Hypothermia is often regarded as a cold injury, because it can be caused or made worse by exposure to cold surroundings.
Being in an environment that is too cold, having certain disorders, or being unable to move increase the risk of harm caused by hypothermia.
The person shivers at first and later 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.
(See also Overview of Cold Injuries.)
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 (see page Effects of submersion in cold water). 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 (glucose) 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° C).
The very young and the very old are at particular risk for developing hypothermia. 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 if they remain 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.
Initial symptoms include intense shivering and teeth chattering. As body temperature falls further,
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. If they are very slow, the person may seem to have no signs of life (no heartbeat or attempts to breathe) even though the heart is beating very weakly. Eventually the heart does stop.
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).
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.
Disorders (such as heart and blood vessel disorders, more common in older people) can also make the body less efficient at shivering and diverting blood and make people less able to compensate for the stress of hypothermia. The body’s ability to produce heat is also decreased by disorders that commonly affect older people, such as underactivity of the thyroid gland (hypothyroidism). 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, particularly when cold. Exercise can increase the body’s production of heat.
Doctors diagnose hypothermia by measuring a body temperature less than 95° F (35° C), typically with a rectal thermometer. Conventional thermometers do not record temperatures below 94° F (about 34° C). Thus, electronic 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. If a person has no signs of life, doctors may use cardiac ultrasonography to determine whether the heart is still beating.
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) outside of a hospital is not recommended, particularly by bystanders, if there are any signs of life, which may be very difficult to detect. For example, it may be difficult, particularly 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. Also, 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).
Doctors may need to help the person breathe by inserting a plastic breathing tube through the mouth into the windpipe (endotracheal intubation) and using mechanical ventilation. If the heart has stopped, CPR is done.
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 heartbeat or other signs of life.