Fever is characterized by activation of effector mechanisms to achieve and maintain a temperature higher than normal. Thus it should be distinguished from hyperthermia, in which the set-point is normal but effector mechanisms have failed to keep body temperature at the set-point. In fever, if the body temperature is below the elevated set-point, heat-conserving mechanisms will be activated as shown in Fig 1. Thus, for example, a patient with a fever may have a set-point of 39 °C
and a body temperature of 38 °C; such a patient will tend to feel cold (behavioral thermoregulation), have peripheral vasoconstriction, and may shiver. Therefore body temperature will tend to rise towards the new elevated set-point. In fever the set-point frequently varies in a cyclical fashion; thus as it falls, the actual body temperature may exceed the set-point and the patient will tend to feel hot, vasodilate, and sweat in order to lose heat.
Fig. 1 Changes in actual and set-point temperatures during a febrile episode.
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