What are hyperthermia and hypothermia?

Quick Answer
Hyperthermia is the elevation of the body core temperature of an organism, while hypothermia is a decrease in that temperature; an extreme in either condition is a medical emergency. They both can be induced therapeutically in certain medical and surgical conditions.
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Causes and Symptoms

Body temperature reflects the level of heat energy in the body of an animal or human being. It is the consequence of the balance between the heat generated by metabolism and the body’s heat exchange with the surrounding environment (ambient temperature). Generally, animal life can be sustained in the temperature range of 0 degrees Celsius (32 degrees Fahrenheit) to 45 degrees Celsius (113 degrees Fahrenheit), but appropriate processes can store animal tissues at much lower temperature. Homeotherms, such as birds and most mammals, have the ability to maintain their high body-core temperatures despite large variations in environmental temperatures. Poikilotherms have slow metabolic rates at rest and, as a result, have difficulty maintaining their inner core temperatures. This form of thermal regulation is called ectothermic (outer heated) and is directly affected by the uptake of heat from the environment; such organisms are often termed cold-blooded. Homeotherms are endothermic (inner heated) and depend largely on their fast and controlled rates of heat production; such organisms are termed warm-blooded. Thus, the lizard, an example of an ectotherm, maintains its body temperature by staying in or out of shade and by assuming a posture toward the sun that will maximize the adjustment for its body heat. At night, the lizard burrows, but its body temperature still drops considerably until the next morning, when it increases with the rising sun.

Body-core temperatures vary considerably among mammals and birds. For example, a sparrow’s inner core temperature is about 43.5 degrees Celsius (110.3 degrees Fahrenheit), a turkey’s is 41.2 degrees Celsius (106.2 degrees Fahrenheit), a cat’s is 36.4 degrees Celsius (97.5 degrees Fahrenheit), and an opossum’s is 34.7 degrees Celsius (94.5 degrees Fahrenheit). In humans, although the temperature of the inner organs varies by only 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit), the skin temperature may vary 10 to 20 degrees Celsius (18 to 36 degrees Fahrenheit) below the core temperature of 37 degrees Celsius (98.6 degrees Fahrenheit), depending on the ambient temperature. This is possible because the cells of the skin, muscles, and blood vessels are not as sensitive as are those of the vital organs.

An elevation in core temperature of homeotherms above the normal range is called hyperthermia, while a corresponding decrease is called hypothermia. Both can be brought about by extremes in the environment, and an extreme in either condition is a medical emergency. Although the human body can withstand a lack of food for a number of weeks and the absence of water for several days, it cannot survive without thermoregulation, which is the maintaining of the inner core temperature. The core temperature has to be kept within strict limits; otherwise, the brain and heart will be compromised, and death will result. Clinically, the body’s core temperature can be monitored by recording the temperature of the rectum, the eardrum, the mouth, the temporal artery, or the esophagus. In elderly people, the body’s ability to cope with extreme temperatures may be impaired, so that exposure to even mildly cold temperatures may lead to accidental hypothermia that can be fatal if not detected and treated properly.

All animals produce heat by oxidation of substrates to carbon dioxide. On the average, about 75 percent of food energy is converted to heat during adenosine triphosphate (ATP) formation and its transfer to the functional systems of the cells. In defense against heat, sweating is the primary physiological mechanism in mammals. Dogs, cats, and other furred carnivores increase evaporative heat loss by panting, while small rodents spread saliva. Human beings have two to three million glands that can produce a high volume of sweat for a short period of time.

Fever may occur for at least four main reasons. Infection by microorganisms is the most familiar because of its large variety of causes. An infection may be bacterial (as with septicemia and abscesses), viral (such as measles, mumps, or influenza), protozoal (such as malaria), or spirochetal (such as syphilis). Fever can also occur because of immunological conditions, such as a response to drug allergies or a reaction to a blood transfusion. Other causes are malignancy, such as Hodgkin disease and leukemia, and noninfectious inflammation, as seen in gout or thrombophlebitis.

Antipyretics are medicines whose consumption results in the lowering of fever. Aspirin, acetaminophen, and ibuprofen are common over-the-counter fever-reducing medications. The mechanism of action of the nonnarcotic antipyretics remains a subject of research. Two hypotheses are considered as possible reasons for the suppression of fever. One hypothesis suggests that the drugs reset the temperature in the hypothalamus of the brain by potentially blocking certain brain chemical activity. The other postulates that a modification of the physiological membrane properties takes place, with subsequent incorporation of drug molecules into the tertiary structure of proteins.

Temperature regulation involves the hypothalamus of the brain and the spinal cord, which monitor the difference between the internal and peripheral (core and skin) temperature, with physiological and psychological adjustments to maintain a constant internal temperature. The brain records the various body temperatures via specialized nerve endings called thermoreceptors. Heat transfer occurs between the skin surface and the environment via radiation (which takes place when environmental temperature is lower than body temperature; 60 to 70 percent of body heat is lost through this method), conduction (which takes place by means of physical contact), convection (which occurs through the movement of air), or evaporation (perspiration or sweating).

