|Classification and external resources|
During Napoléon Bonaparte's retreat from Russia in the winter of 1812 many troops suffered from hypothermia.
Hypothermia is a condition in which core temperature drops below that required for normal metabolism and body functions which is defined as 35.0 °C (95.0 °F). Body temperature is usually maintained near a constant level of 36.5–37.5 °C (98–100 °F) through biologic homeostasis or thermoregulation. If exposed to cold and the internal mechanisms are unable to replenish the heat that is being lost a drop in core temperature occurs. As body temperature decreases characteristic symptoms occur such as shivering and mental confusion.
|Normal||36.5–37.5 °C (98–100 °F)|
|Hypothermia||<35.0 °C (95.0 °F)|
|Fever||>37.5–38.3 °C (100–101 °F)|
|Hyperthermia||>37.5–38.3 °C (100–101 °F)|
|Hyperpyrexia||>40.0–41.5 °C (104–107 °F)|
Normal body temperature in humans is 36.5–37.5 °C (98–100 °F). Hypothermia is defined as any body temperature below 35.0 °C (95.0 °F). It is subdivided into four different degrees, mild 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). This is in contrast to hyperthermia and fever which are 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:
The signs and symptoms vary depending on the degree of hypothermia and may be divided by the three stages of severity.
Symptoms of mild hypothermia 32–35 °C (90–95 °F) may be vague with sympathetic nervous system excitation (shivering, hypertension, tachycardia, tachypnea, and vasoconstriction). These are all physiological responses to preserve heat. Cold diuresis, mental confusion, hyperglycemia, as well as hepatic dysfunction may also be present.
Body temperature of 28–32 °C (82–90 °F) results in shivering becoming more violent. Muscle mis-coordination becomes apparent. 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.
Body temperature drops below approximately 28 °C (82 °F) shivering stops. 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 °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.
As the temperature decreases further physiological systems falter and heart rate, respiratory rate, and blood pressure all decreases. This results in an expected HR in the 30s with a temperature of 28 °C (82 °F).
Twenty to fifty percent of hypothermia deaths are associated with paradoxical undressing. This typically occurs during moderate to severe hypothermia, as the person becomes disoriented, confused, and combative. They may begin discarding their clothing, which, in turn, increases the rate of heat loss.
Rescuers that are trained in mountain survival techniques are taught to expect this, however, some may assume incorrectly that urban victims of hypothermia have been subjected to a 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, fooling the person into feeling overheated.
Hypothermia usually occurs from exposure to low temperatures and is frequently complicated by alcohol. Any condition which decreases heat production, increases heat loss, or impairs thermoregulation however may contribute. Thus risk including: any condition that affects judgment ( hypoglycemia ), the extremes of age, male gender, poor clothing, chronic medical conditions ( such as hypothyroidism and sepsis ), substance abuse, homelessness, and living in a cold environment. It is also occurs frequently in major trauma.
Alcohol consumption increases the risk of hypothermic via its action as a vasodilator. It increases blood flow to the body's extremities, making a person feel warm, while increasing heat loss. Between 33 to 73% of cases of hypothermia are complicated by alcohol.
Hypothermia continues to be a major limitation to diving in cold water. The limitation of finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries. Pressurized heliox breathing mixtures have a much higher thermal mass than air, so for diving below 100 meters, not only is a hot water suit required, but the breathing mixture must be pre-heated, or the symptoms of hypothermia can set in without realization and cause death in minutes.
Other predisposing factors leading to immersion hypothermia include dehydration, inadequate rewarming with repetitive diving, starting 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.
Heat is lost more quickly in water. Water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia. Water temperature of 10 °C (50 °F) often lead to death in one hour, and water temperatures hovering at freezing can lead to death in as little as 15 minutes. Water at a temperature of 26 °C (79 °F) may after prolonged exposure lead to hypothermia.
Heat is primilarily generated in the heart and liver while it is lost through the skin (90%) and lungs (10%). Heat production may be increased 2 to 4 fold through muscle contractions ( ie exercise and shivering ). Rates of heat loss are determined as with any object by convection, conduction, and radiation. The rates of these can be affected by clothing and other environmental conditions.
Many changes to physiology occur as body temperature decreases. These occur in the cardiovascular system leading to the Osborn wave and other dysrhythmias, the central nervous system leading to decreased electrical activity, cold diuresis, and non-cardiogenic pulmonary edema.
