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1. Pathol an abnormally low body temperature, as induced in the elderly by exposure to cold weather
2. Med the intentional reduction of normal body temperature, as by ice packs, to reduce the patient's metabolic rate: performed esp in heart and brain surgery


A condition in which the internal temperature of the (human) body is at least 3.6°F (2.0°C) below an internal temperature of 98.6°F (37°C). Hypothermia represents a continuum of effects that vary with the severity of cold on physiological systems. The human body needs a specific internal temperature that is regulated on a minute-by-minute basis to maintain all normal body functions. The many physiological and behavioral processes involved in maintaining the internal temperature constant are called thermoregulation. See Thermoregulation

Various environmental situations predispose humans to hypothermia, which can occur even in the absence of cold. In fact, hypothermia is more common in temperate regions than in the colder climates. Because of the uniqueness of the situations in which hypothermia can occur, various kinds of hypothermia have been classified, all of which can prove fatal.

Primary hypothermia

Primary hypothermia is a decrease in internal temperature that is caused by environmental factors in which the body's physiological processes are normal but thermoregulation capability is overwhelmed by environmental stress.

Air (formerly exposure) hypothermia is thought to be the most common form. A person exposed to cold air experiences the same processes as a person in cold water, but air hypothermia occurs more slowly. The induction of air hypothermia is more subtle and therefore more dangerous since it can occur over a number of weeks. The degree to which a person reacts to a cold air stress is dependent on such factors as age, physical stamina, the intensity of the cold stress, and the responsiveness of the thermoregulatory system. One of the most convenient ways to determine whether someone is suffering from hypothermia is a noted change in personality: Complaints of fatigue, sluggish speech, and confusion are common, and in some cases the behavior resembles that of intoxication.

Initially, skin temperature falls rapidly, blood vessels to the skin constrict, and shivering begins. After 5–10 min, shivering ceases for about 10–15 min, but this is followed by uncontrollable shivering. In a cold situation, the nervous system causes blood to be redistributed away from the skin as the blood vessels of the skin close down to minimize heat transfer to the cold environment. The decrease in skin temperature coupled with vasoconstriction makes the person feel cold, and sometimes the fingers and toes can become painful. Internally, there is an increase in the levels of hormones that control metabolism, and blood is shunted primarily to the lungs, heart, and brain. The person becomes dehydrated as the inspired air is warmed and humidified. If the tense and shivering muscles do not generate enough heat, the hypothermic process begins and progresses for at least 3–5 h. As hypothermia continues, the arms become rigid, and the person loses the ability to make fine movements. During this period of time the heart rate initially increases, then stabilizes and as the person's internal temperature becomes progressively colder, the heart rate and respiration slow. In severely hypothermic persons, it is very difficult to detect a slow heart rate or determine if the person is breathing. A temperature of 95°F (35°C) is only the beginning of mild hypothermia and shivering can continue for hours, depending on the muscle and fat supplies available. Eventually, the environment becomes overwhelming. At 86°F (30°C), the person loses consciousness and shivering ceases. Death does not occur until the internal temperature drops further: Death results at 68–77°F (20–35°C) because of cardiac standstill.

When a person falls into cold water, a gasping response is triggered by the thermal receptors on the skin. For some individuals, the cold stress may trigger a heart attack. Although as much of the body as possible should be kept out of the water, many victims of immersion hypothermia stay in the cold water because they cannot tell how cold they are. Shivering becomes generalized and, unlike its effect in cold air, may cause a faster drop in internal temperature since the water layer closest to the body is stirred and convective heat loss is promoted. Although the greater conductive property of water relative to air is a major heat sink, physiological and behavioral responses act to minimize the heat loss. Survival in 50°F (10°C) water is possible for several hours at most if the person is dressed in street clothes and a life jacket.

The cooling of the body in submersion hypothermia allows the brain and heart to withstand approximately 45 min of oxygen debt. This is most operative for young children. A child can survive for an extended period of time while completely submerged because the body is undergoing both internal and external cooling. As the child is drowning, cold water is swallowed and enters the lungs, which cools the core. At the same time, the cold water that bathes the skin rapidly cools the periphery. The multiple effects of the internal and external cooling decrease the metabolic rate and give the child a window of safety of approximately 45 min. In warm water, survival is possible for only 5–7 min.

Secondary hypothermia

A decrease in core temperature caused by an underlying pathology that prevents the body from generating enough core heat is referred to as secondary hypothermia. If any of the thermoregulatory systems are altered, the body's ability to generate heat decreases and hypothermia can then develop without warning. Insufficient muscle mass to generate heat, medications that interfere with metabolism, an underlying systemic infection, decreased thyroid hormone production, and paralysis predispose to hypothermia. Premature infants with low body fat and a large surface-to-volume ratio lose heat rapidly and are at risk for becoming hypothermic. The elderly are perhaps the most susceptible to secondary hypothermia. However, whether the process of aging with no associated debility also alters the thermoregulatory system in the elderly remains to be determined.

Clinical hypothermia

Some cardiac surgical procedures require clinically induced cooling to stop the heart from beating. Induced hypothermia lowers the oxygen demand of the body tissues, so that oxygenated blood need not circulate. In the case of coronary bypass surgery, the entire body is cooled, enabling the surgeons to work for an extended period of time on the cold heart.