During cold weather, hikers and climbers are particularly at risk for hypothermia; in extreme cases, body functions are depressed to the extent that victims may be mistaken for dead. Injuries that result from skin exposure to extreme cold are described as frostbite. Frostbite most commonly affects peripheral tissue such as the nose, ears, hands (especially the fingertips), and feet, turning the tissue first unusually red and then unnaturally white. Early symptoms include feelings of coldness, tingling, pain, and numbness. Frostbite takes place when ice crystals form in the skin and (in the most serious cases) in the tissue beneath the skin. If not treated, frostbite may lead to gangrene, the medical condition that results from necrosis (tissue death). The freezing-thawing process causes mechanical disruption (from ice), intracellular and extracellular biochemical changes, and the disruption of the blood corpuscles. Treatment for frostbite includes the removal of wet or restrictive clothing and jewelry. Warm water can be used on the affected body part to restore blood circulation, which will be painful for the victim. Extreme care should be taken when moving the body part with frostbite. The area should never be massaged, as the skin and tissues are very fragile and may be further damaged.

There are several types of hypothermia. Immersion hypothermia occurs when a person falls into cold water. Any movement of the body leads to loss of heat, and the drastic temperature change may trigger a heart attack. Divers are equipped with wet suits to minimize heat loss, but they cool themselves rapidly when they move in cold water and breathe dry air mixtures at the same time.

Submersion hypothermia is actual drowning in cold water. Often during drowning, the person experiences a spasm of the larynx or is able to hold his or her breath. However, the lack of oxygen forces the relaxation of these responses, and water is swallowed. The majority of this water goes into the stomach, but it only takes a very small amount of water in the lungs to cause enough oxygen deprivation to slow or even stop the heart. In cold water, the human body responds by lowering its metabolism, which can allow an individual (especially a child) to survive for a prolonged period without adequate oxygen before death occurs. The exact amount of time that a person can survive in cold-water conditions is unknown. Reports have demonstrated survival in cases of submersion for more than an hour. Unless the person shows evidence of decay, attempts should be made to rescue the individual and involve emergency medical personnel. When removing a victim from the water, care should be taken to remove the individual in a prone (facedown) position to avoid circulatory collapse from the sudden decrease in water pressure. This is a common cause of sudden death for a conscious individual who is rescued from the water.

Clinical reports also indicate hypothermia in alcohol-intoxicated individuals. Shivering is the brain’s hypothalamic response to hypothermia. The hypothalamus stimulates reflexes in the spinal cord that trigger a sequence of skeletal muscle contractions, which lead to an increase in body temperature. Other conditions that promote heat loss in an already-hypothermic person include tight and wet clothing, injury causing hemorrhage, fatigue, and even psychosis.

Treatment and Therapy

Nature protects poikilotherm vertebrates and invertebrates in winter by means of hibernation. Hibernation, which is a condition of dormancy and torpor, occurs when the body temperatures of such animals drop in response to a decrease in environmental temperatures. Animals such as bears, raccoons, badgers, and some birds become drowsy in winter because ambient temperature drops of a few degrees considerably decrease their metabolic rates and physiological functions. For example, the body temperature of a bear is 35.5 degrees Celsius (95.9 degrees Fahrenheit) at an air temperature of 4.4 degrees Celsius (39.9 degrees Fahrenheit) and only 31.2 degrees Celsius (88.1 degrees Fahrenheit) at an air temperature of –4 degrees Celsius (24.8 degrees Fahrenheit).

In humans, however, a significant decrease in body temperature is always a medical emergency requiring immediate attention. The treatment for mild cases of hypothermia may consist only of removing wet clothing before providing additional clothing or blankets. Movement of the victim must be minimized to avoid causing abnormal heart rhythms. Providing warm fluids and attempts at rewarming beyond those mentioned are not recommended for laypersons. Severe hypothermia requires hospitalization in an intensive care unit (ICU), where the body temperature is returned to normal by placing the patient under special heat-reflecting blankets; infusing warm intravenous (IV) fluids; irrigating the stomach, abdominal cavity, and bladder with warm fluids; or circulating the victim’s blood through a machine to warm it.

Hyperthermia can occur in response to illness (fever) or environmental conditions (heat exhaustion or heatstroke). Heatstroke, defined as a core temperature above 40 degrees Celsius (104 degrees Fahrenheit), is a medical emergency that occurs when the thermoregulatory system fails. The patient's body is unable to cool its elevated temperature through normal mechanisms of sweating. A sustained elevated temperature can cause seizures in young children and infants. Adults, especially the elderly, are prone to experience confusion. Treatment while waiting for the arrival of emergency medical services includes placing the victim in a cool environment, removing the victim’s clothing, and placing ice packs on the groin, neck, and underarm regions. Shivering should be avoided. Salt tablets or solutions should not be given by laypersons because of the risk of elevated sodium levels and the possibility of stomach upset.

