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Definition

Hypothermia, a potentially fatal condition, occurs when body temperature falls below 95°F (35°C).

Description

Although hypothermia is an obvious danger for people living in cold climates, many cases have occurred when the air temperature is well above the freezing mark. Elderly people, for instance, have succumbed to hypothermia after prolonged exposure to indoor air temperatures of 50–65°F (10–18.3°C). In the United States, hypothermia is primarily an urban phenomenon associated with alcoholism, drug addiction, mental illness, and cold—water immersion accidents. The victims are often homeless male alcoholics. Officially, 11,817 deaths were attributed to hypothermia in the United States from 1979 to 1994, but experts suspect that many fatal cases go unrecognized. Nearly half the victims were 65 or older, with males dominating every age group. Nonwhites were also overrepresented in the statistics. Among males 65 and older, nonwhites outnumbered whites by more than four to one.

— Howard Baker

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n.

Abnormally low body temperature.

[HYPO– + Greek thermē, heat + –IA¹.]

hypothermic hy'po·ther'mic (-mĭk) adj.

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.

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.

Frostbite

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 also Homeostasis.

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Hypothermia has been reported during marathons, on the mountains, in caving and canoeing, at work on land and at sea, and in and under water. It may occur, even during the summer, among the elderly and the very young, in the operating theatre, and in association with some medical disorders. Geographically hypothermia has in fact been recorded from most parts of the world even at low altitudes in the Sahara and from tropical Kampala.

Thermally the body can be divided into zones. The temperature of the ‘core’, including the vital organs, heart, and brain, is kept stable over a surprising range of environmental conditions. The ‘shell’ is superficial, and its size and temperature varies considerably. Hypothermia is defined as a core temperature below 35°C. However, hypothermia is not a diagnosis, but a sign that changes must have occurred in the body, which vary with the circumstances which have led to cooling. During cold exposure the body ‘burns’ carbohydrate to generate heat and the amount of fuel left depends on the rate of cooling. Also fluid is lost through increased urine output, and there are complex movements of fluids in the body, related in magnitude to the duration of exposure to cold, which reverse during rewarming. There are therefore different types of hypothermia.

(i) In ‘immersion’ hypothermia the cold stress is so great that the body's heat-generating capacity is overwhelmed and the core temperature is forced down. Because energy reserves are available, the person will have very little difficulty in rewarming once removed from the severe environment. Falling into cold water is the commonest accidental cause, and hypothermia deliberately induced for medical reasons is of this type.
(ii) In ‘exhaustion’ hypothermia the body maintains the temperature until energy sources are exhausted. Then, since the heat-generating ability is reduced, even relatively mild cold exposure may cause continued cooling. Thermal protection must therefore take account of every avenue of heat loss, because even small variations of available heat may make the difference between life and death. This is the type most commonly found in mountaineers or hill walkers.
(iii) With ‘urban’ hypothermia, the cold has been mild but prolonged. The core temperature remains normal (35°C or above) possibly for days or weeks, with massive fluid shifts. The temperature eventually drifts down, or faster cooling is precipitated by some other factor such as a fall. This is the most usual type found in the elderly or in association with malnutrition.
(iv) ‘Submersion’ hypothermia occurs in people who have been totally submerged in very cold water. Recovery has been known even after 15-60 min (typical submersion survival is about 3 min). Survival, more likely in children, depends on resuscitation being started immediately on rescue.

The case history distinguishes the different types, though they may overlap. A climber in a snowstorm disabled by a broken leg will cool as rapidly as if immersed, because the shock of the injury increases the rate of heat loss, and the fracture prevents heat generation from voluntary activity or shivering. A deep diver may suffer ‘immersion’ hypothermia, even in a dry pressure chamber, because of the tremendous heat transfer capacity of the compressed oxyhelium gas mixture which he is breathing. A swimmer lost overboard in relatively warm water is a candidate for ‘exhaustion’ hypothermia. A middle-aged man or a child with severe malnutrition is likely to develop ‘urban’ hypothermia, whereas a fit 70-year-old out walking in the hills would be liable to ‘exhaustion’ hypothermia.