In hypothermia, the body's internal temperature decreases, but no solid freezing takes place. In frostbite, which is freezing of the digits or the limbs, there is actual formation of ice crystals. Basically the digits go through various stages of cooling. Initially, in the prefreeze phase, the finger temperature is 37.4–50°F (3–10°C). Next, at 24.8°F (-4°C) ice crystals form outside the cells of the digits, circulation is limited, and cell death takes place if the process is allowed to continue. The cells of the digits and limbs can tolerate low temperatures that would be lethal to brain or nerve cells. However, once they are rewarmed and thawed, they develop an increased sensitivity to the cold and become more susceptible to frostbite. Any part of the body can become frostbitten, but the fingers, toes, ears, nose, and cheeks are most often affected. See Homeostasis



a decrease in body temperature in warm-blooded animals and humans, as a result of heat emission that exceeds the formation of heat in the body. At low environmental temperatures animals and humans are protected against hypothermia by heat insulation (the fat layer, fur, plumage, or clothing). When the heat insulation is insufficient, a physiological reaction to chilling results, involving limitation of heat emission from the skin as a result of blood flow away from the skin to the internal organs, sharp increase in metabolism, increased heat production in the muscles due to movements, work, and muscular trembling. In the cold, hypothermia in humans may develop only after exhaustion of these physiological mechanisms. It may also develop if the individual falls asleep from fatigue or is completely motionless, and it arises readily when there is a disturbance in thermoregulation (drunkenness, shock, narcotic sleep, and blood loss). In cold water, heat emission increases to an enormous degree, and increased heat production cannot compensate for it. At water temperatures of 0°-4° C death from hypothermia may result within 40-60 minutes. Lowering the body temperature to 33°-32° C induces drowsiness and dulled consciousness. Under 30° C there is a progressive decrease in metabolism and blood pressure and a slowing of heartbeat and respiration. At 27°-26° C there is loss of consciousness, and at around 23°-20° C respiration and then heartbeat cease. Physiological hypothermia is observed in some animals during winter hibernation as an adaptive reaction that makes it possible for them to get along for months without food with a small loss of weight. Chilled tissues (for example, of the brain and heart), whose metabolism is sharply decreased by hypothermia, tolerate oxygen deficiency more readily and survive cessation of blood circulation for a longer period. The use of artificial hypothermia in contemporary surgery is based on these facts.


Artificial hypothermia The generalized chilling of a warmblooded organism, accomplished for prophylactic and therapeutic purposes on the basis of inhibition of the mechanisms of thermoregulation, is artificial hypothermia. The first investigations of the effect of cold on the human body and possibilities for the medical use of hypothermia are associated with the name of the English surgeon D. L. Carry (1798). Further study of hypothermia was conducted in the 19th and early 20th centuries on animals. In 1940 the American scientists L. Smith and T. Fay attempted to treat cancer by inducing hypothermia for five to eight days. The attempt was not successful, but it demonstrated the possibility of maintaining vital functions of the human body under general hypothermia at 28°-30° C and under narcosis. Broad application of hypothermia in clinical practice began after 1950, when the Canadian scientist W. Bigelow showed in animal experiments the possibility of safely disconnecting the heart and stopping blood circulation for ten to 15 minutes at temperatures of 26°-28° C. In 1952 the American physicians F. Levy and M. Taufic performed the first operation in the world on an open “dry” heart (that is, a heart disconnected from blood circulation) under conditions of moderate hypothermia, and a few years later heart operations under hypothermia became firmly established in everyday practice. The principal effect of hypothermia is conditioned by the decrease in intensity of metabolic processes under the action of cold and the decrease in the organs’ and tissues’ requirements for oxygen associated with this.

When the mechanisms of thermoregulation are blocked, the oxygen requirement of the body decreases. It has been established that at temperatures of 26°-27° C the general oxygen requirement decreases by 40 percent, the oxygen requirement of the cardiac muscles decreases by 50 percent, and the oxygen requirement of the brain decreases by 33 percent.

Hypothermia can be achieved by immersing the patient in a cold-water bath, wrapping the body in ice bags, using special blankets in which cold water circulates, or placing the patient in a special installation into which cold air is forced. Also used are various methods of chilling the blood outside the body by means of heat exchangers—extracorporeal refrigeration. Craniocerebral hypothermia, which involves chilling the head, is beginning to be widely used. It is most effective in reviving the body or in extremely grave (terminal) conditions.

However, hypothermia is a pathological condition for a warm-blooded organism. Under the influence of hypothermia the functioning of the heart changes, and its irritability increases. Transient disturbances are observed in the functions of the kidneys and liver and in vascular tone. The best means of preventing reciprocal reaction of the body to hypothermia is shallow narcosis with complete relaxation of the muscles induced by curare-like drugs and inhibition of the neuroendocrine system by a complex of pharmacological preparations.

In heart operations hypothermia at temperatures of 29°-30° C is most advisable, because chilling to lower temperatures involves the danger of disrupting heart operation (fibrillation). The permissible period for disconnecting the heart at this temperature is no more than ten minutes.

Hypothermia may be used over a period of many days for therapeutic purposes in cases of the aftereffects of hypoxia, serious craniocerebral trauma, and pronounced hyperthermia. The therapeutic effect of hypothermia depends greatly on its timely, early use.


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Condition of reduced body temperature in homeotherms.