Hypothermia
Classification and external resources
ICD-10	T68.
ICD-9	991.6
DiseasesDB	6542
eMedicine	med/1144
MeSH	D007035

This article is about the adverse condition of Hypothermia. For the deliberately induced medical condition, see Therapeutic hypothermia.

Hypothermia is a condition in which an organism's temperature drops below that required for normal metabolism and body functions. In warm-blooded animals, core body temperature is maintained near a constant level through biologic homeostasis or thermoregulation. However, when the body is exposed to cold, its internal mechanisms may be unable to replenish the heat that is being lost to the organism's surroundings. Extended exposure leads to death.

Hypothermia is the opposite of hyperthermia which is present in heat exhaustion and heat stroke.

Contents 1 Classification 2 Signs and symptoms 2.1 Immersion hypothermia 2.2 Paradoxical undressing 3 Causes 4 Pathophysiology 5 Diagnosis 6 Prevention 7 Management 8 History 9 Society 10 In other animals 11 References 12 External links

[edit] Classification

Hypothermia is defined as any body temperature below 35.0 °C (95.0 °F). It is subdivided into four different degrees, mild around 32–35 °C (90–95 °F); moderate, 28–32 °C (82–90 °F); severe, 20–28 °C (68–82 °F); and profound at less than 20 °C (68 °F).^[1] This is in contrast with hyperthermia or fever which is defined as a rectal temperature of greater than 37.8 °C (100.0 °F). Other cold-related injuries that can either present alone or in combination with hypothermia include:

• Chilblains are superficial ulcers of the skin that occur when a predisposed individual is repeatedly exposed to cold.
• Frostbite involves the freezing and destruction of tissue.
• Frostnip is a superficial cooling of tissues without cellular destruction.^[2]
• Trench foot or immersion foot is due to repetitive exposure to wet, non-freezing temperatures.

[edit] Signs and symptoms

Normal body temperature in humans is 36.5–37.5 °C (98–100 °F). A number of skin conditions may be associated with hypothermia or may occur with normal body temperature. These include: cryopedis and frostbite. Symptoms of hypothermia may be divided into the three stages of severity.

• Stage 1

Body temperature drops by 1–2 °C (1.8–3.6 °F) below normal temperature (down to 35–37 °C or 95–98.6 °F). Mild to strong shivering occurs.^[3][4] The victim is unable to perform complex tasks with the hands; the hands become numb. Blood vessels in the outer extremities constrict, lessening heat loss to the outside air. Breathing becomes quick and shallow. Goose bumps form, raising body hair on end in an attempt to create an insulating layer of air around the body (which is of limited use in humans due to lack of sufficient hair, but useful in other species). Victim may feel sick to their stomach, and very tired. Often, a person will experience a warm sensation, as if they have recovered, but they are in fact heading into Stage 2. Another test to see if the person is entering stage 2 is if they are unable to touch their thumb with their little finger; this is the first stage of muscles not working. They might start to have trouble seeing.

• Stage 2

Body temperature drops by 2–4 °C (3.8–7.6 °F) below normal temperature (33–35 °C or 91–94.8 °F). Shivering becomes more violent. Muscle mis-coordination becomes apparent.^[3][4][5] Movements are slow and labored, accompanied by a stumbling pace and mild confusion, although the victim may appear alert. Surface blood vessels contract further as the body focuses its remaining resources on keeping the vital organs warm. The victim becomes pale. Lips, ears, fingers and toes may become blue.

• Stage 3

Body temperature drops below approximately 32 °C (89.6 °F). Shivering usually stops.^[3][4] Difficulty speaking, sluggish thinking, and amnesia start to appear; inability to use hands and stumbling is also usually present. Cellular metabolic processes shut down. Below 30 °C (86.0 °F), the exposed skin becomes blue and puffy, muscle coordination becomes very poor, walking becomes almost impossible, and the victim exhibits incoherent/irrational behavior including terminal burrowing or even a stupor. Pulse and respiration rates decrease significantly, but fast heart rates (ventricular tachycardia, atrial fibrillation) can occur. Major organs fail. Clinical death occurs. Because of decreased cellular activity in stage 3 hypothermia, the body will actually take longer to undergo brain death.