When a person is in a situation where hypothermia is imminent there is a great temptation to continue to move in order both to keep generating heat from the activity and to escape the situation. This may not be the best option for survival, since the activity increases the rate of heat loss and aggravates exhaustion. The best prospect of survival at sea is not to try to swim but to stay with a capsized boat. Similarly in the hills in bad weather the macho response of trying to battle a way out has resulted in many deaths, whereas those who ‘go to ground’ — taking shelter until the weather improves — usually survive.

A variety of signs and symptoms of hypothermia have been described (see table). However there is great individual variation. For example, loss of consciousness may occur at a core temperature as high as 33°C, but in one case consciousness was still present at a rectal temperature of 24.3°C, and other causes of unconsciousness may confuse the issue. Shivering is considered to cease usually at 30°C — but shivering has been recorded at a core temperature of 24°C. At the other extreme some experimental subjects can cool without shivering and many mountain rescue cases never shiver.

Core temperature °C
37.6	‘Normal’ rectal temperature.
37	‘Normal’ oral temperature.
36	Increased metabolic rate to balance heat loss. Breathing and pulse faster.
35	Shivering at a maximum.
	Reflexes exaggerated; speech disordered; thinking slowed.
34	Usually responsive; normal blood pressure.
	Lower limit compatible with continued exercise.
33-31	Retrograde amnesia; consciousness clouded; blood pressure low; pupils dilated;
	shivering usually ceased.
30-28	Progressive loss of consciousness; increased muscular rigidity;
	slow pulse and respiration; irregular heart beat.
	Susceptible to ventricular fibrillation if heart mechanically irritated.
27	Voluntary movement lost.
26	Pupillary light reflex and deep tendon and skin reflexes lost.
	Victims seldom conscious.
24-21	100% mortality in shipwreck victims in World War II
20	Heart standstill.
17	No electrical activity in brain.
14.4	Lowest known accidental hypothermic patient with recovery.
9	Lowest artificially cooled hypothermic patient with recovery.
4	Monkeys revived successfully.
1-7	Rats and hamsters revived successfully.

Diagnosis requires measurement of core temperature, usually rectal, but since this route may not be practical in the field, the person should be treated as a ‘cold casualty’ if the armpit feels ‘as cold as marble’ to the rescuer's hand. In hypothermia the diagnosis of other accompanying conditions is difficult. Slurred speech, staggering, incoordination or a change in personality may be due to hypothermia and not necessarily brain damage. In hypothermia the reflexes are affected and there is stiffness of the muscles; there are changes in the electrical and mechanical functions of the heart; and the lungs may show clinical and X-ray features similar to pneumonia, though these clear on rewarming. It is therefore important that the patient should be restored to normal body temperature before any diagnosis is made or any irrevocable treatment started.

If the heart stops, the lack of circulation, and therefore of oxygen supply to the brain would ordinarily cause permanent brain damage in about three minutes. Although hypothermia can give some protection for the brain by prolonging the time before oxygen lack causes permanent damage, it is cardiac function that is most important for survival. As the heart cools it becomes more susceptible to ventricular fibrillation (VF) (an uncoordinated electrical activity of the heart which produces no actual pumping of blood). This may be triggered by mechanical irritation (which may be as mild as rolling a patient for bedmaking), by hypoxia of the heart muscle, or by rapid changes in pH or electrolytes in the blood, or in temperature gradients within the heart muscle. Inappropriate rewarming techniques add to the hazard.

