In humans, heat (or “thermal energy”) is more concentrated in the core than it is in the extremities. This is why your hands or feet may be cold even though you core is normothermic. Thus, the core temperature may be 37 but the mean body temperature might be 36 or 35.5. General anesthesia (and neuraxial regional anesthesia) destroy this process by re-distributing heat (thermal energy) equally about the body and core temperature begins to approach mean body temperature. Redistribution of body heat from the core to the periphery is the most common etiology of hypothermia in the first hour after induction of anesthesia and this reduction in temperature is commonly 1-1.5 °Celsius. It should be noted, and may not have been clear in the question, that this is NOT do to heat loss – merely a redistribution of existing heat. After that point heat is lost to the environment through mechanisms such as radiation, convection, evaporation, and conduction.
Four types of heat loss from a patient to the relatively cold environment:
The main mechanism of heat loss is radiation, which was established in a cohort of 13 normal young women who were exposed and awake in a calorimeter chamber measuring basal metabolic rates and perceptions in response to a controlled temperature change from 22-35 C. Radiation accounted for 67% in the cold zones, evaporation 17%, and conduction/convection approximately 16%.
Radiative heat losses occur anytime an object is above absolute zero. The magnitude of the radiation is proportional to the fourth power of the difference between the two sources. Conduction is less important in the OR, because the patient is usually lying on foam or rubber which is well insulated.
Convection can play an important role as air turnover constantly removes heat as the patient’s body exists in equilibrium of heat transfer between itself and the surrounding “layer” of air.
Patients under anesthesia typically do not sweat which is the main way that heat is lost due to evaporation.
Two mechanisms of undesired heat loss in the OR.
- Impaired thermoregulation secondary to anesthesia.
- Low ambient temperature of the operating theatre.
Of these two mechanisms, the anesthetic impairment is the more important mechanism.