What are the physiologic responses to hypothermia?
Vasoconstriction occurs secondary to
sympathetic stimulation. Volatile agents reduce the threshold for
vasocon-striction by 2–4°C. It is about 1°C lower in patients 60–80 years old than in
patients 30–50 years old. Vasoconstriction is more or less an “on-off ”
response.
The threshold for shivering is decreased by
general anesthetics even more than that for vasoconstriction. Two types of
shivering are described. One type is a tonic pat-tern, resembling normal
shivering, with a 4–8 cycles per minute waxing-and-waning component. The second
is a phasic, 5–7 Hz bursting pattern resembling clonus, which is specific to
the postanesthesia care unit (PACU). This lat-ter type of shivering is
secondary to volatile anesthetics and probably results from anesthetic-induced
disinhibition of normal descending control over spinal reflexes. Shivering can
increase the oxygen consumption by 300–500%, thus leading to myocardial
ischemia in susceptible patients. It can also increase the serum potassium
level.
Nonshivering thermogenesis can double the
metabolic heat production in infants but it plays an unimportant role in
adults. It is inhibited by general anesthesia.
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