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.