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Heat disorders: Mild to
grave reactions to high environmental temperature due to inadequate or
inappropriate responses of heat-regulating mechanisms.
Exposure to high ambient
temperature without efficient heat loss may lead to heat cramps,
heatstroke, or heat exhaustion. Acute (eg, 3 to 4 h of strenuous effort)
or prolonged (10 to 12 days) exposure to heat with excessive sweating
uncompensated by fluid intake leads to dehydration, sodium and potassium
depletion, and hypovolemia. Associated vomiting and diarrhea contribute
to fluid loss. Evaporation, the most important source of heat loss,
depends on relative humidity: The higher the humidity, the less
efficient the heat loss. Therefore, high ambient humidity (which
decreases the cooling effect of sweating) and prolonged strenuous
exertion (which increases heat production by muscle) increase the risk
of developing heat disorders. Age, obesity, chronic alcoholism,
debility, and many drugs (eg, anticholinergics, antihistamines,
phenothiazines, numerous psychoactive drugs, alcohol, cocaine) increase
susceptibility to heat disorders, particularly heatstroke. Heatstroke
and heat exhaustion both occur in hot, humid environments, but they are
markedly different disorders.
Prophylaxis
- Prevention
Using common sense is
best. Strenuous exertion in a very hot environment or in an inadequately
ventilated space should be avoided, and heavy insulating clothing should
not be worn. If exertion in a hot environment is unavoidable, fluid and
electrolytes (often lost imperceptibly in very hot, very dry air) should
be replaced by frequently drinking fluids slightly salty to taste (ie,
near isotonic), and evaporation, which helps keep the skin cool, should
be facilitated by wearing open mesh clothing or using fans. Thirst is a
poor indicator of dehydration. During strenuous exercise, fluids should
be drunk every hour regardless of thirst. Salt tablets, which are less
desirable than lightly salted beverages and foods, should not be taken
unless large amounts of fluids are consumed. Depletion of potassium,
magnesium, and calcium is hazardous only when heat exposure is
prolonged.
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Type of
Heat Disorders: |
| A.
Heatstroke - Sunstroke;
Thermic Fever; Siriasis |
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Inadequacy or failure of
heat loss mechanisms resulting in dangerous hyperpyrexia.
Symptoms
and Signs
An abrupt onset is
sometimes preceded by headache, vertigo, and fatigue. Sweating is
usually decreased, and the skin is hot, flushed, and usually dry. The
pulse rate increases rapidly and may reach 160 to 180 beats/min;
respirations usually increase, but BP is seldom affected. Disorientation
may briefly precede unconsciousness or convulsions. The temperature
climbs rapidly to 40 to 41° C (104 to 106° F), causing a feeling of
burning up. Circulatory collapse may precede death; after hours of
extreme hyperpyrexia, survivors are likely to have permanent brain
damage.
Diagnosis
and Prognosis
Sudden development of
hot, dry, flushed skin with a body temperature > 40° C (> 104°
F), a rapid pulse, and confusion or unconsciousness in a person exposed
to a hot environment are usually enough to distinguish heatstroke from
food, chemical, or drug poisoning. An acute infection (eg, septicemia,
meningitis) and toxic shock must be excluded. Drugs (see above) that may
have precipitated the episode should be considered.
Heatstroke is a
life-threatening emergency and, unless promptly and energetically
treated, results in convulsions and death, permanent brain damage, or
renal failure. A core temperature of 41° C (106° F) is a grave
prognostic sign; a temperature even slightly higher is often fatal. Old
age, debility, and alcoholism worsen the prognosis.
Treatment
Heroic measures should be
instituted immediately. If far from a hospital, the patient should be
removed from the heat, wrapped in wet bedding or clothing, and fanned
vigorously to increase evaporative heat loss. Immersion in a lake or
stream, or even cooling with snow or ice, while waiting for
transportation is a good alternative. If the patient starts to shiver,
the cooling process should be slowed, because shivering increases core
temperature. Warning: The temperature should be taken every 10 min and
not allowed to fall below 38.3° C (101° F) to avoid a continued fall
resulting in hypothermia. In the hospital, the core temperature is
monitored continuously to make sure it is stable. Stimulants and
sedatives, including morphine, are avoided; diazepam or a barbiturate
may be given IV if convulsions cannot be controlled otherwise. Frequent
electrolyte determinations should guide IV therapy. After severe
heatstroke, bed rest for a few days is desirable, and temperature may be
labile for weeks.
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| B.
Heat Exhaustion |
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Excessive fluid and
electrolyte loss due to sweating, resulting in hypovolemia and
electrolyte imbalance.
Symptoms
and Signs
Excessive sweating
without concomitant fluid replacement causes heat exhaustion with
increasing fatigue, weakness, and anxiety. Circulatory collapse ensues,
with a slow, thready pulse; low or imperceptible BP; cold, pale, clammy
skin; and disordered mentation followed by a shocklike unconsciousness.
Core temperature ranges from 38.3 to 40.6° C (101 to 105° F). Mild
heat exhaustion, precipitated by prolonged standing in a hot environment
(because blood pools in heat-dilated vessels in the legs), is manifested
by a subnormal body temperature and simple syncope.
Diagnosis,
Prognosis, and Treatment
Heat exhaustion causing
circulatory collapse is more difficult to differentiate from insulin
shock, poisoning, hemorrhage, or traumatic shock than is heatstroke.
Usually, a history of heat exposure, lack of hydration, absence of
another apparent cause, and response to treatment are sufficient for the
diagnosis. Heat exhaustion is usually transient, and the prognosis is
good unless circulatory failure is prolonged.
Normal blood volume must
be restored, and adequate brain perfusion ensured. The patient should be
placed flat or with the head down. Small amounts of cool, slightly salty
fluids or sport electrolyte beverages should be given orally every few
minutes. Isotonic saline IV, cardiac stimulants, and plasma volume
expanders (albumin, dextran) are seldom needed; if used, they are given
cautiously to avoid volume overload.
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| C.
Heat
Cramps |
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Exercise-induced cramps
of striated muscle resulting from excessive fluid intake without sodium
replacement.
Heat cramps occur after
exercise at high ambient temperatures (> 38° C [> 100° F]) when
fluids lost through excessive sweating are replaced only by water. The
result is a relative loss of sodium and occasionally potassium and
magnesium. Heat cramps are common in manual laborers (eg, engine room
personnel, steel workers, miners), in mountaineers or skiers overdressed
for the cold, in tennis players and other weekend athletes, and in
persons not acclimatized to hot, dry climates in which excessive
sweating is almost undetected because of rapid evaporation.
Symptoms
and Signs
Onset is often abrupt,
with muscles of the extremities affected first. Severe pain and
carpopedal spasm may incapacitate the hands and feet. Often episodic,
the cramping makes muscles feel like hard knots. When the cramps affect
only abdominal muscles, the pain may simulate an acute abdomen. Vital
signs are usually normal. The skin may be hot and dry or clammy and
cool, depending on the humidity.
Prophylaxis
and Treatment
Awareness of heat cramps
is usually sufficient to prevent them. In most instances, heat cramps
are prevented and rapidly relieved by drinking fluids or eating foods
containing sodium chloride. Sodium chloride tablets are often used for
prophylaxis but can cause stomach irritation, and overdose may lead to
edema. Use of these tablets should be discouraged. If the patient cannot
eat or drink, 0.9% sodium chloride IV may be necessary.
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[Merck Manual] |
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