What Is Hypothermia?

  • Hypothermia is defined as a core, or internal, body temperature of less than 95 F (35 C). Normal body core temperature ranges from about 98 F to 100 F (36.6 C to 37.7 C); core temperature is best measured by a rectal thermometer; do not rely on an oral, ear, axillary (under the armpit), or skin temperature if hypothermia is suspected.
  • Core body temperatures of 95 F (35 C) and lower can cause the heart and nervous system to begin to malfunction and can, in many instances, lead to severe heart, respiratory and other problems that can result in organ damage and death.
  • Hypothermia has been a military problem ever since Hannibal lost nearly half of his troops while crossing the Pyrenees Alps in 218 B.C., and has continued to plague military campaigns through both world wars and the Korean War.
  • The tragic tales of people falling into icy lakes are poignant examples of hypothermia. Anyone exposed to cold temperatures, whether for work or recreation, may be at risk of becoming hypothermic.
  • Today, with the popularity of an expanding number of winter sports and increasing at-risk populations, hypothermia has slowly become a civilian, urban problem.
  • Hypothermia has been used as a technique to help improve neurologic recovery for people in cardiac arrest. This topic can be further examined by readers looking into the references 2 and 3 as these topics are not covered in this article.

What Causes Hypothermia?

Normal body temperature is the reflection of a delicate balance between heat production and heat loss. Many of the chemical reactions necessary for human survival can occur only in specific temperature ranges. The human brain has a number of ways to maintain vital temperature. When these mechanisms are overwhelmed, heat loss happens faster than heat production, which results in hypothermia.

Primary hypothermia is due to exposure to a cold or frigid environment, with no underlying medical condition, causing disruption in temperature regulation:

The body loses heat by several major mechanisms that may occur at the same time.

  • 55% go 65% is lost to the environment via radiation.
  • Conduction only accounts for 2% to 3% in dry conditions, but this figure can increase to 50% if the victim is immersed in cold water.
  • Convection accounts for 10%, while 2% to 9% is lost to heating inspired air.
  • Twenty percent to twenty-seven percent is lost as a result of evaporation from the skin and lungs.
  • Children cool quicker than adults because their skin provides a larger surface area compared to body mass.

The body also has a variety of methods to increase heat production. But at a certain low level, the body cannot continue heat production, and core body temperature drops quickly. From 98.6 F to 89.6 F (30 C to 32 C), the body begins to shiver, blood vessels contract, and hormones are released to facilitate the generation of heat.

  • Shivering can increase heat generation about two to five times the normal body rate of 40 to 60 kcal per square meter of skin. However, this can only last a few hours under mild to moderate freezing conditions and far less in cold water immersion, the time depending on the water temperature and core body temperature. Eventually fatigue sets in, and the body exhausts its fuel stores.
  • Blood vessels contract or narrow in the arms and legs, which allows warm blood to remain internal and somewhat protected from the cold temperatures to which the skin is subjected.
  • Hormones and other small proteins are released in order to speed up the basal metabolic rate, essentially eating stored fuels in the hopes of producing heat as a byproduct.

When the core body temperature is 89.6 F to 75.2 F (32 C to 24 C), shivering stops and basic metabolism progressively slows down. At a body temperature lower than 75.2 F, almost every mechanism for heat conservation becomes inactive. Core body temperature continues to plummet. In primary hypothermia, the body is unable to generate heat fast enough to compensate for ongoing heat losses. This primarily is a disease of exposure.

  • In general, in cold, dry environments, hypothermia occurs over a period of hours.
  • In cold water, core temperature can drop to dangerous levels in a matter of minutes.
  • The elderly, because of their impaired ability to produce and retain heat, may become hypothermic over a period of days while living in indoor, regulated conditions that other people would find comfortable.
  • The homeless, alcoholics, and mentally ill are prone to hypothermia because they are often unable to find adequate shelter or are unable to recognize when it is time to come in from the cold.

Sometimes the body's temperature control can be altered by disease. In this case, core body temperature can decrease in almost any environment. This condition is called secondary hypothermia. In secondary hypothermia, something goes wrong with the body's heat-balancing mechanisms. People with such diseases as stroke, spinal cord injury, low blood sugar, and a variety of skin disorders can become hypothermic in only mildly cool air.

What Are the Symptoms of Hypothermia?

Although the distinctions among mild, moderate, and severe hypothermia are not often clear, a somewhat constant sequence of events occurs as core body temperatures continue to decline.

