Effect of intravenous versus epidural fentanyl on the minimum local analgesic concentration of epidural bupivacaine in labor

Background The minimum local analgesic concentration (MLAC) has been defined as the median effective local analgesic concentration (ECS,) in a 20-ml volume for epidural analgesia in the first stage of Labor. The aim of this study was to determine the relative local anesthetic sparing efficacies of intravenous and epidural fentanyl by comparison of their effects on the MLAC of bupivacaine. Methods: In this double-blind randomized, prospective study, 84 parturients at less than or equal to 7-cm cervical dilation who requested epidural analgesia were allocated to one of two groups. After lumbar epidural catheter placement, 20 mi bupivacaine (n = 44) or bupivacaine with 3 mu g/ml (60 mu g) fentanyl (n = 40) was administered. The plain bupivacaine group then received 60 mu g intravenous fentanyl. The bupivacaiae-fentanyl group received intravenous saline. The concentration of bupivacaine was determined by the response of the previous patient in that group to a higher or lower concentration using up-down sequential allocation. Analgesic efficacy was assessed using 100-mm visual analog pain scores, with less than or equal to 10 mm within 30 min defined as effective. Results: The MLAC of bupivacaine-intravenous fentanyl was 0.064% wt/vol (95% confidence interval, 0.049-0.080), and the MLAC of bupivacaine-epidural fentanyl was 0.034% wt/vol (95% confidence interval, 0.017-0.050). Epidural fentanyl significantly increased the analgesic potency of bupivacaine by a factor of 1.88 (95% confidence interval, 1.09-3.67) compared with intravenous fentanyl. The epidural fentanyl group demonstrated significantly higher dermatomal spread (P = 0.0064) and increased pruritus (P = 0.01). Conclusions: Epidural fentanyl significantly reduced the MLAC of bupivacaine when compared with intravenous fentanyl for the parturients in this study. The significantly enhanced local anesthetic sparing, dermatomal level, and pruritus with epidural fentanyl suggest a primarily spinal site of action.