abnormally low body temperature in a warm-blooded creature, associated with a general slowing of physiologic activity. Hibernating (hibernation) animals allow their body temperatures to fall to levels only slightly above ambient temperature, in a unique kind of hypothermia from which they can recover rapidly when necessary; similar temperatures would be fatal to nonhibernators.

In human beings, hypothermia may be artificially induced to decrease the metabolic need for oxygen during certain surgical procedures and for the control of some kinds of cancer. The lowering of body temperature in these cases is usually accomplished through the use of ice baths. Similarly induced hypothermia is employed to inhibit metabolic process in organs removed for transplantation, thus prolonging their viability.

Controlled induction of hypothermia has many important medical applications. For example, regional cooling of the heart during coronary artery bypass surgery protects the heart muscle while the revascularization procedure is completed. Profound systemic hypothermia with circulatory arrest is possible by utilizing heat exchange or cooling circuits in modern cardiopulmonary bypass devices. The technique of profound hypothermia allows protection of the brain, heart, and other vital organs when circulation is stopped during the time of circulatory arrest, which may be an hour or longer. Complex aortic aneurysms involving the proximal portion of the aorta (the trunk of the aorta, originating from the heart) and complex congenital defects of the heart can be safely corrected with this technique.

Accidental hypothermia may be life-threatening and constitutes a medical emergency. Low environmental temperature alone may be sufficient to cause hypothermia, as from submersion in icy water or overexposure in heavy winter snows. In cases of hypothermia due to exposure, there is often an underlying condition—such as cerebrovascular disease, hypothyroidism, severe infection, or alcohol or drug intoxication—that predisposes to the drop in body temperature. Hypothermia is serious at body temperatures below 35° C (95° F); below 32.2° C (90° F), the point at which the normal shivering reaction ceases, the condition warrants emergency treatment. Because of the physiologic slowing, pulse, respiration, and blood pressure are depressed; in some cases the victim of hypothermia appears to be dead, although revival may still be possible with appropriate treatment.

Accidental hypothermia is treated by a slow, gradual rewarming of the victim, using blankets and other passive means rather than direct application of heat. The increase in body temperature must not exceed one or two degrees F per hour, as more rapid rewarming can cause the cardiovascular system to collapse.