We compared the minimum local analgesic concentrations (MLAC) of epidural bupivacaine following initial intrathecal or epidural injection.
In a prospective, double-blind study, 115 women requesting epidural analgesia were randomly assigned to receive either an epidural with bupivacaine 20 mg and fentanyl 40 μg or a CSE with intrathecal bupivacaine 2.5 mg and fentanyl 5 μg. Analgesia was assessed using a visual analogue pain score.
When further analgesia was requested, bupivacaine 20 ml was given, and the concentration was determined using the technique of up-down sequential allocation.
The MLAC of bupivacaine in the epidural group was 0.032% wt/vol (95% CI 0.020-0.044) compared with 0.047% wt/vol (95% CI 0.042-0.052) in the CSE group.
Bupivacaine requirements for the second injection were increased following intrathecal analgesia by a factor of 1.45 (p = 0.026) compared with epidural analgesia.