Rescue and care

Profound accidental hypothermia can be very difficult to distinguish from death. The only certain diagnosis of death in hypothermia is failure to recover on rewarming. However hypothermia is seldom present in isolation. Victims may also be injured or have some illness, possibly cold-related. In water, drowning may precede or follow cooling. If neither heart nor breathing activity can be detected and there is evidence that the person was alive within the previous two hours, the rescuers should start resuscitation, though only if this can be continued until the casualty has been rewarmed or has reached hospital: otherwise members of the rescue team will become exhausted and may then become hypothermic casualties themselves. Also, the heart may occasionally still be beating and providing some circulation, even when this cannot be detected; the mechanical effects of active resuscitation attempts may trigger ventricular fibrillation, and the patient would then be in a worse state and needing continuing resuscitation.

— Evan L. Lloyd

Bibliography

• Lloyd, E. L. (1986). Hypothermia and cold stress. Croom Helm, London

See also cold exposure; near-drowning; survival at sea; temperature regulation.

Low body temperature (normal is around 37 °C). Occurs among elderly people far more readily than in younger adults, often with fatal results. Also used in connection with deliberate reduction of body temperature to 28 °C to permit heart and brain surgery.

A condition characterized by an abnormally low body core temperature. It results in rapid, progressive mental and physical collapse. Hypothermia is caused by exposure to cold when a person is suffering from physical exhaustion, and lack of food. It can be aggravated by wet and windy conditions. Victims have a weak pulse, become increasingly irrational, slow to respond, are cold to the touch, and have speech and visual disturbances. Hypothermia can occur all year round even when the weather is not particularly cold. Those involved in water sports are at particular risk, as are cyclists after hard hill climbs. During the climb they are in danger of overheating, but on the descent they are no longer generating much heat but losing it rapidly due to evaporation accelerated by an increase in relative wind speed. Hardened veterans of the Tour de France can be seen at the top of alpine climbs stuffing newspapers down their front in an effort to reduce heat loss. A person with hypothermia should be warmed in a controlled manner. For mild or moderate hypothermia, warm blankets or a warm bath may be all that is required. Those with severe hypothermia will require professional medical attention. Under no circumstances should the body surface be rubbed or alcohol given; although both may make the victim feel warmer, they can cause further heat loss.

n

Body temperature significantly below normal; that is, 98.6° F, 37° C.

For more information on hypothermia, visit Britannica.com.

An abnormally low body core temperature resulting in a rapid and progressive mental and physical collapse. When the core temperature goes below 34.5 °C, the body loses its ability to regulate temperature. Hypothermia is usually caused by prolonged exposure to cold, especially in wet and windy conditions when a person is suffering from physical exhaustion and lack of food. Immersion in water at temperatures below 32 °C, results in hypothermia developing at a rate proportional to either the duration of exposure or the temperature gradient. Sufferers have a weak pulse because heart rate and cardiac output are reduced. They often become irrational and slow to respond. They are cold to touch, and have speech and visual difficulties. Treatment of mild hypothermia includes warming in a controlled manner with warm blankets or a warm bath. Application of external heat direct to the body surface, enforced exercise, rubbing the skin, or taking alcohol can be dangerous because they can result in further heat losses. Severe hypothermia requires expert medical attention; it can be fatal if not treated properly.

Low body temperature.
Hypothermia may be symptomatic of a disease or disorder of the temperature-regulating mechanism of the body, may be due to exposure to cold, or may be induced for certain surgical procedures or as a therapeutic measure. Exposure to cold is well tolerated by animals except those that have poor peripheral circulation, e.g. cows with milk fever, animals under anesthesia and the newborn. Newborn piglets are particularly susceptible, but high mortalities may also occur in lambs during inclement weather and puppies deprived of warmth from the bitch. Even adult sheep may die of hypothermia if they are exposed to cold, wet, windy weather immediately after shearing.

