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 define d as the median effective local analgesic concentration (ECS,) in a 20-ml v olume for epidural analgesia in the first stage of Labor. The aim of this s tudy was to determine the relative local anesthetic sparing efficacies of i ntravenous 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 ana lgesia 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 rec eived 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 concen tration using up-down sequential allocation. Analgesic efficacy was assesse d 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 f entanyl 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 intraveno us 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 bupivacain e when compared with intravenous fentanyl for the parturients in this study . The significantly enhanced local anesthetic sparing, dermatomal level, an d pruritus with epidural fentanyl suggest a primarily spinal site of action .