Multiple: Hypothermia

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Multiple: Hypothermia

Causes and Symptoms
The Future
For more information


Hypothermia is defined as a subnormal body temperature caused by the loss of more body heat than the body can replace. There are several different types of hypothermia. Accidental hypothermia is caused by exposure to cold weather, while intentional or induced hypothermia is a medical technique used to increase a patient's chances of recovery after stroke or cardiac arrest. Primary hypothermia is caused by exposure to a cold or wet environment; secondary hypothermia refers to lowering

of body temperature caused by such diseases as Parkinson disease, multiple sclerosis, or brain tumors. Many elderly people with hypothermia have the secondary form.


The normal temperature of the human body is about 97.9°F (36.6°C). Body temperature is regulated by a part of the brain called the hypothalamus. When a person begins to feel cold, the hypothalamus releases chemicals to step up heat production from the body's energy stores. The muscles begin to shiver, which also releases heat. Shivering increases the body's rate of heat production by two to five times.

The hypothalamus also works to control heat loss by slowing down the flow of blood to the arms, hands, legs, and feet. This process is known as vasoconstriction. Vasoconstriction protects the body by helping to keep the heart and other vital organs in the body core functioning as effectively as possible. In most cases, the person will feel uncomfortable enough to go back indoors or put on more clothing. If the person cannot find shelter or otherwise get warm, the person'scentral nervous system

(CNS) will eventually be affected and the hypothalamus will no longer be able to regulate the body's temperature. When temperature regulation fails, the heart and other vital organs can no longer function properly.

Hypothermia may come on gradually; the person may not know that his or her body temperature is dropping, though others may notice the person is becoming irritable, slurring their speech, becoming clumsy in their movements, or showing poor judgment. The rate at which a person develops hypothermia depends on basic health, environmental temperature and humidity, and the warmth and dryness of their clothing.

Doctors distinguish three stages of hypothermia according to severity:

  • Mild hypothermia: body temperature between 95–90°F (35–32°C).
  • Moderate hypothermia: body temperature between 90–82°F (32–28°C).
  • Severe hypothermia: body temperature below 82°F (28°C).

An Evil Experiment

In the early 1940s Nazi doctors conducted cruel experiments on Russian prisoners of war and concentration camp inmates to see how long it takes people to freeze to death and to try various ways to rewarm people with hypothermia. The reason for these experiments was the high casualty rate suffered by German soldiers during the winter of 1941–1942 following Hitler's invasion of the Soviet Union.

The men used for these experiments were exposed to cold either by being put in vats of icy water or tied naked to stretchers and placed outside in freezing weather. The prisoners put in cold water had probes inserted into their rectums that measured the drop in their body temperature; it was found that they died when their core temperature dropped to 77°F (25°C).

The methods used to warm the men were just as cruel: using sun lamps so hot they burned the skin and forcing scalding-hot water into the stomach, intestines, or bladder. None of the prisoners survived these rewarming treatments.

One of the doctors who conducted these experiments, Sigmund Rascher, was shot on orders of the Nazi leaders in April 1945 before the Allies liberated the concentration camps. Some doctors involved in the experiments were tried for war crimes in 1947 and either executed or sentenced to long prison terms.


The true number of people who develop hypothermia in the United States each year is not known; doctors think that those who are taken to hospital emergency rooms are only a small fraction of the actual number of cases. According to the Centers for Disease Control and Prevention, between 650 and 700 people die from hypothermia each year in the United States, or about 0.2 persons per 100,000. Cold-related deaths are twice as common in men as in women. Fifty-two percent of the victims were over the age of sixty-five. The states with the highest rates of deaths from hypothermia are Alaska, New Mexico, North Dakota, Montana, North and South Carolina, and Arizona. Some states have higher-than-average rates because

they experience rapid temperature changes at certain times of the year or have mountainous regions with rapid changes in overnight temperatures.

Some people are at greater risk of hypothermia, particularly infants and young children and the elderly. People with diabetes and other illnesses that affect blood circulation are also more likely to develop hypothermia when exposed to cold.

Causes and Symptoms

The most common causes of hypothermia are loss of body heat due to exposure to cold weather or to a combination of cold and dampness. Swimmers or people who fall into a body of water can develop hypothermia even when the water is not icy cold. According to the U.S. Coast Guard, a person who falls into water at 40°F (4°C) will become unconscious in about half an hour and die within another hour; at 32°F (0°C), survival time drops to fifteen minutes.

The symptoms of hypothermia include mental as well as physical symptoms:

  • Mild hypothermia: The person shivers vigorously and begins to show confusion and poor judgment. They become moody, may slur their speech, move clumsily, and start breathing heavily.
  • Moderate hypothermia: The person's breathing slows down, central nervous activity is lowered and the body loses its ability to generate additional heat by shivering. The heart may develop an abnormal rhythm. A symptom sometimes seen at this stage is paradoxical undressing, in which the person becomes confused, disoriented, and starts to remove their clothing—which speeds up the rate of heat loss. Between 20 and 50 percent of deaths from hypothermia are thought to result from paradoxical undressing.
  • Severe hypothermia: The risk of heart failure increases; the person may go into coma and be unresponsive when touched. There may be no pulse and the blood pressure may be abnormally low. The lungs may fill with fluid and breathing may be difficult.


