Hypothermia

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Hypothermia

Hypothermia is the intentional or accidental reduction of core body temperature to below 95°F (35°C) which, in severe instances, is fatal. Because humans are endothermic—warm-blooded creatures producing their own body heat—the core body temperature remains relatively constant at 98.6°F (37°C), even in fluctuating environmental temperatures. However, in extreme conditions, a healthy, physically fit person’s core body temperature can rise considerably above this norm and cause heatstroke or fall below it far enough to cause hypothermia. Elderly people, for instance, have succumbed to hypothermia after prolonged exposure to indoor air temperatures of 50 to 65°F (10.0 to 18.3°C). Between 1979 and 2005, deaths from hypothermia in the United States has ranged from about 400 to about 1,100 annually.

Intentional hypothermia

Intentional hypothermia is used in medicine in both regional and total-body cooling for organ and tissue protection, preservation, or destruction. Interrupted blood flow starves organs of oxygen and may cause permanent organ damage or death. The body’s metabolic rate (the rate at which cells provide energy for the body’s vital functioning) decreases 8% with each 1.8°F (1°C) reduction in core body temperature, thus requiring reduced amounts of oxygen. Total-body hypothermia lowers the body temperature and slows the metabolic rate, protecting organs from reduced oxygen supply during the interruption of blood flow necessary in certain surgical procedures. In some procedures, like heart repair and organ transplantation, individual organs are preserved by intentional hypothermia of the organ involved. In open-heart surgery, blood supply to the chilled heart can be totally interrupted while the surgeon repairs the damaged organ. Organ and tissue destruction using extreme hypothermia -212 to -374°F (-100 to -190°C) is utilized in retinal and glaucoma surgery and to destroy pre-cancerous cells in some body tissue. This is called cryosurgery.

History of medical use of intentional hypothermia

Intentional hypothermia is not a modern phenomenon. With it, ancient Egyptians treated high fevers; as did Hipparchus (c. 166-125 BC)—who also understood its analgesic (pain-killing) properties; the Romans; and Europeans of the Middle Ages. Napoleon’s surgeon general used cryoanalgesia when performing amputations. He discovered that packing a limb in ice and snow not only killed most of the pain, it also helped prevent bleeding. Today, intentional hypothermia is most commonly used in heart surgery.

Accidental hypothermia

Accidental hypothermia is potentially fatal. It can happen as simply as falling off a log. Falling into icy water, or exposure to cold weather without appropriate protective clothing, can quickly result in death. Hypothermia is classified into four states. In mild cases of 95 to 89.6°F (35 to 32°C), symptoms include feeling cold, shivering (which helps raise body temperature), increased heart rate, desire to urinate, and some loss of coordination. Moderate 87.8 to 78.8°F (31 to 26°C) hypothermia causes a decrease in, or cessation of, shivering; weakness; sleepiness; confusion; slurred speech; and lack of coordination. Deep hypothermia 77 to 68°F (25 to 20°C) is extremely dangerous as the body can no longer produce heat. Sufferers may behave irrationally, become comatose, lose the ability to see, and often cannot follow commands. In profound cases 66 to 57°F (19 to 14°C), the sufferer will become rigid and may even appear dead, with dilated pupils, extremely low blood pressure, and barely perceptible heartbeat and breathing. This state usually requires complete, professional cardiopulmonary resuscitation for survival.

Primary and secondary hypothermia

Hypothermia is also divided into two types: primary and secondary. Primary hypothermia occurs when the body’s heat-balancing mechanisms are working properly but are subjected to extreme cold, such as exposure to cold air or cold water. Hypothermia due to cold air usually takes at least several hours to develop, but immersion hypothermia will occur within about an hour of entering the water because water draws heat away from the body much faster than air does. Secondary hypothermia affects people whose heat-balancing mechanisms are impaired in some way and, thus, cannot respond adequately to moderate or perhaps even mild cold. Among the reasons for the failure of the body’s heat-balancing are diabetes, malnutrition, bacterial infection, thyroid disease, and the use of medications and other substances that affect the brain or spinal cord, such as alcohol.

Secondary hypothermia is often a threat to the elderly, who may be on medications or suffering from illnesses that affect their ability to conserve heat. Malnutrition and immobility can also put the elderly at risk. Some medical research also suggests that shivering and blood vessel narrowing (two of the body’s defenses against cold) may not be triggered as quickly in older people. For these and other reasons, the elderly can, over a period of days or even weeks, fall victim to hypothermia in poorly insulated homes or other surroundings. In addition, hypothermia can easily be misdiagnosed as a stroke or some other common illness of old age.

Causes of accidental hypothermia

Although overexertion in a cold environment causes most accidental hypothermia, it may occur during anesthesia, primarily due to central nervous system depression of the body’s heat-regulating mechanism; and in babies, elderly, and ill people whose homes are inadequately heated. The human body loses heat to the environment through conduction, convection, evaporation and respiration, and radiation. It generates heat through the metabolic process.

Conduction occurs when direct contact is made between the body and a cold object, and heat passes from the body to that object. Convection is when cold air or water makes contact with the body, become warm, and move away to be replaced by more of the same. The cooler the air or water, and the faster it moves, the faster the core body temperature drops.

Evaporation through perspiration and respiration provides almost 30% of the body’s natural cooling mechanism. Because cold air contains little water and readily evaporates perspiration; and because physical exertion produces sweating, even in extreme cold; heat loss through evaporation takes place even at very low temperatures. The dry air that humans inhale attracts moisture from the lining of the nose and throat. This can happen so quickly that, by the time the air reaches the lungs, it is completely saturated. Combined, evaporation and convection from wet clothes will reduce the body temperature dangerously quickly.

KEY TERMS

Cryoanalgesia —Using cold as a pain reducing anaesthetic.

Cryosurgery —Freezing the tissue to be dissected.

Endothermic —Capable of producing body heat.

When the body is warmer than its environment, it radiates heat into that environment. Radiation is the greatest source of heat loss, and the colder the environment, the greater the potential for heat loss. Most clothing is of little help because body heat radiates into clothing and from clothing into the atmosphere or object with which it comes into contact.

Preventing accidental hypothermia

Although profound hypothermia can be reversed in some instances, even mild states can quickly lead to death. However, through knowledge and common sense, hypothermia is avoidable. Two factors essential in preventing accidental hypothermia are reducing loss of body heat and increasing body heat production.

Appropriate clothing, shelter, and diet are all essential. Apart from new, synthetic fabrics that allow undergarments to wick perspiration away from the body while remaining dry and warm, and outer garments that breathe while keeping wind and moisture out, natural wool fibers contain millions of air pockets that act as excellent insulators. Even when saturated, wool maintains 80% of its dry insulation value. Because it provides no insulation and becomes extremely cold when wet, cotton is often called killer cotton by experienced outdoors people. As 60% of body heat is lost by radiation from the head, hats can be lifesavers. Fingers, toes, hands, and feet lose heat quickly, and excellent quality boots, gloves, and mittens are a must.

Regular consumption of high-energy food rich in carbohydrates aids the body in heat production, while 13 to 17 c (3 to 4 L) of water a day prevents rapid dehydration from evaporation. Exercising large muscles—like those in the legs—is the best generator of body heat; however, overexertion must be avoided as it will only speed the onset of hypothermia.