Page 32 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Hyperthermia, Heat Injuries
On severe physical effort (increased heat pro- ment and/or submerging them into cool water.
duction) and/or in a hot environment (de- However, the body surface must not be al-
creased net heat loss) the thermoregulatory lowed to get too cold, because the resulting va-
mechanisms of the organism are overtasked, soconstriction would delay the reduction in
especially when there is a lack of water and at core temperature. Even successfully treated
high ambient humidity. In contrast to the sit- heat stroke may leave lasting damage in the
uation in fever (→ p. 20), the body’s core tem- thermoregulatory centers. This restricts future
perature can no longer be kept at the (un- tolerance to extreme ambient temperatures.
changed) set level of ca. 378C and hyperther- Malignant hyperthermia (→ B) is the poten-
mia results (→ A, top). On standing upright, tially lethal result of heterogeneous genetic
heat-induced vasodilation causes some of the defects of sarcoplasmic Ca 2+ transport, in
2+
blood to pool in the legs, and the extracellular which the Ca -releasing channel (ryanodine
Temperature, Energy particularly because vasodilation in the skin depolarizing muscle relaxants (suxametho-
volume is reduced by sweating. As a result,
receptor) is affected. Some inhalation anes-
thetics (halothane, enflurane, isoflurane) and
cardiac output (CO) and blood pressure fall,
nium chloride) cause the sudden and excessive
reduces peripheral vascular resistance. Even
2+
from the sarcoplasmic reticu-
at a core temperature below 398C, weakness,
release of Ca
lum, so that generalized, uncoordinated mus-
dizziness, nausea, and loss of consciousness
tion and enormous heat production. The result
pressure (heat collapse; → A1). Blood pressure
is acidosis, hyperkalemia, tachycardia, ar-
will again rise on lying down and after taking
2 may occur as a consequence of reduced blood cle twitches occur with high oxygen consump-
fluids. rhythmia, and rapidly rising hyperthermia. If
A much greater danger arises when the core recognized in time, malignant hyperthermia
temperature reaches 40.58C, because the brain can be successfully treated by discontinuing
cannot tolerate such temperatures. To protect the anesthetics and/or muscle relaxants, ad-
itself against heat stroke the brain can tem- ministering dantrolene, which blocks Ca 2+ re-
porarily be kept cooler than the rest of the lease in skeletal muscle cells, as well as cooling
body because a rising core temperature causes the body.
profuse sweating of the head (even with dehy- Heat cramps occur with strenuous physical
dration), especially the face (→ A2). Blood that work in high ambient temperature (e.g., at a
has been cooled in this way reaches the endo- furnace) if only the loss of water, but not of
cranial venous system and the sinus caverno- salt, is replaced.
sus, where it lowers the temperature of the Sun stroke must be distinguished from hy-
neighboring arteries. This would seem to be perthermia. It is caused by direct sun radiation
the only explanation for the fact that a mara- on head and neck and causes nausea, dizzi-
thon runner in whom a transient rise in core ness, severe headache, cerebral hyperemia,
temperature to 41.98C had been measured did and serous meningitis and may end fatally.
not suffer from heat stroke. Contact or radiant heat may cause first de-
If there is a prolonged rise in core tempera- gree, second degree, or third degree burns
ture to between 40.5 and 438C, the thermoreg- (reddening, blisters, or necroses, respectively)
ulatory center in the midbrain fails (→ p. 20) to the skin. Frequent and intense exposure to
and sweating ceases. Disorientation, apathy, the sun also increases the risk of melanoma.
and loss of consciousness result (heat stroke).
Cerebral edema with accompanying damage to
the central nervous system will, without rapid
help, lead to death; children are especially at
risk because their surface area to body mass
ratio is larger than adults’, and they produce
22 less sweat. Treatment of heat stroke consists
of bringing the person into a cooler environ-
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
All rights reserved. Usage subject to terms and conditions of license.