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Overview |
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Definition |
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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. |
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Etiology |
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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. |
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Risk Factors |
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- 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
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Signs and Symptoms |
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- 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
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Differential
Diagnosis |
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- 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
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Diagnosis |
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Physical Examination |
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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. |
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Laboratory Tests |
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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
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Pathology/Pathophysiology |
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- Efferent fibers of autonomic nervous system are activated to produce
cutaneous vasodilation and sweating. With heat exhaustion, these
thermoregulatory mechanisms become
overwhelmed
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Treatment Options |
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Treatment Strategy |
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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. |
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Drug Therapies |
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- 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
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Complementary and Alternative
Therapies |
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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). |
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Nutrition |
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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
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Herbs |
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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. |
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Homeopathy |
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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
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Patient Monitoring |
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- Assessment of volume status—determines
whether electrolyte replacement is continued; continue administration until
patient is hemodynamically stable
- Rehydration and complications may necessitate hospitalization
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Other
Considerations |
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Prevention |
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- 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
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Complications/Sequelae |
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- 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.
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Prognosis |
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Prognosis is good (24 to 48 hour recovery) if heat stroke is
avoided. |
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Pregnancy |
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Pregnancy may affect baseline temperature, and pregnant women may be more
vulnerable to fluid volume depletion with heat
stress. |
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References |
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Blumenthal M, Goldberg A, Brinkmann J, eds. Herbal Medicine: Expanded
Commission E Monographs. Newton, Mass: Integrative Medicine Communications;
2000:103-105, 419-423.
Cecil RI, Plum F, Bennett JC, eds. Cecil Textbook of Medicine. 20th
ed. Philadelphia, Pa: W.B. Saunders; 1996.
Centers for Disease Control and Prevention. Heat-related illnesses and
deaths—Missouri, 1998, and United States, 1997-1996.
JAMA. 1999;282(3):227-228.
Dambro MR, ed. Griffith's 5 Minute Clinical Consult. Baltimore, Md:
Lippincott Williams & Wilkins; 1999.
Duthie EH, Katz PR, Kersey R, eds. Practice of Geriatrics. 3rd ed.
Philadelphia, Pa: W.B. Saunders; 1998.
Eichner ER. Treatment of suspected heat illness. Int J Sports Med.
1998;19(suppl 2):S150-S153.
Fauci AS, Braunwald E, Isselbacher KJ, et al., eds. Harrison's Principles
of Internal Medicine. 14th ed. New York, NY: McGraw-Hill Book Co; 1998.
Fishbane S. Exercise-induced renal and electrolyte changes. Phys
Sportsmedicine. 1995;23(8):39-40, 42-46.
Furman JA, Assell C. Acute, exercise-induced compartment syndrome,
rhabdomyolysis, and renal failure:—a case report.
Nutr Clin Pract. 1999;14(6):296-298.
Lin MT, Ho, ML, Chandra A, Hsu HK. Serotoninergic mechanisms of the
hypothermia induced by Clerodenron fragrans (Ventenaceae) in the rat.
Am J Chin Med. 1981;9(2):144-154.
McCormick CC, Garlich JD. The interaction of phosphorus nutrition and fasting
on the survival time of young chickens acutely exposed to high temperature.
Poult Sci. 1982;61(2):331-336.
Rakel RE, ed. Conn's Current Therapy. 51st ed. Philadelphia, Pa: W.B.
Saunders Co; 1999.
Rosen P, Barkin R, eds. Emergency Medicine: Concepts and Clinical
Management. 4th ed. St. Louis, Mo: Mosby-Year Book; 1998.
Semenza JC, McCullough JE, Flanders WD, McGeehin MA, Lumpkin JR. Excess
hospital admissions during the July 1995 heat wave in Chicago. Am J Prev
Med. 1999;16(4):269-277.
Simon HB. Hyperthermia. N Eng J Med.
1993;329(7):483-487. |
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Copyright © 2000 Integrative Medicine
Communications This publication contains
information relating to general principles
of medical care that should not in any event be construed as specific
instructions for individual patients. The publisher does not accept any
responsibility for the accuracy of the information or the consequences arising
from the application, use, or misuse of any of the information contained herein,
including any injury and/or damage to any person or property as a matter of
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The reader is advised to check product information (including package inserts)
for changes and new information regarding dosage, precautions, warnings,
interactions, and contraindications before administering any drug, herb, or
supplement discussed herein. | |