In human beings, thermoregulatory mechanisms maintain a core body temperature of approximately 37°C, with a “normal” range of only 0.2°C. (circadian and menstrual influences also cause up to a 1.0°C variation of the core temperature). The body-temperature control system operates via an elegant feedback loop. Widely distributed, distinct heat and cold receptors send afferent thermal input via nerve fibers to the central nervous system and more specifically to the hypothalamus, which is the primary thermoregulatory control center. Hyperthermia results in efferent hypothalamic output that causes cutaneous vasodilation and sweating. Hypothermia generates hypothalamic outputs leading to vasoconstriction, shivering, and, in infants, nonshivering thermogenesis. Overall, the most important mechanism for maintaining normal human body temperature is behavior. The obvious inability of anesthetized patients to add clothing and blankets or adjust the thermostat removes their most important defense against hypothermia.

It has long been recognized that metabolic functions deteriorate when internal temperatures are abnormal and also that both regional and general anesthesia impair thermoregulatory mechanisms. This knowledge is reflected in the American Society of Anesthesiologists’ Standards for Basic Anesthetic Monitoring, which state that “during all anesthetics, the patient’s oxygenation, ventilation, circulation, and temperature should be continually evaluated” and that “every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected.”

Core-temperature monitoring sites include the tympanic membrane, pulmonary artery, nasopharynx, and the distal portion of the esophagus. Whether the bladder is a site permitting accurate measurement of core temperature depends upon urine flow. Core temperature may be measured in the bladder if urine flow is high; however, if urine flow is low, the measurement made may reflect peripheral temperature.

Evidence suggests that perioperative hypothermia contributes to increases in surgical-wound infections, intraoperative blood loss, transfusion requirements, myocardial ischemia, and arrhythmias. The cardiac risks are highlighted in perioperative guidelines published by the American College of Cardiology and American Heart Association. Hypothermia has also been shown to slow the metabolism of anesthetic drugs and to increase the duration of postanesthetic recovery. The postoperative sensation of cold and shivering is unpleasant for patients and is recalled by some as causing more discomfort than was caused by surgery.

Perioperative hypothermia is caused when a patient is in a relatively hypothermic environment while simultaneously experiencing a loss of thermoregulatory control.

The difference between the patient temperature and the operating room ambient temperature drives heat transfer from the patient to the surrounding environment. There are four modes of heat transfer: radiation, convection, evaporation, and conduction. All play some role in heat loss in surgical patients; the greatest amount occurs by radiation.