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 17–1). In fact, unintended perioperative hypothermia has been associated with an increased mortality rate.
Table 17–1.Deleterious Effects of Hypothermia ||Download (.pdf) Table 17–1. Deleterious Effects of Hypothermia
Cardiac arrhythmias and ischemia
Increased peripheral vascular resistance
“Left shift” of the hemoglobin–oxygen saturation curve
Reversible coagulopathy (platelet dysfunction)
Increased postoperative protein catabolism and stress response
Altered mental status
Impaired renal function
Delayed drug metabolism
Impaired wound healing
Increased risk of infection
When there is no attempt to actively warm an anesthetized patient, core temperature usually decreases 1–2°C during the first hour of general anesthesia (phase one), followed by a more gradual decline during the ensuing 3–4 h (phase two), eventually reaching a point of steady state (phase three). With general, epidural, or spinal anesthesia, most of the initial decrease in temperature during phase one is explained by redistribution of heat from warm “central” compartments (eg, abdomen, thorax) to cooler peripheral tissues (eg, arms, legs) from anesthetic-induced vasodilation. This initial heat loss can be greatly reduced by warming the patient preoperatively. Continuous heat loss to the environment is the primary driver for the slower decline during phase two. At steady state, heat loss equals metabolic heat production.
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. Anesthetic agents inhibit central thermoregulation by interfering with these hypothalamic reflex responses. The thermoregulatory impairment from conduction anesthesia that allows continued heat loss is likely also due to altered perception by the hypothalamus of temperature in the anesthetized dermatomes.
Prewarming the patient for half an hour with convective, forced-air warming blankets reduces the phase one decline in core temperature by reducing the central–peripheral temperature gradient.
A cold ambient temperature in the operating room, prolonged exposure of a large wound, and the use of large amounts of room-temperature intravenous fluids or high flows of nonhumidified gases can contribute to hypothermia. Methods ...