  • At temperatures below 95 F (35 C), shivering is seen. Heart rate, breathing rate, and blood pressure increase.
  • As the temperature drops further, pulse, breathing rate, and blood pressure all decrease. People may experience some clumsiness, apathy, confusion, and slurred speech.
  • As core temperature drops lower than 89.9 F (32.2 C), shivering stops and oxygen consumption begins to drop. The victim may be in a stupor. The heart rhythm may become irregular.
  • At temperatures below 82.4 F (28 C), reflexes are lost and cardiac output continues to fall. The risk of dangerously irregular heart rhythms increases, and brain activity is seriously slowed. The pupils are dilated, and the victim appears comatose or dead.

When Should I Call the Doctor About Hypothermia?

Individuals may treat minor cold exposure at home with blankets and home care techniques. Call a doctor to ask about danger signs that might warrant immediate transportation to a medical facility.

Any person who is at risk for hypothermia and is suspected to have sustained a cold exposure should be brought to a hospital's emergency department. Look for these danger signs of cold exposure:

  • Intense shivering, stiffness, and numbness in the arms and legs, stumbling and clumsiness, sleepiness, confusion, and amnesia.
  • Removing clothing inappropriately in a cold environment (paradoxical undressing)
  • Extremities feel very cold when touched by a person with a normal body temperature; skin color changes to a bright red.
  • Cold and change in mental status or unresponsiveness
  • Cardiac arrhythmias
  • The medical adage that "a person is not dead until warm and dead" is based on the concept that victims may appear dead because of cold exposure, but many of these people have made complete recoveries when re-warmed. This situation has occurred with both adults and children, and is more likely to have a successful outcome in children, especially if the cold exposure was rapid, for example, falling into an ice-covered pond or pool. Many health care professionals will follow the adage even if the immersion water temperature is much warmer. Child drowning victims are frequently aggressively treated this way, especially if they are hypothermic. All such victims in this situation need rapid transport so that resuscitation attempts may be made.

How Is Hypothermia Diagnosed?

In severe cases of hypothermia, diagnosis and treatment usually will occur at the same time because it is a medical emergency.

  • The doctor will take a history from either the victim, if possible, or from whoever is present. Some vital information includes the length of exposure, the circumstances of recovery, and any past medical problems that may have influenced this episode.
  • Symptoms vary, so the final diagnosis depends on the core body temperature. It is never taken by mouth. The temperature may be measured rectally or by a tube placed in the esophagus. Temperature will be measured continuously, when such devices are available.
  • A number of blood tests will be performed as hypothermia can affect almost every organ system in the body. X-rays, may be ordered, and an ECG (electrocardiogram) will be done to look at the electrical activity of the heart. The patient might be placed on a monitor to continuously observe their heart rate and watch for arrhythmias.

What Is the Treatment for Hypothermia?

The first priority is to perform a careful check for breathing and a pulse and initiate cardiopulmonary resuscitation (CPR) as necessary.

  • If the person is unconscious, having severe breathing difficulty, or is pulseless, call 911 for an ambulance.
  • Because the victim's heartbeat may be very weak and slow, the pulse check should ideally be continued for at least one minute before beginning CPR. Rough handling of these victims may cause deadly heart rhythms.

The second priority is re-warming.

  • Remove all wet clothes and move the person inside.
  • The victim should be given warm fluids if he or she is able to drink, but do not give the person caffeine or alcohol.
  • Cover the person's body with blankets and aluminum-coated foils or other available protective covers (for example, a sleeping bag). Avoid actively heating the victim with outside sources of heat such as radiators or hot water baths. This may only decrease the amount of shivering and slow the rate of core temperature increase.
  • Strenuous muscle exertion should be avoided; rubbing or massaging the limbs and exertion may trigger cardiac arrest in some hypothermic patients.
  • Some cold exposure (borderline hypothermia), such as cold hands and feet, may be treated with home care techniques, but calling a health care professional for advice is recommended.

What Is the Medical Treatment for Hypothermia?

The doctor will first assess for immediate life threats, which are primarily the lack of breathing or a pulse. If the victim is not breathing, he or she will have a tube placed to help them breathe. If the victim does not have a pulse, chest compressions will be started.

If the heart appears on the cardiac monitor to be beating ineffectively (a condition known as ventricular fibrillation), electricity may be applied to the chest using two paddles in an attempt to defibrillate the heart. This procedure may be tried up to three times at first, and then occasionally as the person's temperature begins to climb.

If necessary, a tube will be placed into the trachea to help the patient breathe, and a catheter may be inserted into the bladder to monitor urine output. An IV line will be started, and warmed fluids will be given to treat the dehydration commonly seen in people with hypothermia.

