Mode and site of analgesic action of epidural buprenorphine in humans.

This study was designed to clarify the site of analgesic action of epidural buprenorphine and its spinal segmental analgesia. Fifty patients undergoing gastrectomy were randomly assigned to five groups according to the dose of buprenorphine and route of administration: epidural saline group, epidural buprenorphine 2- and 4-micrograms/kg groups, and intravenous buprenorphine 2- and 4-micrograms/kg groups. The changes in pressure pain threshold (PPT) and visual analog scale (VAS) were compared within groups of patients receiving either buprenorphine, 2 or 4 micrograms/kg epidurally, and between groups of patients receiving buprenorphine 2 or 4 micrograms/kg either epidurally or intravenously. The PPT near the surgical incision was significantly greater (P < 0.05) in the epidural buprenorphine group compared to the intravenous group during the middle period (between 2 and 6 h after administration) of analgesia obtained after administration of buprenorphine. VAS response significantly (P < 0.05) decreased in the epidural group compared to the intravenous group at the same dose. In the smaller dose buprenorphine epidural group, PPT at the forehead (an index of systemic analgesia) exceeded the preoperative value during the 12 h after administration of buprenorphine. PPT near the surgical incision decreased by 30% from the preoperative value and did not exceed the preoperative value throughout the study. PPT at the forehead significantly exceeded the PPT changes measured near the surgical incision (an index of regional analgesia) (P < 0.05). In the larger dose buprenorphine epidural group, during the early period (up to 2 h after administration), PPT at the forehead increased by 30%-40%, while PPT near the surgical incision decreased by 10%-25% compared to the preoperative value (P < 0.01). However, in the middle period, there were no significant differences in the changes between PPT at the forehead and near the surgical incision. In the late period (more than 6 h after administration), PPT at the forehead maintained the level of preoperative value, whereas PPT near the surgical incision decreased by more than 25% from the preoperative value (P < 0.01). VAS value significantly (P < 0.01) decreased in the larger dose buprenorphine epidural group compared with that in the smaller dose group during the middle period. The larger dose of epidural buprenorphine provided better analgesia than the smaller dose. We conclude that epidural buprenorphine acts predominantly at the supraspinal region and produces spinal segmental analgesia in a dose-related manner.