Paralysis only slightly reduces the febrile response to interleukin-2 during isoflurane anesthesia

Background: Fever sometimes occurs during anesthesia. However, it is rare considering how often pyrogenic causes are likely to be present and how common fever is after surgery. This low incidence results in part from dose-dependent inhibition of fever by volatile anesthetics. Paralysis, however, may contribute by preventing shivering and the associated increase in metabolic heat production. Therefore the authors tested the hypothesis that paralysis during anesthesia decreases the febrile response to pyrogen administration. Methods: Seven volunteers each participated on two study days. They were given 30 IU/g intravenous interleukin-2, followed 90 min later by an additional 70 IU/g dose. Anesthesia was induced 30 min after the second dose and maintained for 6 h with 0.6 minimum alveolar concentration isoflurane. The volunteers were randomly assigned to (1) paralysis with vecuronium or (2) no muscle relaxants. Body heat content and distribution were determined from measured tissue and skin temperatures. Data are presented as mean +/- SD; P < 0.05 was considered significant. Results: There was no clinically important difference in peak core (tympanic membrane) temperatures on the unparalyzed (37.6 +/- 0.9 degrees C) and paralyzed (37.2 +/- 0.6 degrees C) days. Core heat content increased 1.2 +/- 0.7 kcal/kg over the last 5 h of anesthesia on the unparalyzed day, but only by 0.9 +/- 0.4 kcal/kg when the volunteers were paralyzed. Peripheral tissue heat content increased 0.1 +/- 1.1 kcal/kg on the unparalyzed day but decreased 1.1 +/-. 0.7 kcal/kg when the volunteers were paralyzed. Consequently, body heat content increased 1.3 +/- 1.3 kcal/kg on the unparalyzed day but decreased significantly by 0.2 +/- 0.8 kcal/kg when the volunteers were paralyzed. Conclusions: Paralysis prevented shivering from increasing the metabolic rate. Consequently, body heat content decreased during paralysis, whereas otherwise it increased, Thermoregulatory vasoconstriction was nonetheless able to maintain similar peak and integrated core temperatures on each study day. Administration of muscle relaxants thus is not the primary explanation for the relative paucity of intraoperative fever.