hypothermia (redirected from Induced hypothermia)

hypothermia

Abnormally low body temperature, with slowing of physiological activity. It is artificially induced (usually with ice baths) for certain surgical procedures and cancer treatments. Accidental hypothermia can result from falling into cold water or overexposure in cold weather. Underlying conditions such as cerebrovascular disease or intoxication increase the risk from exposure. Hypothermia is serious when body temperature is below 95 °F (35 °C) and an emergency below 90 °F (32.2 °C), at which point shivering stops. Pulse, respiration, and blood pressure are depressed. Even when the victim appears dead, revival may be possible with very gradual passive rewarming (e.g., with blankets). See also frostbite.

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hypothermia

1. Pathol an abnormally low body temperature, as induced in the elderly by exposure to cold weather

2. Med the intentional reduction of normal body temperature, as by ice packs, to reduce the patient's metabolic rate: performed esp in heart and brain surgery

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hypothermia [‚hī·pō′thər·mē·ə]

(physiology)

Condition of reduced body temperature in homeotherms.

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Hypothermia

A condition in which the internal temperature of the (human) body is at least 3.6^°F (2.0^°C) below an internal temperature of 98.6^°F (37^°C). Hypothermia represents a continuum of effects that vary with the severity of cold on physiological systems. The human body needs a specific internal temperature that is regulated on a minute-by-minute basis to maintain all normal body functions. The many physiological and behavioral processes involved in maintaining the internal temperature constant are called thermoregulation. See Thermoregulation

Various environmental situations predispose humans to hypothermia, which can occur even in the absence of cold. In fact, hypothermia is more common in temperate regions than in the colder climates. Because of the uniqueness of the situations in which hypothermia can occur, various kinds of hypothermia have been classified, all of which can prove fatal.

Primary hypothermia

Primary hypothermia is a decrease in internal temperature that is caused by environmental factors in which the body's physiological processes are normal but thermoregulation capability is overwhelmed by environmental stress.

Air (formerly exposure) hypothermia is thought to be the most common form. A person exposed to cold air experiences the same processes as a person in cold water, but air hypothermia occurs more slowly. The induction of air hypothermia is more subtle and therefore more dangerous since it can occur over a number of weeks. The degree to which a person reacts to a cold air stress is dependent on such factors as age, physical stamina, the intensity of the cold stress, and the responsiveness of the thermoregulatory system. One of the most convenient ways to determine whether someone is suffering from hypothermia is a noted change in personality: Complaints of fatigue, sluggish speech, and confusion are common, and in some cases the behavior resembles that of intoxication.

Initially, skin temperature falls rapidly, blood vessels to the skin constrict, and shivering begins. After 5–10 min, shivering ceases for about 10–15 min, but this is followed by uncontrollable shivering. In a cold situation, the nervous system causes blood to be redistributed away from the skin as the blood vessels of the skin close down to minimize heat transfer to the cold environment. The decrease in skin temperature coupled with vasoconstriction makes the person feel cold, and sometimes the fingers and toes can become painful. Internally, there is an increase in the levels of hormones that control metabolism, and blood is shunted primarily to the lungs, heart, and brain. The person becomes dehydrated as the inspired air is warmed and humidified. If the tense and shivering muscles do not generate enough heat, the hypothermic process begins and progresses for at least 3–5 h. As hypothermia continues, the arms become rigid, and the person loses the ability to make fine movements. During this period of time the heart rate initially increases, then stabilizes and as the person's internal temperature becomes progressively colder, the heart rate and respiration slow. In severely hypothermic persons, it is very difficult to detect a slow heart rate or determine if the person is breathing. A temperature of 95^°F (35^°C) is only the beginning of mild hypothermia and shivering can continue for hours, depending on the muscle and fat supplies available. Eventually, the environment becomes overwhelming. At 86^°F (30^°C), the person loses consciousness and shivering ceases. Death does not occur until the internal temperature drops further: Death results at 68–77^°F (20–35^°C) because of cardiac standstill.

When a person falls into cold water, a gasping response is triggered by the thermal receptors on the skin. For some individuals, the cold stress may trigger a heart attack. Although as much of the body as possible should be kept out of the water, many victims of immersion hypothermia stay in the cold water because they cannot tell how cold they are. Shivering becomes generalized and, unlike its effect in cold air, may cause a faster drop in internal temperature since the water layer closest to the body is stirred and convective heat loss is promoted. Although the greater conductive property of water relative to air is a major heat sink, physiological and behavioral responses act to minimize the heat loss. Survival in 50^°F (10^°C) water is possible for several hours at most if the person is dressed in street clothes and a life jacket.

The cooling of the body in submersion hypothermia allows the brain and heart to withstand approximately 45 min of oxygen debt. This is most operative for young children. A child can survive for an extended period of time while completely submerged because the body is undergoing both internal and external cooling. As the child is drowning, cold water is swallowed and enters the lungs, which cools the core. At the same time, the cold water that bathes the skin rapidly cools the periphery. The multiple effects of the internal and external cooling decrease the metabolic rate and give the child a window of safety of approximately 45 min. In warm water, survival is possible for only 5–7 min.

Secondary hypothermia

A decrease in core temperature caused by an underlying pathology that prevents the body from generating enough core heat is referred to as secondary hypothermia. If any of the thermoregulatory systems are altered, the body's ability to generate heat decreases and hypothermia can then develop without warning. Insufficient muscle mass to generate heat, medications that interfere with metabolism, an underlying systemic infection, decreased thyroid hormone production, and paralysis predispose to hypothermia. Premature infants with low body fat and a large surface-to-volume ratio lose heat rapidly and are at risk for becoming hypothermic. The elderly are perhaps the most susceptible to secondary hypothermia. However, whether the process of aging with no associated debility also alters the thermoregulatory system in the elderly remains to be determined.

Clinical hypothermia

Some cardiac surgical procedures require clinically induced cooling to stop the heart from beating. Induced hypothermia lowers the oxygen demand of the body tissues, so that oxygenated blood need not circulate. In the case of coronary bypass surgery, the entire body is cooled, enabling the surgeons to work for an extended period of time on the cold heart.

Frostbite

In hypothermia, the body's internal temperature decreases, but no solid freezing takes place. In frostbite, which is freezing of the digits or the limbs, there is actual formation of ice crystals. Basically the digits go through various stages of cooling. Initially, in the prefreeze phase, the finger temperature is 37.4–50^°F (3–10^°C). Next, at 24.8^°F (-4^°C) ice crystals form outside the cells of the digits, circulation is limited, and cell death takes place if the process is allowed to continue. The cells of the digits and limbs can tolerate low temperatures that would be lethal to brain or nerve cells. However, once they are rewarmed and thawed, they develop an increased sensitivity to the cold and become more susceptible to frostbite. Any part of the body can become frostbitten, but the fingers, toes, ears, nose, and cheeks are most often affected. See Homeostasis