Hypothermia

Hypothermia is often defined as any body temperature below . With this method it is divided into degrees of severity based on the core temperature. • Frostbite: the freezing and destruction of tissue, which happens below the freezing point of water • Frostnip: a superficial cooling of tissues without cellular destruction • Trench foot or immersion foot: a condition caused by repetitive exposure to cold water at non-freezing temperatures 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, Increased urine production due to cold, mental confusion, and liver dysfunction may also be present. 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. 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 . 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. 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. Vasodilation increases blood flow to the skin, resulting in heat being lost to the environment. 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. 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. The actual cause of death in cold water is usually the bodily reactions to heat loss and to freezing water, rather than hypothermia (loss of core temperature) itself. For example, plunged into freezing seas, around 20% of victims die within two minutes from cold shock (uncontrolled rapid breathing and gasping, causing water inhalation; massive increase in blood pressure and cardiac strain, leading to cardiac arrest; and panic, causing ineffectual flailing of the limbs); another 50% die within 15–30 minutes from cold incapacitation (inability to use or control limbs and hands for swimming or gripping, as the body "protectively" shuts down the peripheral muscles of the limbs to protect its core). Exhaustion and unconsciousness cause drowning, claiming the rest within a similar time. ==Pathophysiology==

Heat is primarily generated in muscle tissue, including the heart, and in the liver, while it is lost through the skin (90%) and lungs (10%). Heat production may be increased (to over 1200 W in trained endurance athletes) through muscle contractions (i.e. exercise and shivering). The rate of heat loss is determined, as with any object, by convection, conduction, and radiation. Many changes to physiology occur as body temperatures decrease. These occur in the cardiovascular system leading to the Osborn J wave and other dysrhythmias, decreased central nervous system electrical activity, cold diuresis, and non-cardiogenic pulmonary edema. Research has shown that glomerular filtration rates (GFR) decrease as a result of hypothermia. In essence, hypothermia increases preglomerular vasoconstriction, thus decreasing both renal blood flow (RBF) and GFR. ==Diagnosis==

and Osborn J waves in a person with hypothermia. Note what could be mistaken for ST elevation. Accurate determination of core temperature often requires a special low temperature thermometer, as most clinical thermometers do not measure accurately below . Others recommend a 60-second check. The Osborn J may look very similar to those of an acute ST elevation myocardial infarction. Thrombolysis as a reaction to the presence of Osborn J waves is not indicated, as it would only worsen the underlying coagulopathy caused by hypothermia. == Prevention ==

Staying dry and wearing proper clothing help to prevent hypothermia. Synthetic and wool fabrics are superior to cotton as they provide better insulation when wet and dry. Some synthetic fabrics, such as polypropylene and polyester, are used in clothing designed to wick perspiration away from the body, such as liner socks and moisture-wicking undergarments. Clothing should be loose fitting, as tight clothing reduces the circulation of warm blood. In planning outdoor activity, prepare appropriately for possible cold weather. Those who drink alcohol before or during outdoor activity should ensure at least one sober person responsible for safety is present. Covering the head is effective, but no more effective than covering any other part of the body. While common folklore says that people lose most of their heat through their heads, heat loss from the head is no more significant than that from other uncovered parts of the body. However, heat loss from the head is significant in infants, whose head is larger relative to the rest of the body than in adults. Several studies have shown that for uncovered infants, lined hats significantly reduce heat loss and thermal stress. Children have a larger surface area per unit mass, and other things being equal should have one more layer of clothing than adults in similar conditions, and the time they spend in cold environments should be limited. However, children are often more active than adults, and may generate more heat. In both adults and children, overexertion causes sweating and thus increases heat loss. The United States Coast Guard promotes using life vests to protect against hypothermia through the 50/50/50 rule: If someone is in water for 50 minutes, they have a 50 percent better chance of survival if they are wearing a life jacket. A heat escape lessening position can be used to increase survival in cold water. Babies should sleep at and housebound people should be checked regularly to make sure the temperature of the home is at least . ==Management==

