Cutaneous Warming

Operating room temperature is the most critical factor influencing heat loss because it determines the rate at which metabolic heat is lost by radiation and convection from the skin and by evaporation from within surgical incisions. Consequently, increasing room temperature is one way to minimize heat loss. However, room temperatures exceeding 23°C are generally required to maintain normothermia in patients undergoing all but the smallest procedures^[168] ; most operating room personnel find such temperatures uncomfortably warm. Infants may require ambient temperatures exceeding 26°C to maintain normothermia. Such temperatures are sufficiently high to impair the performance of operating room personnel and decrease their vigilance.

The easiest method of decreasing cutaneous heat loss is to apply passive insulation to the skin surface. Insulators readily available in most operating rooms include cotton blankets, surgical drapes, plastic sheeting, and reflective composites ("space blankets"). A single layer of each reduces heat loss approximately 30%, with no clinically

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Figure 40-19 Insulators readily available in most operating rooms include cotton blankets, surgical drapes, plastic sheeting, and reflective composites ("space blankets"). A single layer of each reduces total cutaneous heat loss approximately 30%, without any clinically important differences among the insulation types. Data are presented as means ± SD. (Redrawn from Sessler DI, McGuire J, Sessler AM: Perioperative thermal insulation. Anesthesiology 74:875–879, 1991.)

The reduction in heat loss from all the commonly used passive insulators is similar because most of the insulation is provided by the layer of still air trapped beneath the covering. Consequently, adding additional layers of insulation further reduces heat loss only slightly. For example, one cotton blanket reduces heat loss by about 30%, but three cotton blankets reduce heat loss by only 50%. Furthermore, warming the cotton blankets provides little benefit, and the benefit is short-lived ( Fig. 40-20 ). ^[170] These data indicate that simply adding additional layers of passive insulation or warming the insulation before application will usually be insufficient in patients becoming hypothermic while covered with a single layer of insulation.

Cutaneous heat loss is roughly proportional to surface area throughout the body.^[171] (The popular perception that a large fraction of metabolic heat is lost from the head is false in adults. Loss from the head can be substantial in small infants,^{[172] [173]} but the loss is high mostly because the head represents a large fraction of the total surface area.) Consequently, the amount of skin covered is more important than which surfaces are insulated. It does not make sense, for example, to cover the head and leave the arms exposed; the arms have more surface area than the head does and account for more heat loss.

Passive insulation alone is rarely sufficient to maintain normothermia in patients undergoing large operations; active warming will be required in these cases. Because about 90% of metabolic heat is lost through the skin surface, only cutaneous warming will transfer sufficient heat to prevent hypothermia. Most infrared systems are little more effective than passive insulation.^{[139] [169]} Consequently, for intraoperative use, circulating water

Figure 40-20 Total cutaneous heat loss when the volunteers were covered with a single warmed (open circles) or unwarmed blanket (open squares) or three warmed (filled circles) or unwarmed (filled squares) blankets. These data indicate that warming cotton blankets is of little benefit and that adding additional blankets only slightly decreases cutaneous heat loss. Data are presented as means ± SD. (Redrawn from Sessler DI, Schroeder M: Heat loss in humans covered with cotton hospital blankets. Anesth Analg 77:73–77, 1993.)

Studies consistently report that circulating-water mattresses are nearly ineffective.^[174] Presumably, they are unable to maintain normothermia because little heat is lost from the back into the 5 cm of foam insulation covering most operating room tables. Furthermore, the combination of heat and decreased local perfusion (resulting when the patient's weight reduces capillary blood flow) increases the propensity for pressure/heat necrosis ("burns"). ^[175] Such tissue injury can occur even when water temperature does not exceed 40°C.^[176] Circulating water is more effective—and safer—when placed over patients rather than under them and, in that position, can almost completely eliminate metabolic heat loss.^[139] Metabolic heat production will increase mean body temperature approximately 1°C/hr when cutaneous heat loss is eliminated. Recently developed circulating-water garments transfer large amounts of heat by increasing the warmed surface area or using materials that facilitate conduction.^{[177] [178]}

The most common perianesthetic warming system is forced air. The best forced-air systems transfer more than 30 W across the skin surface, which rapidly increases mean body temperature.^{[139] [179]} Forced air usually maintains normothermia even during the largest operations^{[58] [117] [180]} and is superior to circulating-water mattresses.^[181] Resistive heating (electric blankets) is as effective as forced air but is much less expensive because it does not require a disposable product ( Fig. 40-21 ).^[182] Nonetheless, carbonfiber resistive heaters should be avoided because they frequently cause burns. Figure 40-22 shows the relative effect of common patient warming methods and the

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Figure 40-21 Core temperature as a function of time in patients assigned to the circulating-water, forced-air, and carbon-fiber (resistive heating) groups. Temperature changes in the control group differed significantly from changes in the other groups after 150 elapsed minutes. Temperatures in the circulating-water and carbon-fiber groups never differed significantly. Results are presented as means ± SEM. Despite similar efficacy, carbon-fiber heating is associated with patient burns and should be avoided. (Redrawn from Negishi C, Hasegawa K, Mukai S, et al: Resistive-heating and forced-air warming are comparably effective. Anesth Analg 96:1683–1687, 2003.)