Chapter 88

1. Adults thermoregulate with their environment by cutaneous vasomotor adjustments, sweating, shivering, and environmental behavioral adaptation (dressing appropriately, modifying environmental temperature).

2. Neonates do not shiver but can generate heat via nonshivering thermogenesis.

3. Heat loss occurs via sweating and cutaneous vasodilation. Heat conservation results from cutaneous vasoconstriction and behavioral adaptation.

4. Although not precisely defined, core temperature reflects mean temperature of the well-perfused organs (eg, brain, heart, kidney, and lungs).

5. Hypothermia that develops during general anesthesia typically follows a predictable pattern: (1) an initial rapid decrease in core temperature of between 0.5°C and 1.5°C during the first hour after induction, believed to be the result of internal redistribution of heat; (2) a more gradual linear decline in core temperature, usually lasting 2 to 3 hours, that results from cutaneous heat loss exceeding metabolic heat production (typically 0.5°C/h to 1°C/h); and (3) a plateau phase when core temperature stabilizes after 3 to 4 hours that results from the thermoregulatory balance of continual heat production and loss.

6. Following redistribution-mediated decreases in temperature, body heat loss occurs via radiation (60% of heat loss) and convection (30%), with less than 10% occurring via evaporation, and a negligible amount via conduction.

7. Postanesthesia shivering is most likely mediated via a normal thermoregulatory response to hypothermia.

8. Hypothermia during regional anesthesia is caused by the depression of regional thermal afferent input and efferent responses, such as vasoconstriction and shivering, loss of heat to the operating room environment, and redistribution of heat within the body.

9. Hypothermia during anesthesia may be prevented or treated by prewarming, the control of ambient temperature, skin insulation, warm intravenous solutions, heating and humidifying inspired gases, the application of a forced-air convective heating system, and the use of new-generation circulating-water convective heating systems.

10. There is an abundance of experimental evidence for the neuroprotective effects of hypothermia; clinical applications include deep hypothermic circulatory arrest for cardiac surgery, patients having suffered from witnessed cardiac arrest from ventricular fibrillation, and possibly perinatal asphyxia.

Hypothermia is a common perioperative occurrence resulting in a number of clinical consequences that range in significance from mild to serious (Table 88-1).1 With normal body temperature being 37°C, and taking into account a 1°C diurnal variation with an additional variation of 0.5°C in females depending on the menstrual cycle, perioperative hypothermia can be defined as a core temperature of less than 36°C.2 As with other mammals, humans require a nearly constant internal body temperature to maintain optimal homeostatic function. Significant deviation in this internal temperature can result in alterations in numerous metabolic and physiologic functions that, if not reversed, may contribute to significant morbidity and eventually mortality. The human body has evolved a sophisticated thermoregulatory control system by which to maintain core temperature (usually to within 0.2°C of its ideal target temperature). Anesthetics profoundly affect these control mechanisms, and when coupled with adverse environmental conditions (such as a cool operating room) and other heat-losing perioperative events, the risk of developing hypothermia easily becomes apparent. An understanding of ...

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