Aggressiveness of treatment is matched to the degree of hypothermia. The UK National Health Service advises against putting a person in a hot bath, massaging their arms and legs, using a heating pad, or giving them alcohol. These measures can cause a rapid fall in blood pressure and potential cardiac arrest. Rewarming Rewarming can be done with a number of methods including passive external rewarming, active external rewarming, and active internal rewarming. Passive external rewarming involves the use of a person's own ability to generate heat by providing properly insulated dry clothing and moving to a warm environment. Passive external rewarming is recommended for those with mild hypothermia. Active external rewarming is recommended for moderate hypothermia. There was a theoretical concern that external rewarming rather than internal rewarming may increase the risk. As most hypothermic people are moderately dehydrated due to cold-induced diuresis, warmed intravenous fluids to a temperature of are often recommended. However, people with severe hypothermia may not respond to pacing or defibrillation. It is not known if further defibrillation should be withheld until the core temperature reaches . In Europe, epinephrine is not recommended until the person's core temperature reaches , while the American Heart Association recommends up to three doses of epinephrine before a core temperature of is reached. Once a temperature of has been reached, normal ACLS protocols should be followed. ==Prognosis==

It is usually recommended not to declare a person dead until their body is warmed to a near normal body temperature of greater than , This is summarized in the common saying "You're not dead until you're warm and dead." Exceptions include if there are obvious fatal injuries or the chest is frozen so that it cannot be compressed. The cold water lowers the metabolism, allowing the brain to withstand a much longer period of hypoxia. While survival is possible, mortality from severe or profound hypothermia remains high despite optimal treatment. Studies estimate mortality at between 38% and 75%. In those who have hypothermia due to another underlying health problem, when death occurs it is frequently from that underlying health problem. ==Epidemiology==

Between 1995 and 2004 in the United States, an average of 1,560 cold-related emergency department visits occurred per year and in the years 1999 to 2004, an average of 647 people died per year due to hypothermia. Of deaths reported between 1999 and 2002 in the US, 49% of those affected were 65 years or older and two-thirds were male. Most deaths were not work related (63%) and 23% of affected people were at home. Hypothermia was most common during the autumn and winter months of October through March. In the United Kingdom, an estimated 300 deaths per year are due to hypothermia, whereas the annual incidence of hypothermia-related deaths in Canada is 8,000. == History ==

retreat from Russia in 1812. '', J. M. W. Turner Hypothermia has played a major role in the success or failure of many military campaigns, from Hannibal's loss of nearly half his men in the Second Punic War (218 B.C.) to the near destruction of Napoleon's armies in Russia in 1812. Men wandered around confused by hypothermia, some lost consciousness and died, others shivered, later developed torpor, and tended to sleep. Others too weak to walk fell on their knees; some stayed that way for some time resisting death. The pulse of some was weak and hard to detect; others groaned; yet others had eyes open and wild with quiet delirium. Civilian examples of deaths caused by hypothermia occurred during the sinkings of the RMS Titanic and RMS Lusitania, and more recently of the MS Estonia. Antarctic explorers developed hypothermia; Ernest Shackleton and his team measured body temperatures "below 94.2°, which spells death at home", though this probably referred to oral temperatures rather than core temperature and corresponded to mild hypothermia. One of Scott's team, Atkinson, became confused through hypothermia. Nazi human experimentation during World War II amounting to medical torture included hypothermia experiments, which killed many victims. There were 360 to 400 experiments and 280 to 300 subjects, indicating some had more than one experiment performed on them. Various methods of rewarming were attempted: "One assistant later testified that some victims were thrown into boiling water for rewarming". In 2024, at least six babies in Gaza Strip died of hypothermia under the harsh rain and cold, which they had to endure in flimsy camps due to the bombing of their homes and forced displacement inflicted upon them by the Israel Defense Forces. Three of the babies died in the coastal zone of Al-Mawasi, which their families had been forced to evacuate to, given its designation as a "safe zone". ==Medical use==

Various degrees of hypothermia may be deliberately induced in medicine for purposes of treatment of brain injury, or lowering metabolism so that total brain ischemia can be tolerated for a short time. Deep hypothermic circulatory arrest is a medical technique in which the brain is cooled as low as 10 °C, which allows the heart to be stopped and blood pressure to be lowered to zero, for the treatment of aneurysms and other circulatory problems that do not tolerate arterial pressure or blood flow. The time limit for this technique, as also for accidental arrest in ice water (which internal temperatures may drop to as low as 15 °C), is about one hour. ==Other animals==

Hypothermia can happen in most mammals in cold weather and can be fatal. Baby mammals such as kittens are unable to regulate their body temperatures and have a risk of hypothermia if they are not kept warm by their mothers. Many animals other than humans often induce hypothermia during hibernation or torpor. Water bears (Tardigrade), microscopic multicellular organisms, can survive freezing at low temperatures by replacing most of their internal water with the sugar trehalose solution, preventing the crystallization that otherwise damages cell membranes. ==See also==