Diagnosis of hypothermia is usually based on a measurement of body temperature in the emergency room, as many of the physical and mental

symptoms of hypothermia can be caused by such other conditions as stroke, alcohol intoxication, medication side effects (common in the elderly), and mental illness. The use of a special low-temperature probe inserted into the rectum or the bladder is thought to give a more accurate reading than a standard thermometer.


The first line of emergency treatment for hypothermia includes preventing further heat loss, raising the body core temperature, and preventing heart failure. To lower the risk of the person's developing an abnormal heart rhythm, rescuers are advised to move the person as gently as possible and to begin rewarming the person in the field before taking him or her to the hospital. Wet clothing is removed and replaced with dry clothing and blankets or a dry sleeping bag. Hot water bottles or chemical heat packs are used to warm the body; in extreme emergencies, rescuers can warm the person by skin-to-skin contact.

In the hospital, emergency room doctors will take the person's core temperature to determine the severity of the hypothermia; the lower the body temperature, the more careful the doctors must be in rewarming the patient. Sometimes the person's core temperature continues to drop after rewarming is started; this complication, known as after drop, is thought to result from cooler blood in the patient's extremities being recirculated back into the body's core organs during rewarming.

Various techniques have been used to rewarm people with hypothermia, ranging from wrapping the patient in heated blankets, immersing him or her in warm water in a device known as a Hubbard tank, or giving warmed and humidified oxygen through a face mask or endotracheal tube. Another method that is used is injection of intravenous fluids heated to 113°F (45°C).


The prognosis for recovery from hypothermia depends on its severity. Most people survive mild hypothermia without significant after-effects. The death rate for moderate hypothermia is close to 21 percent; for severe hypothermia, it is close to 40 percent.

Hypothermia can lead to a number of long-term health complications, including frostbite, pneumonia, other infections, disorders of the pancreas and bladder, and lung damage.


Accidental hypothermia can be prevented by dressing sensibly for cold-weather activities and by avoiding the use of alcohol and other substances that interfere with good judgment. In the summer, people should be careful not to drink before operating a boat, as falling into water even in summertime temperatures can still cause hypothermia.

In addition, persistent shivering is a sign that the body is losing too much heat; this is an important signal to go back inside as soon as possible. A tip that can help people remember how to dress for winter is the word COLD:

  • Cover: Keep head, neck, and face covered with a warm hat, hood, or scarf. Mittens are better than gloves for protecting the hands because they keep the fingers closer together.
  • Overexertion: Avoid exercise or other activities that cause heavy sweating, because the combination of moisture on the skin, clothing that becomes damp from sweat, and the cold outside can cause rapid loss of body heat.
  • Layer: Loose-fitting layered clothing holds in body heat better than tight-fitting garments. Water-repellent outerwear is a good choice for wet and windy weather.
  • Dry: People should stay as dry as possible outdoors, and check mittens and boots from time to time to make sure that snow cannot get inside and melt. It is a good idea to pack an extra pair of dry socks and mittens just in case.

The CDC recommends carrying emergency supplies of food (granola and crackers are good choices), blankets, matches and candles, and extra clothing in the car during the winter in case the car stalls or is stranded in snow. Additional safety precautions include checking the weather forecast before setting out on a trip and letting others know the expected arrival time.

The Future

Hypothermia is thought to be a growing problem in North America because of the increasing numbers of people participating in outdoor sports as well as the increasing numbers of mentally ill and substance-addicted homeless people. Public health doctors and social workers often have trouble convincing homeless people to go into public shelters in the

cold weather, however, because they claim they are afraid of drug abusers who also use the shelters.

SEE ALSO Frostbite


After drop: A term that doctors use to refer to lowering of the body's core temperature that continues while the person is being rewarmed.

Hypothalamus: The part of the brain that controls body temperature, hunger, thirst, and response to stress.

Paradoxical undressing: A symptom sometimes seen in people with moderate or severe hypothermia, thought to be caused by a malfunction of the hypothalamus. The person becomes confused, disoriented, and begins to remove clothing.

Vasoconstriction: A narrowing of the blood vessels in response to cold or certain medications.

For more information


Centers for Disease Control and Prevention (CDC). Extreme Cold: A Prevention Guide to Promote Your Personal Health and Safety. Atlanta, GA: CDC, 2007. Available online in PDF format at (accessed November 5, 2008).

Giesbrecht, Gordon G. Hypothermia, Frostbite, and Other Cold Injuries: Prevention, Survival, Rescue and Treatment, 2nd ed. Seattle, WA: Mountaineers Books, 2006.


Fagan, Kevin. “Shame of the City: A Rugged Refuge.” San Francisco Chronicle, December 2, 2003. Available online at (accessed April 3, 2008). This article is the third of a five-part series on homelessness in San Francisco.

McNeil, Donald G. “Chill Therapy Is Endorsed for Some Heart Attacks.” New York Times, July 8, 2003. Available online at (accessed April 3, 2008). This article describes the use of induced hypothermia in the treatment of people with stroke or cardiac arrest.

WEB SITES Hypothermia, (accessed November 5, 2008).

Nova. Deadly Ascent. (accessed November 5, 2008). This is a website with interactive features that explores hypothermia and other health conditions related to mountain climbing.

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Multiple: Hypothermia

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