Accurate determination of core temperature often requires a special low temperature thermometer, as most clinically thermometers do not measure accurately below 34.4°C (94°F). A low temperature thermometer can either be placed rectally, esophageal, or in the bladder. The classical ECG finding of hypothermia is the Osborne J wave well ventricular fibrillation frequently occurs at <28°C (82.4°F) and asystole at <20°C (68°F). The Osborn J may look very similar to those of an acute ST elevation myocardial infarction. Thrombolysis is not indicated, as it would only worsen the underlying coagulopathy caused by hypothermia.
As a persons heart rate may be very slow, prolonged palpation could be required before detecting a pulse. In 2005 American Heart Association recommended at least 30 - 45 seconds to verify the absence of a pulse before initiating CPR.
Appropriate clothing helps to prevent hypothermia. Synthetic and wool fabrics are superior to cotton as they provide 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.
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. The heat escape lessening position can be used to increase survival in cold water.
|Mild: > 34 °C (93 °F)||Passive rewarming|
|Moderate: 30–34 °C (86–93 °F)||Active external rewarming|
|Severe: < 30 °C (86 °F)||Active internal rewarming|
Aggressiveness of treatment is matched to the degree of hypothermia. Treatment ranges from noninvasive, passive external warming, to active external rewarming, to active core rewarming. In severe cases resuscitation begins with simultaneous removal from the cold environment and concurrent management of the airway, breathing, and circulation. Rapid rewarming is then commenced. A minimum of patient movement is required as aggressive handling may increase risks of a dysrhythmia.
Hypoglycemia is a frequent complicate and therefore needs to be tested for and treated. Intravenous thiamine and glucose is often recommended as many causes of hypothermia are complicated by Wernicke's encephalopathy.
Rewarming can be achieved using a number of different methods including passive external rewarming, active external rewarming, and active internal rewarming.
This 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.
This involves applying warming devices externally such as hot water bottles or warmed forced air (Bair Hugger). In austere environments this is often done by placing a hot water bottle in both armpits. Another method is to place the person in a tub with hot water (of 44°C), and place their arms and legs outside of the tub/hot water.
This 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.
As most people are moderately dehydrated due to hypothermia induced cold diuresis intravenous access and intravenous fluids are often helpful ( 250 - 500 cc 5 percent dextrose and normal saline is often recommended at a temperature of 40 -45 C ) .
Rewarming collapse ( or rewarming shock ) is a sudden drop in blood pressure in combination with a low cardiac output which may occur during active treatment of a severely hypothermic person. There is theoretical concerns that external rewarming rather than internal rewarming may increase the risk. However recent studies have not supported these concerns.
For ventricular fibrillation a single defibrillation should be attempted. Neither pacing nor defibrillation is recommended for other dysrhythmia. If a single defibrillation is not effective CPR should be continued ( extra force will be required ) well medication and further defibrillation should be withheld until the core temperature reaches 30 C. Once a temperature of 30 C is reached normal ACLS protocols are followed as appropriate.
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. The cold water lowers 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% - 75%. If there are obvious fatal injuries or chest is too frozen, compression resuscitation is futile.
In the past hypothermia was occurring most frequently in homeless people but increasing recreational exposure in cold environments now is the main cause of hypothermia. Between 1995 and 2004 in the United States an average of 1560 cold related emergency department visits occurred per year and in the years 1999 to 2004 an average 647 people died per year due to hypothermia.
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 BC to the near decimation of Napoleon's armies in Russia in 1812. Loss of life to hypothermia continued through the first and second world wars. 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.
Classification and other resource links
|Being homeless can make you become exposed to hypothermia|
Hypothermia is a condition when a person is so cold that the body temperature drops below normal. Hypothermia is any body temperature lower than
When hypothermia begins, a person starts shivering and can't stop. The person cannot do complicated things with his or her hands. The person might feel sick in his stomach and very tired, wanting to go to sleep, anywhere. Sometimes a person will feel suddenly warm, as if they are getting better, but this just means they are getting worse. They are also unable to touch their thumb with their little finger, because their muscles stop working. They might not be able to see things clearly.
The person begins shivering even more. Their muscles obviously do not work. They move slowly and with difficulty, growing a little confused and walking . The person becomes pale, and lips, ears, fingers, and toes might become blue. This is because the body is trying to keep the most important organs warm.
Body temperature drops even more, but the person usually stops shivering. They cannot talk clearly; they think slowly, and they cannot move their hands. Slowly, the skin that is to the cold air becomes blue and . They cannot walk, and start behaving strangely. Their pulse becomes slower, but the heart may beat faster. Finally, the person dies.