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Perioperative hypothermia has been associated with an increase in morbidity and mortality. Central blood temperature, also known as core body temperature, ranges on average from 36 to 37°C. Throughout the day, due to the circadian cycle, core body temperature typically varies by 1°C, with a peak arising in the mid-afternoon and the nadir ensuing in the early morning.
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General hypothermia (which can be categorized into mild, moderate, or severe) is defined as a 1°C decline from normal body temperature; that is, a core body temperature less than 35°C. More specifically, mild hypothermia is defined as a core body temperature 32°C-35°C, with moderate hypothermia being 28°C-32°C, and severe hypothermia being characterized by less than 28°C.
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Hypothermia is caused by (1) heat loss, (2) a decrease in heat production, or (3) inhibition of the body’s innate thermoregulatory mechanisms. All three of these mechanisms can occur during general or regional anesthesia, resulting in the common occurrence of hypothermia during surgery.
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The human body naturally auto-regulates its own temperature. Thermoregulatory receptors in the body relay information to the hypothalamus, the main area in the brain that activates varied thermogenesis mechanisms. These mechanisms include shivering, vasoconstriction, and piloerection. Shivering produces heat by continual muscle contraction. Neonates, however, who are unable to shiver effectively, depend on nonshivering thermogenesis via metabolism of brown fat or dietary thermogenesis to stay warm. Vasoconstriction can help prevent cutaneous heat loss by allowing heat to be maintained in the core compartment of the body. However, hyperthermia causes sweating and vasodilatation to dissipate heat. Piloerection aids in preventing air, and thus heat, from escaping the body.
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General anesthesia interferes with hypothalamic thermoregulation via centrally and peripherally acting mechanisms. For example, volatile anesthetics (propofol and older opioids) foster heat loss through vasodilation. In addition, these drugs interfere with thermoregulation at the level of the hypothalamus. Regional anesthetics can similarly lead to hypothermia by causing vasodilatation and subsequent redistribution of heat. Thermoregulatory impairment in the hypothalamus also occurs because of altered dermatome perception. Through all of these mechanisms, anesthesia affects the body’s capability to auto-regulate its own temperature, predisposing to perioperative hypothermia.
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PERIOPERATIVE HYPOTHERMIA
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The operating room environment, surgical procedure, and anesthetic drugs all contribute to perioperative hypothermia. Operating rooms are often kept cold for surgeon comfort. Once in the operating room, a large portion of the patient’s body surface area is often exposed. Depending on the surgery, exposed viscera can also lead to substantial heat loss. Large amounts of cold antiseptic, intravenous, and irrigating solutions can likewise lower the core body temperature. Cool anesthetic gases inspired by the patient during the surgery constantly cause heat loss (as the body loses heat to the cold vapors) unless preventive measures are taken. Furthermore, anesthetic drugs such as volatile anesthetics can cause vasodilation, causing heat to be transferred from the core compartment of the body to the ...