KEY POINTS
- The normal interthreshold range is only a few tenths of a degree centigrade.
- General anesthetics slightly increase the sweating threshold, but profoundly
and synchronously reduce the vasoconstriction and shivering thresholds.
- Hypothermia during anesthesia develops in three phases: (1) a rapid decrease
in core temperature during the first hour resulting from redistribution of body heat
from the core to peripheral thermal compartments; (2) a slower, linear decrease in
core temperature from heat loss exceeding heat production; and (3) a core temperature
plateau that results from thermal equilibrium or re-emergence of thermoregulatory
vasoconstriction.
- Neuraxial anesthesia inhibits behavioral and autonomic thermoregulatory
control; consequently, hypothermia is nearly as common and severe during spinal or
epidural anesthesia as during general anesthesia.
- The three most important complications associated with mild hypothermia
are a threefold increase in morbid myocardial events, a threefold increase in the
risk of surgical wound infection and prolonged hospitalization, and increased blood
loss and transfusion requirements.
- Airway heating and humidification and circulating-water mattresses are
nearly ineffective. Intravenous fluid warming should be used when large volumes
of fluid are being given. However, fluid warming alone will not prevent hypothermia.
Forced-air heating is the most effective, commonly available, inexpensive, noninvasive
warming method.
- Any increase in body temperature is hyperthermia; fever is a regulated
hyperthermia that is mediated by endogenous pyrogens. Anesthetics and opioids inhibit
fever.
- Core temperature-monitoring sites include the pulmonary artery, tympanic
membrane (with a thermocouple), distal end of the esophagus, and nasopharynx. "Intermediate"
sites that can be used in most patients include the axilla, mouth, and bladder.
Rectal and skin temperatures may fail to track core temperature during malignant
hyperthermia crises. Infrared aural canal thermometers are insufficiently accurate
for clinical use.
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