During this time, the process of re-warming is begun. There are three categories of re-warming:

  • Passive external re-warming (PER): This method is ideal for mild hypothermia. In order to be effective, the person must be able to generate enough heat to maintain a good rate of spontaneous re-warming. The victim is placed in a suitably warm environment and covered with insulation. Core temperature is expected to increase a few degrees per hour with this method. At a core temperature below 86 F (30 C), spontaneous shivering is lost. The person has no ability to increase his or her own temperature, and PER is ineffective.
  • Active external re-warming (AER) is a controversial technique in which heat is applied to the skin. Although common sense would suggest that this would be an effective method of re-warming, it has complications. When applied to the entire body, the warmth causes the brain to dilate the blood vessels in the arms and legs from their highly narrowed state. This action can bring cold blood that was previously trapped in the arms and legs back to the core of the body and actually lower its temperature. This same blood may also carry with it a large amount of toxins, including acids, which can flood the core and cause a dangerous acidosis. For these reasons and others, if AER is employed, it is usually directed over the trunk of the body only. Many clinicians only utilize warm air instead of direct warm compresses with AER.
  • Active core re-warming (ACR) is the most effective way to rapidly increase core temperature. It avoids many of the dangers associated with external re-warming. ACR is used when the person's heart is unstable, when body temperature is below 89.9 F (32.2 C), and when the person is re-warming too slowly or not at all or in cases of secondary hypothermia. ACR may be performed in a variety of ways.
    • Airway: Warmed, humidified air is given either through the breathing tube or a closely fitted oxygen mask.
    • Warmed IV fluids are administered
    • Warm fluids are put into the bladder via a Foley catheter
    • Warm fluids are circulated into the peritoneal cavity
    • Peritoneal dialysis: Warmed fluid is placed into the abdomen through an incision and later removed. This cycle is repeated every 20-30 minutes. The major benefit here is that the liver may be quickly rewarmed and thus able to clear the body of toxins.
    • Heated irrigation: Tubes may be placed between the ribs, and heated water applied over the lungs and heart. Its effects are questionable.
    • Diathermy: This is a method in which ultrasound and low-frequency microwave radiation is employed to deliver heat to deeper tissues; it is not used often in environmentally-caused hypothermia.
    • Extracorporeal: Employing one of a variety of methods, blood is circulated from the person's body through a warmer and then back into the bloodstream. This is the most rapid means currently available; however, it is not available in many hospitals.

What Is the Follow-up for Hypothermia?

People who experience accidental hypothermia with body temperatures in the range of 95 F to 89.9F (35 C to 32.2 C) and are otherwise healthy usually re-warm easily and most patients can be safely sent home.

Those whose core temperatures are below 89.9 F (32.2 C) are admitted to the hospital. Underlying medical disorders are investigated and cardiac monitoring performed.

Patients who attempt suicide by hypothermia need a psychiatric referral.

How Do I Prevent Hypothermia?

Prepare well before embarking on any cold weather activities.

  • Be aware of the environmental conditions you will encounter.
  • Make sure people are conditioned physically and have adequate nutrition and rest.
  • Travel with a partner.
  • Wear multiple layers of clothing, loosely fitted. Cover the head, wrists, neck, hands, and feet and try to remain dry; wool, silk or polypropylene layered clothing is better than cotton clothing.
  • In an emergency, drink cold water rather than ice or snow.
  • Be wary of wind and wet weather because they increase the rate of heat loss.
  • Keep the homes of the elderly heated to at least 70 F (21.1 C), especially the sleeping area.

What Is the Prognosis for Hypothermia?

People with accidental hypothermia in the range of 95 F to 89.9 F (35 C to 32.2 C) and who are otherwise healthy usually re-warm easily and can be safely sent home. Those with lower core body temperatures are usually admitted to the hospital.

People with uncomplicated hypothermia do better as a group than do people with hypothermia and another associated disease. In fact, outcome depends more on the underlying disease process than the person's initial temperature or the re-warming method employed.

Age is not always a risk factor, although elderly people tend to have more associated medical problems. People with mild to moderate hypothermia usually have a complete recovery.

People with poor outcomes usually have had a cardiac arrest, a very low or no blood pressure, and the need to have breathing assisted with a tube - all before arriving at the hospital.

Hypothermia Symptoms

Recognizing Hypothermia

Warnings signs of hypothermia:


  • shivering, exhaustion
  • confusion, fumbling hands
  • memory loss, slurred speech
  • drowsiness


  • bright red, cold skin
  • very low energy

CDC.gov. Winter Weather: Hypothermia.

Medically reviewed by Avrom Simon, MD; Board Certified Preventative Medicine with Subspecialty in Occupational Medicine


MedscapeReference.com. Hypothermia.

MedscapeReference.com. Hypothermia for Neuroprotection After Brain Injury Falls Short in Latest Trial.

MedscapeReference.com. The Role of Mild Induced Hypothermia in Cardiac Arrest.