Hypothermia is a condition in which core temperature drops below that required for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of 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.
Hypothermia is the opposite of hyperthermia which is present in heat exhaustion and heat stroke.
Normal body temperature in humans is . Hypothermia is defined as any body temperature below . It is subdivided into four different degrees, mild ; moderate, ; severe, ; and profound at less than . This is in contrast to hyperthermia and fever which are defined as a rectal temperature of greater than .
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.
- Trench foot or immersion foot is due to repetitive exposure to wet, non-freezing temperatures. Cold diuresis, mental confusion, hyperglycemia, as well as hepatic dysfunction may also be present.
Body temperature of 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 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 , 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.
A number of skin conditions may be associated with hypothermia or may occur with normal body temperature. These include: cryopedis and frostbite.
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. Therefore a number of underlying conditions increase ones 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 ( ethanol specifically producing peripheral vasodilation ) which incapacitates and more quickly leads to body temperature equilibrating with that of the surroundings, homelessness, and living in a cold environment.
It is also occurs frequently in major trauma. Alcohol consumption increases the risk of becoming 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.
Hypothermia continues to be a major limitation to diving in cold water. Water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia. Water temperature of 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 may after prolonged exposure lead to hypothermia.
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