Segmental analgesic effect and reduction of halothane MAC from epidural fentanyl in humans.

To clarify the site of action of epidural fentanyl, we compared the effects of epidural and intravenous fentanyl on the change in pressure pain threshold (PPT) and the minimum alveolar concentration (MAC) of halothane. Seventy patients who underwent gastrectomy in the PPT study group and 84 female patients who underwent hysterectomy in the MAC study group were assigned randomly to seven groups in each study. The seven groups each received a bolus injection of 1, 2, or 4 micrograms/kg of fentanyl, either intravenously or epidurally, and of saline solution epidurally. Compared with intravenous fentanyl, epidural fentanyl significantly increased (P less than 0.01) PPT around surgical incisions by approximately 50%, 100%, and 150% of preadministration levels 1 h after administration of 1, 2, and 4 micrograms/kg, respectively, and significantly reduced (P less than 0.05) halothane MAC at the same doses. These data suggest that the more potent analgesic and anesthetic effects of epidural fentanyl, compared with intravenous fentanyl, are due mainly to the segmental analgesia produced by its spinal analgesic action.