A randomised double-blinded controlled trial of the effect of diluent volume on the efficacy of a single dose of epidural ropivacaine for labour analgesia

BACKGROUND: Few studies have systematically investigated the effect of diluent volume on the efficacy of epidural local anaesthetics for labour analgesia and no data are available for ropivacaine. Our aim was to compare epidural ropivacaine given in three different volumes for labour analgesia. METHODS: In a double-blinded study, we randomly assigned 60 labouring parturients with cervical dilatation <5 cm to receive epidural ropivacaine 30 mg diluted to 5 mL (group 5, n=20), 10 mL (group 10, n=20) or 20 mL (group 20, n=20) for analgesia. We measured visual analogue scale pain scores at the peak of contraction for 30 min plus onset and duration of analgesia. RESULTS: Pain scores at 30 min were similar among group 5 (median 22 [interquartile range 0-44] mm), group 10 (9 [0-21] mm) and group 20 (37 [0-51] mm), P=0.35. The number of patients who achieved a decrease in pain score to < or =30% of baseline was similar among group 5 (12/20 (60%)), group 10 (17/20 (85%)) and group 20 (11/19 (58%)), P=0.13. Onset and duration of analgesia, the incidence of motor block and patient satisfaction were similar among groups. CONCLUSIONS: Under the conditions of this study, within the range of 5-20 mL, the volume and concentration were not significant factors influencing the efficacy of a single 30-mg bolus of epidural ropivacaine given for labour analgesia. This suggests that the most important factor influencing analgesic efficacy was drug mass.