Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe hypothermia, there may be hallucinations and paradoxical undressing, the victim's self-removal of their own clothing, as well as an increased risk of the heart's stopping.
Hypothermia has two main types of causes. It classically occurs from exposure to cold weather and cold water immersion. It may also occur from any condition that decreases heat production or increases heat loss. Commonly, this includes alcohol intoxication but may also include low blood sugar, anorexia, and advanced age. Body temperature is usually maintained near a constant level of through thermoregulation. Efforts to increase body temperature involve shivering, increased voluntary activity, and putting on warmer clothing. Hypothermia may be diagnosed by either a person's symptoms in the presence of risk factors or measuring a person's core temperature.
The treatment of mild hypothermia involves warm drinks, warm clothing, and voluntary physical activity. In those with moderate hypothermia, heating blankets and warmed intravenous fluids are recommended. People with moderate or severe hypothermia should be moved gently. In severe hypothermia, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass may be useful. In those without a pulse, cardiopulmonary resuscitation (CPR) is indicated along with the above measures. Rewarming is typically continued until a victim's temperature is greater than . If there is no improvement at this point or the blood potassium level is greater than 12 millimoles per litre at any time, resuscitation may be discontinued.
Hypothermia is the cause of at least 1,500 deaths a year in the United States alone. It is more common in older people and men. One of the lowest documented body temperatures which someone with accidental hypothermia has survived is in a 2-year-old boy from Poland named Adam. Survival after more than six hours' CPR has been described. When ECMO or bypass is used, survival is around 50%. Deaths due to hypothermia have played an important role in many wars.
The term is from Greek ῠ̔πο (ypo), meaning "under", and θέρμη (thérmē), meaning "heat". The opposite of hypothermia is hyperthermia, an increased body temperature due to failed thermoregulation.
Classification + Hypothermia classificationSwiss systemSymptomsBy degreeTemperatureStage 1Awake and shivering Mild Stage 2Drowsy and not shivering Moderate Stage 3Unconscious, not shivering Severe Stage 4No vital signs Profound <
Hypothermia is often defined as any body temperature below . With this method it is divided into degrees of severity based on the core temperature.
Another classification system, the Swiss staging system, divides hypothermia on the basis of the presenting symptoms, as is preferred when it is not possible to determine an accurate core temperature.
Other cold-related injuries that can be present either alone or in combination with hypothermia include:
Chilblains: condition caused by repeated exposure of skin to temperatures just above freezing. The cold causes damage to small blood vessels in the skin. This damage is permanent and the redness and itching will return with additional exposure. The redness and itching typically occurs on cheeks, ears, fingers, and toes. Frostbite: the freezing and destruction of tissue, which happens below the freezing point of water Frostnip: a superficial cooling of tissues without cellular destructionMarx 2010 p.1862 Trench foot or immersion foot: a condition caused by repetitive exposure to cold water at non-freezing temperatures
The normal human body temperature is often stated as . Hyperthermia and fevers are defined as a temperature of greater than .
Signs and symptoms Signs and symptoms vary depending on the degree of hypothermia, and may be divided by the three stages of severity. People with hypothermia may appear pale and feel cold to touch. Infants with hypothermia may feel cold when touched, with bright red skin and an unusual lack of energy.
Behavioural changes such as impaired judgement, impaired sense of time and place, unusual aggression, and numbness can be observed in individuals with hypothermia; they can also deny their condition and refuse any help. A hypothermic person can be euphoric and hallucinating.
Cold stress refers to a near-normal body temperature with low skin temperature; signs include shivering. Cold stress is caused by cold exposure and can lead to hypothermia and frostbite if not treated.
Mild Symptoms of mild hypothermia may be vague, with sympathetic nervous system excitation (shivering, high blood pressure, fast heart rate, fast respiratory rate, and contraction of blood vessels). These are all physiological responses to preserve heat. Increased urine production due to cold, mental confusion, and liver dysfunction may also be present.Marx 2010 p.1869 Hyperglycemia may be present, as glucose consumption by cells and insulin secretion both decrease, and tissue sensitivity to insulin may be blunted. Sympathetic activation also releases glucose from the liver, but in many cases this appears to result more often from hypoglycemia, especially in people with alcoholic intoxication. Hypoglycemia is also found in many people with hypothermia, as hypothermia may be a result of hypoglycemia,eMedicine Specialties > Emergency Medicine > Environmental >Hypothermia Author: Jamie Alison Edelstein, MD. Coauthors: James Li, MD; Mark A Silverberg, MD; Wyatt Decker, MD. Updated: October 29, 2009 which dysregulates body temperature by decreasing energy demand and impairs judgment.