Both hyperthermia and hypothermia have been used in medical treatments. Cancer treatment traditionally consists primarily of radiation therapy, surgery, and chemotherapy, but more recent experimental approaches for cancer treatment include immunotherapy and hyperthermia. Hyperthermia has been used with chemotherapy or radiation therapy to improve blood flow to the targeted cancer cells, increase the drug concentration inside those cells, and damage or kill them while preserving the healthy cells nearby. This preservation is attributed to the differences in the structure of cancer cells, which are effectively damaged by heat. Further studies are needed to determine the temperature that is needed to damage or kill the cancer cells without harming other cells. Most body tissues can withstand temperature up to 44 degrees Celsius for up to one hour, with the exception of the brain, spinal cord, and nerves, which have been found to sustain irreversible damage at lower temperatures and shorter exposure time than other tissues.

Ultrasonic irradiation, which uses a thin, stainless-steel tube to induce hyperthermia, is regarded as a promising technique. The heat generated by the energy produced improves the local blood supply by dilating the blood vessels. This dilation, together with an acceleration of enzyme activity, helps cells obtain fresh nutrients and rid themselves of waste products. The other advantage of ultrasonic irradiation is the vibrations that it creates. In the case of a hardened and calcified brain tumor, for example, these vibrations can crush the tumor, which can then be removed via vacuum.

Induced hypothermia is selectively used in various heart and brain surgeries. The patient’s body temperature is lowered by the use of cooling blankets and other devices. The core temperature is monitored by bladder and rectal electronic devices. The hypothermia is considered neuroprotective. It is uncertain if the brain protection is from lowered oxygen metabolism or prevention of harmful chemical reactions. The goal of the majority of surgical procedures is to keep the patient’s temperature within the normal range. Hypothermia has been associated with increased blood loss, higher infection rates, and other poor outcomes in many surgical situations. Therapeutic hypothermia has also been used after cardiac arrest to preserve heart tissue by decreasing its oxygen demand.

Perspective and Prospects

Both hypothermia and hyperthermia have long had a commanding role in medicine. An Egyptian papyrus roll that can be dated back to 3000 BCE describes the treatment of a breast tumor with hyperthermia. Heat has been used as a therapeutic agent since the days of Hippocrates (c. 460–c. 370 BCE), who stated that a patient who could not be cured by heat was incurable. In the seventeenth century, the Japanese used hyperthermia to treat syphilis, arthritis, and gout, using hot water to increase the body temperature to about 39 degrees Celsius.

The medical use of hyperthermia owes much of its modern-era development to Georges Lakhovsky (1880–1942), a Russian Jew with a physics background who did most of his work in Paris, France. Although he is not generally given the credit for it, he was the first person to design and build a short-wave diathermy machine, which was first used to create artificial fever in 1923. Lakhovsky's work was done primarily on patients with malignant tumors at the Hospital de la Salpetriere and the Hospital Saint-Louis. The first machine that he developed used frequencies from 0.75 megahertz to 3,000 megahertz, a range very much in use in today’s clinical hyperthermia. In 1931, he started using a new machine that emitted radio waves of multiple wavelengths. He had partial success with his treatment, as reported to the Pasteur Institute and the French Academy of Sciences. Other scientists in this field include the German physicians W. Busch and P. Bruns, who applied it to erysipelas infection in 1886, and the Swedish gynecologist N. Westermark, who applied it with partial success to nonoperable carcinomas of the cervix uteri in 1898. The combination of hyperthermia and immunotherapy was first applied by William B. Coley, a New York surgeon who managed to cause complete regression of malignant melanoma in patients by inducing artificial fever created by inoculation of infected erysipelas cells.

The practice of hyperthermia in conjunction with chemotherapy, radiation, and surgery for the treatment of cancers is still in its infancy. Research has produced mixed results. Studies have shown that heat is able to kill cancer cells left behind following surgery, and it can kill those cells that tend to be radiation resistant. Unlike radiation, hyperthermia has no known cumulative toxicity. The challenge is to determine the effectiveness of hyperthermia alone compared to with other treatments, the amount of heat needed to kill cancer cells and preserve normal cells, and the proper hyperthermia techniques, such as ultrasound-induced hyperthermia.

The application of hyperthermia in cases of acquired immunodeficiency syndrome (AIDS) has not yet provided decisively positive results. The process involves circulating the patient’s blood through a chamber heated to approximately 10 degrees Fahrenheit higher than the body temperature. Although the AIDS virus is killed, many of the patients’ other enzymes are found to lose their activity. Consequently, United States health officials have opposed and criticized blood-heating therapy for AIDS patients until more convincing results are produced.

The role of hyperthermia in treating metastatic cancer, in combination with radiation and drugs that sensitize cells to heat and radiation, is increasing. This technique has been made feasible by the technological advancements in deep-heating machines, such as the Magnetrode and the BSD annular phased array, which allow the sequential regional hyperthermia of large body regions such as the thorax and the abdomen. The challenge is to prevent heatstroke during the procedure.

Induced-hypothermia brain surgery protects brain tissue. In stroke patients, a temperature elevation of 1 degree Celsius (1.8 degrees Fahrenheit) can increase the size of the damaged tissue by as much as 50 percent.

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