DEVELOPMENT OF HYPOTHERMIA DURING GENERAL ANESTHESIA

Inadvertent hypothermia during anesthesia is by far the most common perioperative thermal disturbance. Hypothermia results from a combination of anesthetic-impaired thermoregulation and exposure to a cold operating room environment.

Heat Transfer

Heat can be transferred from a patient to the environment in four ways: (1) radiation, (2) conduction, (3) convection, and (4) evaporation. Among these mechanisms, radiation and convection contribute most to perioperative heat loss. All surfaces with a temperature above absolute zero radiate heat; similarly, all surfaces absorb radiant heat from surrounding surfaces. Heat transfer by this mechanism is proportional to the fourth power of the absolute temperature difference between the surfaces. It is likely that radiation is the major type of heat loss in most surgical patients.^[49]

Conductive heat loss is proportional to the temperature difference between two adjacent surfaces and the strength of the thermal insulation separating them. In general, conductive losses are negligible during surgery because patients usually only directly contact the foam pad (an excellent thermal insulator) covering most operating room tables.

Conductive loss of heat directly to air molecules is limited by the development of a layer of still air adjacent to the skin that serves as an insulator. When this layer is disturbed by air currents, the insulative properties diminish substantially, and heat loss therefore increases. This increase is termed convection and is proportional to the square root of air speed; it is the basis of the familiar "wind chill" factor. Air speed in operating rooms—even those with high rates of air turnover—is typically just about 20 cm/sec, which only slightly increases loss in comparison to that in still air. Nonetheless, convective loss is usually the second most important mechanism by which heat is transferred from patients to the environment. Presumably, convective loss increases substantially in operating rooms equipped to provide laminar flow. However, the actual augmentation has not been quantified and may be less than expected from the increase in air speed because surgical draping provides considerable thermal insulation.

Sweating increases cutaneous evaporative loss enormously but is rare during anesthesia. In the absence of sweating, evaporative loss from the skin surface is limited to less than 10% of metabolic heat production in adults. In contrast, infants lose a higher fraction of their metabolic heat from transpiration of water through thin skin. The problem becomes especially acute in premature infants, who may lose a fifth of their metabolic heat production through transcutaneous evaporation. ^{[50] [51]} Simple thermodynamic calculations and clinical measurements indicate that only trivial amounts of heat are lost from the respiratory system.^[52] However, evaporation inside a surgical wound can contribute substantially to total heat loss.^[53]