Epidural bupivacaine dilution for labour

We have compared the effects of three epidural infusions in a randomised, double-blind study of 56 primigravid mothers in labour. An initial dose of bupivacaine 0.5% 8 ml was followed by infusion of either bupivacaine 0.125%, 0.062% or 0.031%. All solutions contained fentanyl 0.0002% and were run at 7.5 ml/hour. Women receiving the most dilute solution had significantly less analgesia (p less than 0.001) for the first 4 hours of the study, but pain scores were then similar for the three groups. No obvious benefit was gained by using very dilute bupivacaine.