Preventing Redistribution Hypothermia

The initial 0.5°C to 1.5°C reduction in core temperature is difficult to prevent because it results from redistribution of heat from the central thermal compartment to cooler peripheral tissues.^[57] Consequently, surface warming usually fails to prevent hypothermia during the first hour of anesthesia.^{[58] [117]} Lack of efficacy during this period results because the central-to-peripheral flow of heat is massive and because transfer of applied cutaneous heat to the core requires nearly an hour, even in vasodilated patients.

Although redistribution cannot be treated effectively,^{[58] [117]} it can be prevented. Redistribution results when anesthetic-induced vasodilation allows heat to flow peripherally down the normal temperature gradient. Skin surface warming before induction of anesthesia does not significantly alter core temperature (which remains well regulated), but it does increase body heat content. Most of the increase is in the legs, the most important component of the peripheral thermal compartment. When peripheral tissue temperature is sufficiently increased, subsequent inhibition of normal tonic thermoregulatory vasoconstriction produces little redistribution hypothermia because heat can flow only down a temperature gradient ( Fig. 40-18 ).^{[158] [159]} Although substantial amounts of heat must be transferred across the skin surface, active

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Figure 40-18 During the preinduction period (-120 to 0 minutes), volunteers were either actively warmed or passively cooled (no warming). At induction of anesthesia (time = 0 minutes), active warming was discontinued and volunteers were exposed to the ambient environment. Initial tympanic membrane temperatures were similar before each preinduction treatment. During the 60 minutes after induction of anesthesia, core temperature decreased less when volunteers were prewarmed. (ΔT = −1.1°C ± 0.3°C) than when the same volunteers were not warmed (ΔT = −1.9°C ± 0.3°C). Data are presented as means ± SD. (Redrawn from Hynson JM, Sessler DI, Moayeri A, et al: The effects of pre-induction warming on temperature and blood pressure during propofol/nitrous oxide anesthesia. Anesthesiology 79:219–228, 1993.)

The "afterdrop" associated with discontinuation of cardiopulmonary bypass is a type of redistribution hypothermia that results from a substantial core-to-peripheral tissue temperature gradient. As might be expected, it is more impressive after bypass at 17°C^[161] than at 27°C to 31°C.^[162] Cutaneous warming during and after bypass reduces core temperature afterdrop by 60%. However, heat balance data indicate that this reduction results primarily because cutaneous warming prevents the typical decrease in body heat content after discontinuation of bypass rather than by reducing redistribution.^[163]