in which the capacity to withstand heat stress is improved.^{14,43,75,176,177} Physiologic responses to heat stress are summarized in Table 4. Exercise heat exposure produces progressive changes in thermoregulation that involve sweating, skin circulation, thermoregulatory setpoint, cardiovascular alterations, and endocrine adjustments.^29,43,178 Individual differences affect the onset and decay of acclimatization.^29,45,179 The rate of acclimatization is related to aerobic conditioning and fitness; more conditioned athletes acclimatize more quickly.^43,45,180 The acclimatization process begins with heat exposure and is reasonably protective after 7 to 14 days, but maximum acclimatization may take 2 to 3 months.^45,181,182 Heat acclimatization diminishes by day 6 when heat stress is no longer present.^180,183 Fluid replacement improves the induction and effect of heat acclimatization.^184–187 Extra salt in the diet during the first few days of heat exposure also improves acclimatization; this can be accomplished by encouraging the athlete to eat salty foods and to use the salt shaker liberally during meals.

Table 4. Physiologic Responses After Heat Acclimatization Relative to Nonacclimatized State

Cumulative Dehydration

Cumulative dehydration develops insidiously over several days and is typically observed during the first few days of a season during practice sessions or in tournament competition. Cumulative dehydration can be detected by monitoring daily prepractice and postpractice weights. Even though a small decrease in body weight (less than 1%) may not have a detrimental effect on the individual, the cumulative effect of a 1% fluid loss per day occurring over several days will create an increased risk for heat illness and a decrease in performance.¹¹⁰

During intense exercise in the heat, sweat rates can be 1 to 2.5 L/h (about 1 to 2.25 kilograms [2 to 5 pounds] of body weight per hour) or more, resulting in dehydration. Unfortunately, the volume of fluid that most athletes drink voluntarily during exercise replaces only about 50% of body-fluid losses.¹⁸⁸ Ideally, rehydration involves drinking at a rate sufficient to replace all of the water lost through sweating and urination.^60,77 If the athlete is not able to drink at this rate, he or she should drink the maximum tolerated. Use caution to ensure that athletes do not overhydrate and put themselves at risk for the development of hyponatremia. However, hydration before an event is essential to help decrease the incidence of heat illnesses. For more information on this topic, see the “National Athletic Trainers' Association Position Statement: Fluid Replacement in Athletes.”⁵²

Cooling Therapies

The fastest way to decrease body-core temperature is immersion of the trunk and extremities into a pool or tub filled with cold water (between 1°C [35°F] and 15°C [59°F]).^39,88,91,97 Conditions that have been associated with immersion therapy include shivering and peripheral vasoconstriction; however, the potential for these should not deter the medical staff from using immersion therapy for rapid cooling. Shivering can be prevented if the athlete is removed from the water once rectal temperature reaches 38.3°C to 38.9°C (101°F to 102°F). Peripheral vasoconstriction may occur, but the powerful cooling potential of immersion outweighs any potential concerns. Cardiogenic shock has also been a proposed consequence of immersion therapy, but this connection has not been proven in cooling heat-stroke patients.³⁹ Cold-water immersion therapy was associated with a zero percent fatality rate in 252 cases of exertional heat stroke in the military.⁸⁹ Other forms of cooling (water spray; ice packs covering the body; ice packs on axillae, groin, and neck; or blowing air) decrease body-core temperature at a slower rate compared with cold-water immersion.⁹⁷ If immersion cooling is not being used, cooling with ice bags should be directed to as much of the body as possible, especially the major vessels in the armpit, groin, and neck regions (and likely the hands and feet), and cold towels may be applied to the head and trunk because these areas have been demonstrated on thermography^173,189 to have the most rapid heat loss.

SPECIAL CONCERNS

Most research related to heat illness has been performed on normal, healthy adults. Child athletes, older athletes, and athletes with spinal-cord injuries have been studied less frequently. The following are suggestions for special populations or those with special conditions.

