Opioids inhibit febrile responses in humans, whereas epidural analgesia does not - An explanation for hyperthermia during epidural analgesia

Background: Epidural analgesia is frequently associated with hyperthermia d uring labor and in the postoperative period. The conventional assumption Is that hyperthermia is caused by the technique, although no convincing mecha nism has been proposed. However, pain in the "control" patients is inevitab ly treated with opioids, which themselves attenuate fever. Fever associated with infection or tissue injury may then be suppressed by opioids in the " control" patients while being expressed normally in patients given epidural analgesia. The authors therefore tested the hypothesis that fever in human s is manifested normally during epidural analgesia, but is suppressed by lo w-dose intravenous opioid. Methods: The authors studied eight volunteers, each on four study days. Fev er was induced each day by 150 IU/g intravenous interleukin 2, Volunteers w ere randomly assigned to: (1) a control day when no opioid or epidural anal gesia was given; (2) epidural analgesia using ropivacaine alone; (3) epidur al analgesia using ropivacaine in combination with 2 mug/ml fentanyl; or (4 ) intravenous fentanyl at a target plasma concentration of 2.5 ng/ml. Results: Fentanyl halved the febrile response to pyrogen, decreasing integr ated core temperature from 7.0 +/- 3.2 degreesC.h on the control day, to 3. 8 +/- 3.0 degreesC.h on the intravenous fentanyl day. in contrast, epidural ropivacaine and epidural ropivacaine-fentanyl did not inhibit fever, The f raction of core-temperature measurements that exceeded 38 degreesC was halv ed by intravenous fentanyl, and the fraction exceeding 38.5 degreesC was re duced more than fivefold. Conclusions: These data support the authors' proposed mechanism for hyperth ermia during epidural analgesia. Fever during epidural analgesia should thu s not be considered a complication of the anesthetic technique per se.