Minimum local analgesic dose of intrathecal bupivacaine in labor and the effect of intrathecal fentanyl

Background: Combining bupivacaine with fentanyl for intrathecal analgesia in labor is well recognized, but dosages commonly used are arbitrarily chosen and may be excessive. This study aimed to determine the median effective dose (ED50) of intrathecal bupivacaine, defined as the minimum local analgesic dose (MLAD), and then use this to assess the effect of different doses of fentanyl. Methods: In this double-blind, randomized, prospective study, 124 parturients receiving combined spinal epidural analgesia at 2-6-cm cervical dilatation were allocated to one of four groups to receive bupivacaine alone or with 5, 15, or 25 mug fentanyl, using the technique of up-down sequential allocation. Analgesic effectiveness was assessed using 100-mm visual analog pain scores, with less than or equal to 10 mm within 15 min defined as effective. MLAD was calculated using the formula of Dixon and Massey. Pruritus and duration of spinal analgesia were also recorded. Results: Minimum local analgesic dose of intrathecal bupivacaine was 1.99 mg (95% confidence interval, 1.71, 2.27). There were similar significant reductions in MLAD (P < 0.001) for all bupivacaine-fentanyl groups compared with bupivacaine control. There was a dose-dependent increase in both pruritus and duration of spinal analgesia with increasing fentanyl (P < 0.0001). Conclusion: Under the conditions of this study, the addition of intrathecal fentanyl 5 mug offers a similar significant bupivacaine dose-sparing effect as 15 and 25 mug. Analgesia in the first stage of labor can be achieved using lower doses of fentanyl, resulting in less pruritus but with a shortening of duration of action.