Epidural meperidine after cesarean section: the effect of diluent volume

We investigated the effect of diluent volume on analgesia and systemic absorption from epidural meperidine after cesarean section in a randomized, double-blind study. At the first request for postoperative analgesia, 36 parturients were given epidural meperidine 25 mg diluted with saline to either 2 mL (12.5 mg/mL), 5 mL (5 mg/mL), or 10 mL (2.5 mg/mL). Visual analog pain scores measured in the first 30 min were greater in the 2-mL group compared with both the 5-mL group (P = 0.028) and the 10-mL group (P = 0.031). Onset of analgesia (time for visual analog pain scores to decrease by 50%) was also slower in the 2-mL group (17.5 min) compared with the 5-mL group (9 min; P = 0.015) and the 10-mL group (12 min; P = 0.003); there were no differences between the 5-mL group and the 10-mL group. Duration of analgesia and plasma concentrations of meperidine were similar among groups. No adverse side effects were recorded. Previous work has suggested that injection of epidural opioids in large volumes increases the potential risk of respiratory depression from cephalad spread of the drug. Therefore, we conclude that analgesia is optimum when epidural meperidine is administered diluted to 5 mL.