Children (Prepubescents)

Exercise in hot environments and heat tolerance are affected by many physiologic factors in children. These include decreased sweat gland activity,¹⁹⁰ higher skin temperatures,^191–193 decreased cardiac output (increased heart rate and lower stroke volume) due to increased peripheral circulation,¹⁹⁴ decreased exercise economy,¹⁹⁵ decreased ability to acclimatize to heat (slower and takes longer),¹⁹² smaller body size (issues related to body surface-to-mass ratio), maturational differences,¹⁹⁰ and predisposing conditions (obesity, hypohydration, childhood illnesses, and other disease states).^190,192,196

• Decrease the intensity of activities that last longer than 30 minutes,¹⁹⁷ and have the athlete take brief rests⁵⁰ if the WBGT is between 22.8°C and 27.8°C (73°F and 82°F); cancel or modify the activity if the WBGT is greater than 27.8°C (82°F).^31,69–73 Modification could involve longer and more frequent rest breaks than are usually permitted within the rules of the sport (eg, insert a rest break before halftime).

• Encourage children to ingest some fluids at least every 15 to 30 minutes during activity to maintain hydration, even if they are not thirsty.¹⁹⁷

• Use similar precautions as listed earlier for adults.

Older Athletes (>50 Years Old)

The ability of the older athlete to adapt is partly a function of age and also depends on functional capacity and physiologic health status.^198–206

• The athlete should be evaluated by a physician before exercise, with the potential consequences of predisposing medical conditions and illnesses addressed.^9,34–36 An increase has been shown in the exercise heart rate of 1 beat per minute for each 1°C (1.8°F) increase in ambient temperature above neutral (23.9°C [75°F]).²⁰⁷ Athletes with known or suspected heart disease should curtail activities at lower temperatures than healthy athletes and should have cardiovascular stress testing before participating in hot environments.

• Older athletes have a decreased ability to maintain an adequate plasma volume and osmolality during exercise,^198,208 which may predispose them to dehydration. Regular fluid intake is critical to avoid hyperthermia.

Athletes with Spinal-Cord Injuries

As sport participation for athletes with spinal-cord injuries increases from beginner to elite levels, understanding the disability,^209,210 training methods, and causes of heat injury will help make competition safer.²¹¹ For example, the abilities to regulate heart rate, circulate the blood volume, produce sweat, and transfer heat to the surface vary with the level and severity of the spinal-cord lesion.^{208,212–218}

• Monitor these athletes closely for heat-related problems. One technique for determining hyperthermia is to feel the skin under the arms of the distressed athlete.²¹¹ Rectal temperature may not be as accurate for measuring core temperature as in other athletes due to decreased ability to regulate blood flow beneath the spinal-cord lesion.^218–220

• If the athlete is hyperthermic, provide more water, lighter clothing, or cooling of the trunk,^211,213 legs,²¹¹ and head.²¹³

HOSPITALIZATION AND RECOVERY

After an episode of heat stroke, the athlete may experience impaired thermoregulation, persistent CNS dysfunction,^221,222 hepatic insufficiency, and renal insufficiency.^39,223 For persons with exertional heat stroke and associated multisystem tissue damage, the rate of recovery is highly individualized, ranging up to more than 1 year.^8,86,221 In one study, 9 of 10 patients exhibited normal heat-acclimatization responses, thermoregulation, whole-body sodium and potassium balance, sweat-gland function, and blood values about 2 months after the heat stroke.⁸ Transient or persistent heat intolerance was found in a small percentage of patients.⁸³ For some athletes, a history of exertional heat stroke increases the chance of experiencing subsequent episodes.³⁹

An athlete who experiences heat stroke may have compromised heat tolerance and heat acclimatization after physician clearance.^35,224,225 Decreased heat tolerance may affect 15% to 20% of persons after a heat stroke-related collapse,^226,227 and in a few individuals, decreased heat tolerance has persisted up to 5 years.^35,224,228 Additional heat stress may reduce the athlete's ability to train and compete due to impaired cardiovascular and thermoregulatory responses.^{115,228–230}

After recovery from an episode of heat stroke or hyponatremia, an athlete's physical activity should be restricted^8,86 and the gradual return to sport individualized by his or her physician. The athlete should be monitored on a daily basis by the ATC during exercise.⁸⁶ During the return-to-exercise phase, an athlete may experience some detraining and deconditioning not directly related to the heat exposure.^8,86 Evaluate the athlete over time to determine whether there has been a complete recovery of exercise and heat tolerance.^8,86

CONCLUSIONS

Athletic trainers and other allied health providers must be able to differentiate exercise-associated muscle (heat) cramps, heat syncope, exercise (heat) exhaustion, exertional heat stroke, and exertional hyponatremia in athletes.

This position statement outlines the NATA's current recommendations to reduce the incidence, improve the recognition, and optimize treatment of heat illness in athletes. Education and increased awareness will help to reduce both the frequency and the severity of heat illness in athletes.