Clinical Note

Hypothermia and Cold-Induced Tissue Damage

The body is better able to withstand cooling than heating of the core temperature for limited periods because metabolism slows and irreversible damage is less. When the body temperature falls below 33°C (91°F), mental confusion and sluggishness result, and the thermoregulatory ability of the central nervous system is lost below 30° C (86°F). At this point, shivering stops and consciousness is lost. This is followed by muscular rigidity and collapse. Further cooling leads to slow atrial fibrillation and, eventually, ventricular fibrillation and death.

Hypothermia is frequently seen in derelict alcoholics. Not only do they not have shelter from the cold, but the alcoholic stupor also decreases the generation of heat by muscular activity. Hypothermia is also common following accidental immersion in cold water or wilderness exposure in a cold climate. Immersion can lead to a rapid loss of body heat because the water in contact with the skin is a much better heat conductor than air, and convection is increased by the physical exertion of struggling to swim. In the wilderness, hypothermia can develop rapidly because of changing weather conditions; it usually constitutes the greatest danger to survival. The effects of any temperature drop are compounded by the wind, and evaporative losses can be very high due to rapid respiration and sweating while attempting to escape the situation. The clothing becomes soaked by the perspiration and any precipitation that may be occurring, which decreases the insulation around the body.

Hypothermia is also purposely induced in surgical procedures in which the blood flow to vital organs may be interrupted, as in the case of heart surgery when a heart-lung machine is used. In these situations, hypothermia is induced by packing ice around the patient's body until the desired level of hypothermia is attained. Because the metabolic rate is markedly decreased in hypothermia, tissue damage due to interruptions of the circulation is minimized.

Hypothermia is, of course, treated by rewarm-ing, but this should be done slowly. If heat is applied to the skin rapidly, the resulting cutaneous vasodilation can lead to large volumes of chilled blood entering the general circulation, which further decreases core temperature and can produce fatal cardiac arrhythmias.

There are remarkable accounts of complete recovery from hypothermia in which there has been cardiac arrest or fibrillation for more than an hour. Thus, even if all the usual signs, including apparent rigor mortis, are present, the physician should not rule out the possibility of reversible hypothermia. If there is doubt, the body should be slowly rewarmed to 37°C before the patient is pronounced dead.

In addition to the dangers of systemic hypothermia, local tissue damage due to freezing (frost bite) may occur with prolonged cold exposure. Freezing of the digits or facial tissue is the most common type of frost bite and is facilitated by the vasoconstriction that occurs in response to the cold. Tissue damage can also occur without freezing, as in the condition of immersion foot (trench foot), in which the feet are immersed in cold water for long periods of time. Because of the reduced blood flow in the foot, the nerves and muscles are damaged irreversibly.

Mild hypothermia is also seen in a number of diseases, including myxedema, pituitary insufficiency, Addison's disease, hypoglycemia, and cerebrosvascular disease, and as a consequence of the effects of drugs or alcohol.

climates. Some primitive peoples have adapted to living at or near freezing temperatures with very little clothing. For example, Australian aborigines spend much of the year at low temperatures and yet shiver relatively little; however, when the air is cold, their body temperature falls markedly during the night. Long-distance swimmers are noted for their ability to withstand water temperatures of 15 to 20°C for many hours, as well as for their characteristically heavy layer of cutaneous fat. While they swim in cold water their heat production is increased by the physical activity, and they experience extreme cutaneous vasoconstriction.

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