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Fever is an adaptation mechanism of the body in response to internal and external environmental stressors, and is a key indicator of immune system activation. Normal body temperature is maintained by peripheral nerves that transmit signals back to the hypothalamus. Fever occurs when cytokines cause an increase in body temperature in association with a rise in hypothalamic set point, and consists of three clinical phases: chill, fever, and flush. Elevated body temperatures can broadly be classified as hyperthermia syndromes and infectious and noninfectious fever (see Table 34-1). Hyperthermia occurs when thermoregulatory mechanisms fail and when heat production exceeds heat loss through either overproduction of heat or decrease in heat loss. Such examples of heat overproduction include thyrotoxicosis, pheochromocytoma, adrenal crisis, or salicylate toxicity via interruption of the citric acid cycle and uncoupled oxidative phosphorylation. Heatstroke or anticholinergic toxicity is mediated by a deficiency in mechanisms of heat dissipation. Some hyperthermia syndromes fall into both categories, such as postanesthesia neuroleptic malignant syndrome (NMS), and may cause profound hyperpyrexia. It is important to differentiate fever from hyperthermia because hyperthermia due to thermoregulatory failure is treated by a lowering of the body temperature by physical mechanisms (conduction, convection, evaporation); antipyretics are not effective. Both noninfectious and infectious causes of elevated body temperatures in the intensive care unit (ICU) will be discussed in detail in following sections.
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The average body temperature set point is 37.0°C (98.6°F) and may vary by 0.5–1.0°C according to time of day or hormonal milieu: it is highest at 6:00 am and for women at the time of ovulation. Fever is defined in multiple ways. It is an isolated core body temperature >38.0°C (100.4°F) or two consecutive elevations of greater than 38.3°C. In neutropenic patients, fever may be defined as a single temperature greater than 38.3°F (101.0°F) or greater than 38.0°C (100.4°F) for 60 minutes. The American College of Critical Care Medicine and the Infectious Diseases Society of America (ACCM/IDSA) define a fever as a rise in body temperature greater than 38.3°C (101°F) and recommend any new fever be investigated.1 However, in immunocompromised or elderly individuals, a lower cutoff may be appropriate, as these patients may not be able to mount substantial febrile responses. Additionally, the fever response may ...