[edit] Immersion hypothermia

Hypothermia of both the extremities and body core continues to be a major limitation to diving in cold water.^[3] Cooling in the extremities is often the limitation to operations.^[3] The limitation of finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries.^[3][5]

For divers breathing heliox below 100 meters wearing hot water suits, the inhaled gas must be heated, or the symptoms of hypothermia can set in without the divers realizing it.^[3]

Other predisposing factors leading to immersion hypothermia include dehydration, inadequate rewarming with repetitive operations, starting operations while wearing cold, wet dry suit undergarments, sweating with work, inadequate thermal insulation (for example, thin dry suit undergarment), lack of heated breathing gas with deep heliox diving, and poor physical conditioning.^[3]

Moderate and severe cases of hypothermia require immediate hospitalization. In a hospital, external treatments, such as heated blankets are used to warm patients with mild hypothermia, as well as internal treatments such as injected warm fluids. For severe cases of hypothermia, patients may undergo lavage (washing) of the bladder, stomach, chest and abdominal cavities with warmed fluids.^[6] These patients are at high risk for arrhythmias (irregular heartbeats), and care must be taken to minimize jostling and other disturbances until they have been sufficiently warmed, as these arrhythmias are very difficult to treat while the victim is still cold.

An important tenet of treatment is that a person cannot be considered dead until he/she has been adequately warmed. Remarkable accounts of recovery after prolonged cardiac arrest have been reported in patients with hypothermia, such as children who have been submerged in cold water for more than 15 minutes (called mini-hibernation).^[7] It is presumed that this is because the low temperature prevents some of the cellular damage that occurs when blood flow and oxygen are lost for an extended period of time.

There is considerable evidence that children who suffer near-drowning accidents in water near 0°C (32°F) can be revived over an hour after losing consciousness.^[8] The cold water considerably lowers metabolism, allowing the brain to withstand a much longer period of hypoxia.

[edit] Paradoxical undressing

Twenty to fifty percent of hypothermal deaths are associated with a phenomenon known as paradoxical undressing. This typically occurs during moderate to severe hypothermia, as the victim becomes disoriented, confused, and combative. The hypothermic victim may begin discarding their clothing, which, in turn, increases the rate of heat loss.^[9] There have been several published case studies of victims throwing off their clothes before help reached them.^[10]

Rescuers that are trained in mountain survival techniques have been taught to expect this effect. However, the phenomenon still regularly leads police to assume incorrectly that urban victims of hypothermia have been subjected to a sexual assault. (M. A. Rothschild et al., "Terminal burrowing behavior", p. 1)

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, fooling the victim into feeling overheated.

[edit] Causes

Hypothermia usually occurs from exposure to low temperatures however anything which decreases heat production, increases heat loss, or impairs thermoregulation can cause it.^[11] It is well recognized in major trauma.

[edit] Pathophysiology

Many changes to physiology occur as body temperature decreases. These occur in the cardiovascular system leading to the Osborn wave and other dysrhythmias, the CNS leading to decreased electrical activity, cold diuresis, and non-cardiogenic pulmonary edema.^[12]

[edit] Diagnosis

Accurate determination of core temperature often require a special low temperature thermometer placed rectally, esophageal, or in the bladder. An ECG may show the classic finding of hypothermia, an Osborn wave.

[edit] Prevention

Appropriate clothing helps to prevent hypothermia. Wearing cotton in chilly weather is a particular hypothermia risk as it retains water, and water quickly conducts heat away from the body. Even in dry weather, cotton clothing can become damp from perspiration, and chilly after the wearer stops exercising. Synthetic and wool fabrics provide far better insulation when wet and dry more quickly. 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.

