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Hypothermia is an under-recognized condition, especially in the elderly as well as in pediatric age groups and can result in significant morbidity and mortality, especially in the peri-operative period. An understanding of body temperature control mechanism and the relevant clinical features help to establish the correct diagnosis.
Hypothermia occurs when core body temperature is less than 95°F. It represents a lot of balance between heat generation and heat dissipation. The human thermo–regulatory system usually maintains a body core temperature near 98.6°F. Variations of approximately 33.8°F with a circadian cycle and of approximately 32.9°F with the menstrual cycle, are superimposed on the normal core temperature.
At a temperature less than 95°F, the thermo-regulatory response is defective and the mental process is slowed, the behavior is defective and automatic defense is lost. At temperature < 89.6°F, shivering is lost and the patient develops a coma and after an initial decrease in heart rate, cardiac output and mean arterial pressure (MAP) are preserved. At 89.6°F, renal cellular based metabolic rate decreases and the respiratory quotient are as low as 0.65. At about 82.4-86°F, the susceptibility to ventricular fibrillation is significantly increased. At temperature < 82.4°F, progressive bradycardia supervenes and at temperature < 64.4°F, asystole may supervene. The hepatic function also decreases and so does the adrenal cortical activity and the patient may also develop adynamic ileus.
At Temperatures < 95°F, mental processes are slowed and behavior is mostly affected by the thermo-regulatory response. Also, the main autonomic defenses against cold are vasoconstriction and shivering. At temperature < 90°F, the ability to shiver is lost and at 82.4°F, coma often supervenes. Shivering can be treated by warming the surface or by the administration of clonidine (75 mg IV) or pethidine (25 mg IV) in adults. Approximately 90% of heat is lost through the surface of the skin, with radiation and convection, contributing more than evaporation.
The body can tolerate dry cold much better than wet cold (immersion). Wetness increases heat loss considerably. The direct effects of cold are prominent in fatty tissues and myelinated nerve fibers. Indirect effects are mostly ischaemic in nature, due to vascular damage.
The essence of the therapeutic approach for these patients lies in the return of core body temperature to normal. Supportive therapy should continue until normothermia is achieved. The administration of antibiotics is a controversial issue. Few experts recommend the administration of antibiotics for 72 hours, pending culture reports.
The anterior hypothalamus, the primary thermo-regulatory control center of the body receives a signal from the cold and heat receptors, which is widely distributed in the body. Shivering may not provide enough heat to overcome the ambient temperature gradient or energy supplies eventually become depleted and thermogenesis declines. Neonates and infants present a special problem of hypothermia, because of a larger body surface area. Elderly patients also prone to develop hypothermia, because of the capacity of normal physiologic thermo-regulatory mechanisms to produce and conserve heat declines, as the age increases.
An often quoted dictum in the treatment of hypothermia is “organism is not dead, till it is warm”.
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Hypothermia is classified as mild, if core body temperature is between 91.4-95°F, moderate when it is between 86- 91.4°F and severe if core body temperature is < 86°F. Therefore the patient should be carefully monitored with either a rectal probe with a full range of 68 to 104°F or with an ear thermister (an apparatus for measuring very small changes in temperature) or with a nasopharyngeal probe.
Hypothermia has some benefits also. By lowering the core temperature from 98.6 to 71.6°F, the cerebral metabolic rate of oxygen consumption (CMRO) decreases linearly to 25% of normal. Hypothermia has long been a useful adjunct in cardiac surgery, but prolonged hypothermia may be detrimental in the light of increased viscosity of blood at lower temperatures and attendant sludging of flow. Hence, moderate hypothermia is advocated with core temperature being lowered to not less than 89.6- 91.4°F. This causes myocardial depression and CMRO is substantially decreased. In the management of brain trauma, there is a better outcome (62 vs 38%) after moderate hypothermia of 89.6- 91.4°F, which was started after 10 hours and maintained for 24 hours.