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Definition
Fever is defined as an abnormally high body temperature or a regulated rise to a new set point of body temperature. While a body temperature above 100°F(37.8°C) is considered to be a fever by some clinicians, a significant fever is usually defined as an oral temperature of 102°F (39°C) or a rectal temperature of 103°F(39.5°C). Fever is a sign of inflammation and represents the body's response to microbial invasion or to a disease process.
Hyperthermia is defined as abnormally high body temperature caused by disruption of the body's thermoregulatory mechanisms. Hyperthermia occurs when the body's metabolic heat production or environmental heat load exceeds the normal heat loss capacity (or when normal heat loss is impaired).
Description
The normal temperature of the human body fluctuates between 97°F (36.1°C) and 100°F (37.8°C), with the average being 98.6°F (37°C). There is a diurnal pattern of temperature variation in humans, with body temperature being lowest in the morning and highest in late afternoon. In women of childbearing age, there is a small rise in temperature following ovulation during the menstrual cycle, and during the first trimester of pregnancy.
The body's thermoregulatory mechanisms include changes in muscle tone, vascular tone, and sweat production, which serve to balance body heat produced by metabolism with heat lost to the environment. This balance is regulated by the hypothalamus, a small structure in the brain below the thalamus.The body's temperature regulation can be upset by environmental factors (external heat and humidity); by disorders or conditions that inhibit sweat production or evaporation; and by infectious diseases.
Fever occurs when the hypothalamus raises the body's temperature to a new set point. White blood cells called monocyte-macrophages release proteins called pyrogens when the cells encounter pathogenic microorganisms. The pyrogens act on the hypothalamus, causing it to reset the body's "thermostat" upward. The vessels that supply the skin with blood often narrow as the body's temperature is rising. This process, which is called vasoconstriction, reduces sweating and causes the body to retain more heat than it loses. Vasoconstriction in the skin and extremities allows the body to move blood toward its core, which increases the rate of metabolism and induces shivering. The chills that often accompany a fever are caused by this movement of blood to the body's core, which leaves the skin surface and extremities relatively cool. When the infection or disease process resolves, the hypothalamus attempts to reset the body's thermoregulation at its normal set point. The body's cooling mechanisms switch on as the blood returns to the extremities and skin surface, and sweating occurs.
Fever is an important component of the immune system's inflammatory response, though its role is not completely understood. A rise in body temperature has several effects. The pyrogens that trigger the resetting of the body's thermostat also increase the production of macrophages, which fight off invading bacteria or viruses. Higher temperatures also inhibit the growth of some bacteria, while at the same time activating the immune system. In addition, the increased heart rate that accompanies vasoconstriction also speeds the arrival of white blood cells to the sites of inflammation.
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Author Info: Lisette Hilton, The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Nursing and Allied Health, 2002 |
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