Temperature Monitoring and Perioperative

Most clinically available thermometers accurately report the temperature of whatever tissue is being measured. The difficulty is that no reliably core-temperature-measuring sites are completely noninvasive and easy to use—especially in patients not undergoing general anesthesia. Nonetheless, temperature can be reliably measured in most patients. Body temperature should be measured in patients undergoing general anesthesia exceeding 30 min in duration and in patients undergoing major operations during neuraxial anesthesia. Core body temperature is normally tightly regulated. All general anesthetics produce a profound dose-dependent reduction in the core temperature, triggering cold defenses, including arteriovenous shunt vasoconstriction and shivering. Anesthetic-induced impairment of normal thermoregulatory control, with the resulting core-toperipheral redistribution of body heat, is the primary cause of hypothermia in most patients. Neuraxial anesthesia also impairs thermoregulatory control, although to a lesser extent than does general anesthesia. Prolonged epidural analgesia is associated with hyperthermia whose cause remains unknown. IN previous articles, I have reviewed heat balance in surgical patients, 1 complications associated with perioperative thermal perturbations, 2 and the etiology and treatments of postoperative shivering. 3 Heier and Caldwell 4 have reviewed the effects of hypothermia on the response to neuromuscular blocking drugs. Furthermore, an entire book is devoted to the emerging field of therapeutic hypothermia. 5 In this article, I will belatedly review temperature monitoring and the effects of general and regional anesthesia on thermoregulatory control. Surgery typically involves exposure to a cold environment, administration of unwarmed intravenous fluids, and evaporation from within surgical incisions. However, these factors alone would not usually cause hypothermia; instead, thermoregulatory defenses would normally maintain core temperature in the face of comparable environmental stress. That hypothermia is typical in unwarmed surgical patients reflects a failure of effective thermoregulatory defenses. Understanding the effects of anesthetics on normal thermoregulatory control is thus the key to perioperative thermal perturbations because ineffective thermoregulation—much more than cold exposure—underlies most temperature changes observed in surgical patients. I will first briefly review temperature monitoring and normal thermoregulation, and then discuss the effects of general and neuraxial anesthesia on temperature control.