• induced h. — deliberate reduction of the temperature of all or part of the body; sometimes used as an adjunct to anesthesia in surgical procedures involving a limb, and as a protective measure in cardiac and neurological surgery. The hypothermia may be continued only for the duration of the operation or it may be prolonged depending on the reason for its use.
• neonatal h. — a significant cause of loss especially in lambs born outdoors in changeable weather with a high chill factor combining low temperature, rain and wind and no shelter from the wind. Piglets are the neonates most susceptible to cold.
• symptomatic h. — pathological reduction of body temperature as a result of decreased heat production or increased heat loss. Hypothyroidism, severe blood loss with circulatory failure, and damage to the heat-producing cells of the hypothalamus can lead to decreased heat production.
• therapeutic h. — the application of cold to acute injuries induces vasoconstriction and limits edema and muscle spasm.

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Lifesaving treatments for victims of cold water
Hypothermia is a potentially deadly lowering of the body’s core temperature. It is a threat to any boater who falls into water colder than 85°F (29°C).The colder the water, the quicker is the onset of hypothermia. If you fall into water of 65°F (18°C)—a normal summertime temperature over much of the United States—you will lapse into unconsciousness in about two hours if you’re not wearing protective clothing.Your chances of recovery depend on how long you’ve been in the water. In all cases of hypothermia, the object is to re-warm the body from the inside out, which is a problem on a boat. Signs of mild hypothermia include slurred speech, blue lips, and violent shivering. The victim must be removed from the water and wind and wrapped in warm dry clothes or bedding. Warm, not hot, fluids may be taken, but definitely no alcohol.A victim of medium hypothermia displays loss of muscle control, incoherent speech, drowsiness, exhaustion, or even aggression—most of the symptoms of drunkenness, in fact. Gentle external warming is required. If too much heat is applied externally, the colder blood will be driven to the body core, doing potentially irreparable damage. Warm wet towels on the head, neck, chest, abdomen, and groin are recommended, but these have to be rewarmed frequently. Sharing a sleeping bag is a practical way to provide gentle heat; both victim and volunteer should be naked, with contact mainly at the chest and back.If you can time your breathing so the victim inhales your exhaled breath, some valuable heat will be introduced to the lungs. Do not under any circumstances massage the arms or legs.Severe hypothermia is characterized by collapse and unconsciousness, difficulty in breathing, and heart failure. It is a medical emergency and requiring skilled assistance if possible.Use cardiopulmonary resuscitation (CPR) as a last resource if there has been no pulse or breathing for 1 to 2 minutes. Although CPR may save a life, it can also precipitate heart arrhythmia or arrest in deeply hypothermic patients.It’s unlikely that you have a bathtub on your boat, but if you do, immersing the patient in warm water may be another practical alternative. The suggested temperature is 105°F to 110°F (40°C to 43°C), or slightly warmer if the victim is clothed. It is imperative that the legs and arms be kept out of the water. Adjust the water temperature frequently because the victim will cool it down rapidly.See also HELP Posture

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Dansk (Danish)
n. - hypotermi, unormal lav legemstemperatur

Nederlands (Dutch)
onderkoeling

Français (French)
n. - hypothermie

Deutsch (German)
n. - Hypothermie, Unterkühlung

Ελληνική (Greek)
n. - (παθολ.) υποθερμία

Italiano (Italian)
ipotermia

Português (Portuguese)
n. - hipotermia (f) (Med.)

Русский (Russian)
гипотермия

Español (Spanish)
n. - hipotermia

Svenska (Swedish)
n. - hypotermi (kroppstemperatur under det normala)

中文（简体） (Chinese (Simplified))
降低体温

中文（繁體） (Chinese (Traditional))
n. - 降低體溫

한국어 (Korean)
n. - 저체온, 저체온증

日本語 (Japanese)
n. - 低体温

العربيه (Arabic)
‏(الاسم) حرارة الجسم المخفضه إلى ما تحت المستوى الطبيعي لتعويق العمليات الايضيه وتسيل الجرا‏

עברית (Hebrew)
n. - ‮מיעוט חום (בגוף)‬

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