When there is no attempt to actively warm an anesthetized patient, core temperature usually decreases 1°C to 2°C during the first hour of general anesthesia (phase one), followed by a more gradual decline during the ensuing 3 to 4 h (phase two), eventually reaching a point of steady state.
In the normal, unanesthetized patient the hypothalamus maintains core body temperature within very narrow tolerances, termed the interthreshold range, with the threshold for sweating and vasodilation at one extreme and the threshold for vasoconstriction and shivering at the other.
Anesthetics inhibit central thermoregulation by interfering with these hypothalamic reflex responses.
Postoperative hypothermia should be treated with a forced-air warming device, if available; alternately (but less satisfactorily) warming lights or heating blankets can be used to restore body temperature to normal.
Nearly 50% of patients who experience an episode of malignant hyperthermia (MH) have had at least one previous uneventful exposure to anesthesia during which they received a recognized triggering agent. Why MH fails to occur after every exposure to a triggering agent is unclear.
The earliest signs of MH during anesthesia include muscle rigidity, tachycardia, unexplained hypercarbia, and increased temperature.
Susceptibility to MH may be increased in several musculoskeletal diseases. These include central-core disease, multi-minicore myopathy, and King–Denborough syndrome.
Treatment of an MH episode is directed at terminating the episode and treating complications such as hyperthermia and acidosis. The mortality rate for MH, even with prompt treatment, ranges from 5% to 30%. First and most importantly, the triggering agent must be stopped; second, dantrolene must be given immediately.
Dantrolene, a hydantoin derivative, directly interferes with muscle contraction by inhibiting calcium ion release from the sarcoplasmic reticulum. The dose is 2.5 mg/kg intravenously every 5 min until the episode is terminated (upper limit, 10 mg/kg). Dantrolene should be continued for 24 h after initial treatment.
Propofol, etomidate, benzodiazepines, ketamine, thiopental, methohexital, opiates, droperidol, nitrous oxide, nondepolarizing muscle relaxants, and all local anesthetics are safe for use in MH-susceptible patients.
THERMOREGULATION & HYPOTHERMIA
Anesthesia and surgery predispose patients to hypothermia, usually defined as a body temperature less than 36°C. Unintentional perioperative hypothermia is more common in patients at the extremes of age, and in those undergoing abdominal surgery or procedures of long duration, especially with cold ambient operating room temperatures; it will occur in nearly every such patient unless steps are taken to prevent this complication.
Hypothermia (in the absence of shivering) reduces metabolic oxygen requirements and can be protective during cerebral or cardiac ischemia. Nevertheless, hypothermia has multiple deleterious physiological effects (Table 52–1). In fact, unintended perioperative hypothermia has been associated with an increased mortality rate.
TABLE 52–1Deleterious effects of hypothermia. ||Download (.pdf) TABLE 52–1 Deleterious effects of hypothermia.
Cardiac arrhythmias and ischemia
Increased peripheral vascular resistance
“Left shift” of the hemoglobin–oxygen saturation curve
Reversible coagulopathy (platelet dysfunction)