Heat exhaustion is an elevation of the body's core temperature without an elevation of the brain's hypothalamic set point, as with fever, and occurs as a result of the body's inability to sufficiently dissipate heat. Under normal conditions, the hypothalamus (or more specifically, the preoptic nucleus of the anterior hypothalamus) controls the body's temperature by a process of heat loss and maintains thermal homeostasis. Heat is dissipated primarily through the skin; however, the lungs also contribute to heat loss. In heat exhaustion, heat generation exceeds heat loss. The body's metabolic rate produces more heat than is needed to maintain its core temperature and thermoregulatory mechanisms are overwhelmed.

Precautions are key to prevention of heat exhaustion. An annual average of 381 deaths occurred in the United States between 1979 and 1996 as a result of extreme temperatures. Approximately half of these deaths occurred among people age 65 or older.

Heat exhaustion is generally characterized by hyperthermia accompanied by water depletion, salt depletion, or both. Most frequently, it is caused by water depletion, usually from inadequate intake of fluids. Salt-depletion heat exhaustion may ensue when large volumes of sweat are replaced with fluids that contain too little salt. Thermoregulatory mechanisms may be overwhelmed by excessive metabolic production, environmental heat, or impaired heat dissipation. Please see section entitled Nutrition for more information regarding the consequences of inadequate fluid intake.

• Dehydration 
• Elderly 
• Children <5 years of age 
• Chronically disabled or ill persons 
• Athletes, military personnel, or people who labor outdoors in hot climates 
• Obesity
• Cardiovascular disease 
• Respiratory disease 
• Alcohol consumption 
• Physical exertion in hot environments 
• Medications interfering with the body's heat regulatory system—antipsychotics, tranquilizers, medications with anticholinergic effects (e.g., antihistamines, tricyclic antidepressants) 
• Social isolation 
• Lack of acclimatization to a warm or humid environment

• Malaise, fatigue 
• Headache 
• Core temperature may be elevated, but less than 104°F (40°C); > 104°F indicates heat stroke 
• Clinical dehydration such as orthostatic hypotension and tachycardia 
• Mental function intact, but impaired judgment possible 
• Vertigo, syncope 
• Nausea, vomiting 
• Muscle cramps 
• No coma or seizure (if present, may indicate that heat exhaustion has progressed to heat stroke) 
• Diaphoresis—may be excessive or may be surprisingly absent 

• Heat stroke (core temperature in excess of 104°F [40°C]) 
• Fever 
• Heat cramps (which do not include systemic symptoms) 
• Drug-induced hyperthermia (e.g., amphetamines, monoamine oxidase inhibitors, cocaine, phencyclidine, tricyclic antidepressants) 
• Serotonin syndrome 
• Neuroleptic malignant syndrome 
• Endocrinopathy (e.g., thyrotoxicosis, pheochromocytoma) 
• Hypothalamic disorders 

A rapid check of blood pressure, pulse, and orthostatic changes, and a history of recent urinary output, help assess the level of dehydration. Rectal core temperature is typically less than 104°F (40°C). Any signs of mental deterioration are indicative of progression to heat stroke, and emergency measures must be taken.

These tests not only aid diagnosis, but also are crucial to guiding electrolyte and fluid replacement.

• Serum electrolyte measurement—to assess volume depletion 
• Blood urea nitrogen (BUN) and creatinine—to assess volume depletion 
• Hematocrit—to assess volume depletion 
• Urinary electrolytes—to assess volume depletion 
• Hepatic transaminases—to differentiate from heat stroke 

• Efferent fibers of autonomic nervous system are activated to produce cutaneous vasodilation and sweating. With heat exhaustion, these thermoregulatory mechanisms become overwhelmed 

A patient with clear signs of heat exhaustion and an unclear diagnosis for heat stroke should be treated for heat stroke. If cooling is delayed, mortality rates increase. Primary measures for heat exhaustion are rest in a cool environment and rehydration.

• 0.1% oral saline electrolyte solution—for mild heat exhaustion
• 0.9% or 0.45% intravenous saline electrolyte solution—for severe heat exhaustion with volume depletion or electrolyte imbalances
• 5% dextrose solution in first liter of intravenous solution, if needed

The treatment of heat exhaustion relies largely on prevention. The Centers for Disease Control and Prevention recommend that long-term prevention efforts for heat exhaustion include regular physician-approved exercise (Semenza et al. 1999). There is some suggestion that underconditioned athletes participating in prolonged, strenuous exercise on hot days are at increased risk for developing heat exhaustion and acute renal failure. Regular exercise and heat acclimation may increase the athlete's ability to tolerate cardiovascular stress and adapt to decreased plasma volume (Fishbane 1995).