Moderate As hypothermia progresses, symptoms include amnesia, confusion, slurred speech, decreased reflexes, and loss of fine motor skills.
Severe As the temperature decreases, further physiological systems falter and heart rate, respiratory rate, and blood pressure all decrease. This results in an expected heart rate in the 30s at a temperature of .
There is often cold, inflamed skin, hallucinations, lack of reflexes, fixed dilated pupils, low blood pressure, pulmonary edema, and shivering is often absent. Pulse and respiration rates usually decrease significantly, but fast heart rates (ventricular tachycardia, atrial fibrillation) can also occur. Atrial fibrillation is not typically a concern in and of itself.
Paradoxical undressing Twenty to fifty percent of hypothermia deaths are associated with paradoxical undressing. This typically occurs during moderate and severe hypothermia, as the victim becomes disoriented, confused, and combative. They may begin discarding their clothing, so increasing the rate of heat loss.
Rescuers who are trained in mountain survival techniques are taught to expect this; however, people who die from hypothermia in urban environments and are found undressed are sometimes incorrectly assumed to have been subjected to sexual assault.
One explanation for the effect is a cold-induced malfunction of the hypothalamus, the part of the brain that regulates body temperature. Another explanation is that the muscles contracting peripheral blood vessels become exhausted (known as a loss of vasomotor tone) and relax, leading to a sudden surge of blood (and heat) to the extremities, causing the victim to feel overheated.
Terminal burrowing An apparent self-protective behaviour, known as "terminal burrowing", or "hide-and-die syndrome", occurs in the final stages of hypothermia. Those affected will enter small, enclosed spaces, such as underneath beds or behind wardrobes. It is often associated with paradoxical undressing. Researchers in Germany claim this is "obviously an autonomous process of the brain stem, which is triggered in the final state of hypothermia and produces a primitive and burrowing-like behavior of protection, as seen in hibernating mammals". This happens mostly in cases where temperature drops slowly.
Causes
Hypothermia usually occurs from exposure to low temperatures, and is frequently complicated by alcohol consumption. Any condition that decreases heat production, increases heat loss, or impairs thermoregulation, however, may contribute. Thus, hypothermia risk factors include: substance use disorders (including alcohol use disorder), homelessness, any condition that affects judgment (such as hypoglycemia), the extremes of age, poor clothing, chronic medical conditions (such as hypothyroidism and sepsis), and living in a cold environment. Hypothermia occurs frequently in major trauma, and is also observed in severe cases of anorexia nervosa. Hypothermia is also associated with worse outcomes in people with sepsiswhile most people with sepsis develop fevers (elevated body temperature), some develop hypothermia.
In urban areas, hypothermia frequently occurs with chronic cold exposure, such as in cases of homelessness, as well as with immersion accidents involving alcohol, other drugs, or mental illness. While studies have shown that people experiencing homelessness are at risk of premature death from hypothermia, the true incidence of hypothermia-related deaths in this population is difficult to determine. In more rural environments, the incidence of hypothermia is higher among people with significant comorbidities and less ability to move independently. With rising interest in wilderness exploration and outdoor and water sports, the incidence of hypothermia secondary to accidental cold exposure may become more frequent in the general population.
Alcohol Alcohol consumption increases the risk of hypothermia in two ways: vasodilation and dysregulation of temperature-controlling systems in the brain. Vasodilation increases blood flow to the skin, resulting in heat being lost to the environment. This produces the effect of feeling warm, when one is actually losing heat. Alcohol also affects the temperature-regulating system in the brain, decreasing the body's ability to shiver and use energy that would normally aid the body in generating heat. The overall effects of alcohol lead to a decrease in both body temperature and ability to generate body heat in response to cold environments. Alcohol is a common risk factor for death due to hypothermia. Between 33% and 73% of hypothermia cases are complicated by alcohol.
Water immersion
Hypothermia continues to be a major limitation to swimming or diving in cold water. The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries.
Other factors predisposing to immersion hypothermia include dehydration, inadequate rewarming between repetitive dives, starting a dive while wearing cold, wet dry suit undergarments, sweating with work, inadequate thermal insulation, and poor physical conditioning.
Heat is lost much more quickly to water than to air. Thus, water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia, although this is not usually the direct clinical cause of death for those not rescued from accidental immmersion in cold water. A water temperature of can kill a human adult in as little as one hour, and water temperatures near freezing can kill in as little as 15 minutes. During the sinking of the Titanic, most people who entered the water died in 15–30 minutes.