Heat loss on land is very difficult to predict due to multiple variables such as clothing type and quantity, amount of insulating fat on the victim, environmental humidity or personal dampness such as after exertion, the circumstances surrounding the hypothermic episode, and so on. Heat is lost much faster in water, hence the need for wetsuits or drysuits in cold-weather activities such as kayaking.^[3] Water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia very quickly. For example, a water temperature of 10 °C (50 °F) can be expected to lead to death in approximately 1 hour, and water temperatures hovering at freezing can lead to death in as little as 15 minutes.^[13] On the other end of the scale, in water even a temperature as high as 26 °C (80 °F) may eventually (after many hours) lead to mild hypothermia.^[14]

Alcohol consumption prior to cold exposure may increase one's risk of becoming hypothermic. Alcohol acts as a vasodilator, increasing blood flow to the body's extremities, thereby increasing heat loss.^[15] Ironically, this may cause the victim to feel warm while rapidly losing heat to the surrounding environment.

The United States Coast Guard promotes using life vests as a method of protection against hypothermia through the 50/50/50 rule: If someone is in 50 °F water for 50 minutes, he/she has a 50 percent better chance of survival if wearing a life jacket.^[16] The heat escape lessening position can be used to increase survival in cold water.

[edit] Management

Treatment involves removal from the cold environment along with simultaneous management of the airway, breathing, and circulation and rapid rewarming. Intravenous thiamine and glucose is often recommended as many causes of hypothermia are complicated by Wernicke's encephalopathy and / or hypoglycemia.^[17]

Rewarming can be achieved using a number of different methods including passive external rewarming, active external rewarming, and active internal rewarming.^[18] Passive external rewarming involves the use of a person's own heat generating ability through the provision of properly insulated dry clothing and moving to a warm environment. Active external rewarming involves applying warming devices externally such as hot water bottles or warmed forced air (Bair Hugger). Active core rewarming involves the use of intravenous warmed fluids, irrigation of body cavities with warmed fluids (the thorax, peritoneal, stomach, or bladder), use of warm humidified inhaled air, or use of extracorporeal rewarming such as via a heart lung machine.^[19]

[edit] History

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 218 BCE to the near decimation of Napoleon's armies in Russia in 1812. Losses of lives to hypothermia continued in one degree or another through the first and second world wars.^[20] Civilian examples of deaths caused by hypothermia are found during the sinking RMS Titanic, RMS Lusitania and RMS Empress of Ireland.

During the second world war Nazi Germany conducted numerous cold experiments on human prisoners.

[edit] Society

Cryonics is the induction of hypothermia after death in the hopes that at some point in the future science and medicine will have evolved to such a point that the person could be brought back to life.

[edit] In other animals

Many animals often induce hypothermia during hibernation or torpor.

[edit] References

[edit] External links

Look up hypothermia in Wiktionary, the free dictionary.

• Princeton Outdoor Guide to Hypothermia
• eMedicine: Hypothermia
• Wilderness Management of Hypothermia

v • d • e Consequences of external causes (T66-T88, 990-999) Temperature/radiation elevated: Hyperthermia · Heat syncope · Radiation poisoning reduced: Hypothermia · Immersion foot · Chilblain Air Hypoxia/Asphyxia · Barotrauma (Aerosinusitis, Decompression sickness) · Altitude sickness/Chronic mountain sickness Food Starvation Maltreatment Physical abuse · Sexual abuse · Psychological abuse Emesis Motion sickness · Seasickness · Airsickness · Space adaptation syndrome Other Electric shock · Anaphylaxis · Angioedema Hypersensitivity (Allergy, Arthus reaction) Subcutaneous emphysema Certain early complications of trauma embolism (Air, Fat) Crush syndrome/Rhabdomyolysis Contracture/Volkmann's contracture/Compartment syndrome Complications of surgical and medical care transfusion reactions: Transfusion hemosiderosis · TRALI · TACO · TA-GvHD · FNHTR · Acute/Delayed hemolytic transfusion reaction other: Serum sickness · Malignant hyperthermia · Herxheimer reaction · Graft-versus-host disease · Tumor lysis syndrome