Fluid Replacement:

As illustrated by case reports, adequate hydration during exercise prevents heat exhaustion and its complications (Furman and Assell 1999). Dehydration can augment the deleterious effects of exercise-induced metabolic stress and cause the following (Fishbane 1995):

• Impairment of the kidneys' ability to maintain fluid volume and electrolyte balance
• Decreased plasma volume
• Imbalanced electrolytes
• Accumulation of potassium
• Lactic acidosis 

Replacing fluid volume throughout the period of exercise lowers core temperature (Fishbane 1995). Fluid replacement should start before exposure to heat or strenuous exercise and continue throughout the periods of exposure and cool-down. The sensation of thirst is not a reliable indicator of the need for fluids. Alcohol and caffeine should be avoided due to their dehydrating effects.

Phosphorous and other Minerals:

In addition to fluid replacement, an animal study of young chicks with induced heat exhaustion showed that phosphorus improved heat tolerance (McCormick and Garlich 1982). While this result cannot be generalized to humans, it may be a prudent preventive step to include electrolytes in fluid replacement products for people at risk of heat exhaustion. Some clinicians also suggest that mineral supplementation may be valuable for endurance athletes including:

• Calcium
• Magnesium
• Potassium

Foods high in these nutrients include:

• Dark leafy greens
• Nuts
• Seeds
• Whole grains
• Sea vegetables
• Blackstrap molasses
• Bananas

.

Herbs traditionally used as antipyretics include (Blumenthal et al. 2000):

• Elder flower (Sambucus nigra)
• Yarrow (Achillea millefolium)

The use of these herbs as an adjunctive treatment for heat exhaustion would be useful to examine in future scientific studies.

• Cayenne Pepper (Capsicum spp): It is also interesting to note that capsaicin, an active constituent found in the herb Capsicum, may lower body temperature by stimulating sweat glands. Many cultures, particularly those in hot climates, incorporate red pepper into their cuisine. 

• Chou-mou-li (Clerodenron fragrans) has long been used in traditional Chinese medicine for the treatment of heat-related conditions. An animal study investigating the thermoregulatory effects of this formula demonstrated hypothermic changes in cold to moderate temperatures but not at warm temperatures. According to the authors, Clerodenron fragrans appears to decrease metabolic heat production by decreasing metabolism and increasing cutaneous circulation via vasodilation. Although it is not entirely clear from the report, it seems possible that Clerodenron fragrans may be used adjunctively for heat exhaustion once treatment, including exposure to cold temperatures, has begun (Lin et al. 1981).

Two other herbs with similar traditional use induce reduction in temperature by stimulating peripheral vasodilation (Lin et al. 1981):

• Euchresta formosana (Subprostrata)
• Scutellaria baicalensis (Chinese Skullcap)

Until further study, it remains unclear if any of these substances add benefit over and above usual care for heat exhaustion.

The use of homeopathic remedies has yet to be validated through scientific evaluation in the treatment of heat exhaustion. Some common remedies used for overheated conditions include:

• Belladonna (Deadly nightshade) - often used for fever, particularly if patient is flushed with bright red skin and dulled mentation, even stuporous; although mouth and skin are dry, the patient for whom this is appropriate does not usually feel thirsty 

• Glonoine (Nitroglycerin) – used for fever if flushed and sweaty; patient for whom this is considered appropriate may complain of hot face but cold extremities; headache and confusion may be present in appropriate patient who may also be irritable 

• Assessment of volume status—determines whether electrolyte replacement is continued; continue administration until patient is hemodynamically stable
• Rehydration and complications may necessitate hospitalization

• Increase fluid intake
• Air-conditioned environments (including malls and public libraries for those without home air-conditioning units)
• Avoid alcohol
• Exercise or work outdoors during cooler parts of the day only
• Cool baths
• Check on persons vulnerable to heat exhaustion
• Wear loose, lightweight clothing
• The American College of Sports Medicine recommends canceling competitive events when the wet bulb-globe thermometer (WBGT) index is at 84 °F (28°C). Others urge caution when above 77°F (25°C).
• Athletes should consume 500 ml of water before event and 200 to 300 ml at regular intervals; consider salt and mineral supplements for endurance ahletes
• Patient education—for prevention and to learn to recognize early symptoms

• Free water deficits must be replaced slowly enough to not increase serum osmolality more than 2 mOsm/hour, or cerebral edema may result in seizure.
• When temperatures reach ³90°F (32.2° C) and humidity is above 35%, fans can actually contribute to heat stress by increasing the movement of hot air, much like a convection oven.
• Heat stroke and all of the potential complications including seizures, rhabdomyolysis, renal failure, and death.

Prognosis is good (24 to 48 hour recovery) if heat stroke is avoided.

Pregnancy may affect baseline temperature, and pregnant women may be more vulnerable to fluid volume depletion with heat stress.

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