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Hypothermia is a condition in which an organism’s temperature drops below that required for normal metabolism and bodily functions. In warm-blooded animals, core body temperature is maintained near a constant level through biologic homeostasis. But, 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.
Hypothermia is the opposite of hyperthermia, the condition that causes heat exhaustion and heat stroke.
Normal body temperature in humans is 36.8°C (98.6°F). Hypothermia can be divided in three stages of severity.
Body temperature drops by 1-2°C (1.8-3.6°F) below normal temperature (35-37°C or 95-98.6°F). Mild to strong shivering occurs.[1][2] 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.
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.[1][2][3] 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 32 °C (89.6 °F). Shivering usually stops.[1][2] 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.
Hypothermia of both the extremities and body core continues to be a major limitation to diving in cold water.[1] Cooling in the extremities is often the limitation to operations.[1] The fingers decrease dexterity due to pain or numbness, safety, work capacity, and increase the risk of developing nonfreezing cold injury.[1][3]
In divers breathing heliox below 100 meters wearing hot water suits, the inspired gas must be heated or the symptoms of hypothermia can set in without the divers realizing it.[1]
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