Regulation of Body Temperature. Body temperature is regulated by feedback mechanisms predominantly mediated by the preoptic nucleus of the anterior hypothalamus.

1. Causes of Increased Body Temperature (Table 3-7)

2. Perioperative Temperature Changes. Anesthesia and surgery in a cool environment makes perioperative hypothermia a likely occurrence (Table 3-8).

3. Sequence of Temperature Changes during Anesthesia

   1. Under general anesthesia, tonic vasoconstriction is attenuated and heat contained in the core compartment will move to the periphery, thus allowing the core temperature to decrease toward the anesthetic-induced lowered threshold for vasoconstriction. This core to peripheral heat redistribution is responsible for the 1°C to 5°C decrease in core temperature that occurs during the first hour of general anesthesia (Fig. 3-22).

   2. Protection from heat loss early in a surgical procedure is important to reduce the temperature gradient from the environment to the peripheral compartment because significant heat energy has been shunted to the periphery.

   3. After the first hour of general anesthesia, the core temperature usually decreases at a slower rate. This decrease is nearly linear and occurs because continuing heat loss to the environment exceeds the metabolic production of heat.

   4. After 3 to 5 hours of anesthesia, the core temperature often stops decreasing (see Fig. 3-22). This type of thermal steady state is especially likely in patients who are well insulated or effectively warmed.

Beneficial Effects of Perioperative Hypothermia. Oxygen consumption is decreased by approximately 5% to 7% per degree Celsius of cooling. Thus, even moderate decreases in core temperature of 1°C to 3°C below normal provide substantial protection against cerebral ischemia and arterial hypoxemia.

Adverse Consequences of Perioperative Hypothermia (Table 3-9)

Perioperative Temperature Measurement. It is recommended that intraoperative core temperature be maintained at greater than or equal to 36°C. Measuring the temperature of the lower 25% of the esophagus (about 24 cm beyond the corniculate cartilages or site of the loudest heart sounds heard through an esophageal stethoscope) gives a reliable approximation of blood and cerebral temperature.

Prevention of Perioperative Hypothermia

1. Passive or active airway heating and humidification contribute little to perioperative thermal management in adults because less than 10% of metabolic heat is lost via ventilation.

2. The administration of unwarmed fluids can markedly decrease body temperature. Warming fluids to near 37°C is useful for preventing hypothermia, especially if large volumes of fluid are being infused.

3. Covering the skin with surgical drapes or blankets can decrease cutaneous heat loss. A single layer of insulator decreases heat loss by approximately 30%, but additional layers do not proportionately increase the benefit.

4. Active warming is needed to prevent intraoperative hypothermia. Forced air warming is probably the most effective method available, although any method or combination of methods that maintains core body temperature near 36°C is acceptable (Fig. 3-23).