Perioperative Thermoregulation
Daniel I. Sesler, M.D.
Thermoregulatory control of body temperature is characterized by thresholds (triggering core temperature), gains (incremental response), and maximum response intensity. General anesthetics slightly increase the sweating threshold, but markedly and synchronously reduce the vasoconstriction and shivering thresholds. Consequently, the interthreshold range (temperatures not triggering thermoregulatory defenses) increases by a factor of ten to twenty.
Impaired central thermoregulation results in core hypothermia during surgery. Interestingly, the major cause of hypothermia is not heat loss to the environment, but core-to-peripheral redistribution of body heat. In patients who become sufficiently hypothermic, re-emergence of thermoregulatory vasoconstriction limits further hypothermia. Core temperature in unwarmed surgical patients typically decreases 1‑1.5_C, although larger decreases are common during large operations.
Neuraxial (spinal and epidural) anesthesia also impairs central thermoregulatory control via mechanisms that remain unclear. Regional anesthesia also causes a sympathectomy that prevents lower-body vasoconstriction and paralysis that prevents lower-body shivering. Consequently, hypothermia during neuraxial anesthesia is as common, and nearly as serious, as during general anesthesia.
Even mild hypothermia causes a surprising number of severe complications. For example, just 1.5_C of core hypothermia triples the risk of morbid myocardial outcomes. Hypothermia similarly triples the risk of surgical wound infection and prolongs the duration of hospitalization 20%. Hypothermia also impairs normal coagulation, and therefore increases surgical blood loss. Other complications caused by mild perioperative hypothermia include impaired drug metabolism, prolonged post-anesthetic recovery, protein wasting, and thermal discomfort and shivering.
Although fever is common in before and after surgery, it is rare during general anesthesia. The reason is that volatile anesthetics profoundly inhibit fever. Opioids also inhibit fever, although to a lesser extent. Fever, by definition, is an actively maintained hyperthermia. It is by far the most common thermal disturbance and is frequently treated with surface cooling. But as one might expect, surface cooling fails to reduce core temperature — but does provoke shivering, hypertension, and intense thermal discomfort. Other approaches, such as pharmacologic inhibition of thermoregulatory control, are thus preferable.
Time of Presentation
0830
