Hypothermia is a dangerously low body temperature.

• Being surrounded by too cold of an environment, having certain disorders, being unable to move, or a combination can cause body temperature to become too low.
• The person shivers but then may become confused and lose awareness.
• Getting warm and dry can lead to recovery unless the body temperature is very low.
• If the body temperature is very low, doctors may warm the person with warmed oxygen and heated fluids given intravenously or passed into the bladder, stomach, abdominal cavity or chest cavity through plastic tubes. Doctors also provide heat to the outside of the body.

Hypothermia causes about 600 deaths each year in the United States. Hypothermia also increases the risk of death in people with heart, blood vessel, and nerve disorders.

Hypothermia results when the body loses more heat than can be replaced by increasing the amount of heat generated by the body through exercise or by increasing warming from external sources, such as a fire or the sun. Wind increases heat loss, as does sitting or lying on a cold surface or being immersed in water. Sudden immersion in very cold water may cause fatal hypothermia in 5 to 15 minutes. However, a few people, mostly infants and young children, have survived for as long as 1 hour completely submerged in ice water. The shock can shut off all systems, essentially protecting the body. Hypothermia may also occur after prolonged exposure in only moderately cool water.

People at greatest risk are those who are lying immobile in a cold environment—such as people who have had a stroke or a seizure or who are unconscious due to intoxication, those with a low blood sugar level, or those with an injury. Because they are not moving, these people generate less heat and also are unable to leave the cold environment.Such people are at risk of becoming hypothermic even when the surrounding temperature may be only as cold as 55 or 60° F (about 13 to 16° Celsius [C]). The very young and the very old are at particular risk. People in these age groups often do not compensate for cold as well as young adults and are dependent on others to anticipate their needs and keep them warm. Very old people may become hypothermic while indoors from sitting immobile in a cold room for hours. Infants lose body heat rapidly and are particularly susceptible to hypothermia. Sometimes a disorder, such as a widespread infection or underactivity of the thyroid gland (hypothyroidism), causes or contributes to hypothermia.

Symptoms

Initial symptoms include intense shivering and teeth chattering. As body temperature falls further, shivering stops and movements become slow and clumsy, reaction time is longer, thinking is blurred, and judgment is impaired. These symptoms may develop so gradually that people, including companions of the affected person, do not realize what is happening. People may fall, wander off, or simply lie down to rest. When shivering stops, people become more sluggish and slip into a coma. The heart and breathing rates become slower and weaker. Eventually the heart stops.

The lower the body temperature is, the higher the risk of death. Death may occur at body temperatures below 88° F (about 31° C) but is most likely to occur below 83° F (about 28° C).

Spotlight on Aging
Aging takes a toll on the body's ability to adapt to the cold. With aging, the body becomes less efficient at shivering and at diverting blood away from the surface of the body. Also, the layer of fat just under the skin thins, so there is less insulation to prevent heat loss. The body's ability to produce heat is also decreased by disorders that commonly affect older people, such as hypothyroidism. The body's ability to retain heat is decreased by disorders such as diabetes. A person who is less able to move around because of an injury or a disorder such as a stroke or arthritis is also at a greater risk of dangerous cooling, because the decreased movement generates less heat-producing muscle activity. Alcohol and certain drugs, such as antidepressants, increase the risk as well. Hypothermia is almost always preventable. Older people are advised to take the following precautions: Maintain a warm environment. Older people sometimes keep their home at a lower-than-desirable temperature as a means of saving money, but the thermostat should be set at 68° F or higher. It is especially important that the bedroom be kept warm. Fuel assistance programs and home winterization programs may help defray costs. Wear several layers of clothing. Clothing made of wool or synthetic materials such as polypropylene are especially useful because these materials insulate even when they become wet. Because the body loses a large amount of heat from the head, wearing a hat is important. Fingers and toes must also be protected. Eat warm foods and drink warm fluids. Food provides the body with fuel to be burned, and warm fluids provide heat and prevent dehydration. Avoid alcoholic beverages. Alcohol dilates blood vessels in the skin, which makes the body temporarily feel warm but actually causes greater heat loss. Exercise regularly. Exercise can increase the body's production of heat.

Diagnosis

Doctors diagnose hypothermia by measuring body temperature, typically with a rectal thermometer. Conventional thermometers do not record temperatures below 94° F (about 34° C). Thus, special thermometers are needed to measure temperatures in severe hypothermia. Blood and sometimes other tests are done to see whether a disorder such as an infection or hypothyroidism caused hypothermia.

Treatment

In the early stages, drying the body, changing into warm, dry clothing, being covered with warm blankets, and drinking hot beverages can bring about recovery. In people who are found unconscious, further heat loss is prevented by wrapping them in a warm, dry blanket and, if possible, removing wet clothing and moving them to a warm place while arrangements are made for immediate transportation to a hospital. Cardiopulmonary resuscitation (CPR) is only recommended outside of a hospital after very careful consideration for the following reasons:

• It is difficult for untrained people to detect very faint respirations and heartbeats.
• Often, even if no pulse can be felt and no heartbeat can be heard, the heart may be beating. Applying chest compressions to a cold beating heart often disturbs heart rhythm, resulting in death.
• A severely hypothermic person must be handled gently, because a sudden jolt may cause an irregular heart rhythm (arrhythmia) that could be fatal.

In the hospital, doctors warm the person with warmed oxygen given by inhalation and heated fluids given intravenously or passed into the bladder, stomach, abdominal cavity, or chest cavity through plastic tubes inserted into those areas. In addition, the blood may be warmed through the process of hemodialysis (in which the blood is pumped out of the body, through a filter with a heating attachment, and back into the body) or with a heart-lung machine (which pumps blood out of the body, heats the blood, adds oxygen, and then returns the blood to the body).

Because some people with hypothermia who have arrived at the hospital with no signs of life have recovered, doctors may continue resuscitation efforts until the person is warmed but still shows no signs of life. If certain blood test results are extremely abnormal, the person is already dead.

Last full review/revision September 2012 by Daniel F. Danzl, MD