RELATIVE ANALGESIC POTENCIES OF ROPIVACAINE AND BUPIVACAINE FOR EPIDURAL ANALGESIA IN LABOR

Abstract

S384 INTRODUCTION: Ropivacaine, a recently introduced amide local anesthetic, has been associated with decreased cardiotoxicity [1] and motor block [2] relative to epidural bupivacaine. These features are of potential advantage in the laboring obstetric population. It is difficult to draw conclusions regarding side effects in the absence of information concerning the relative analgesic potencies of the two agents. In order to evaluate the pharmacodynamic contributions of the various epidural analgesics, a clinical model was devised to determine the relative potencies of local anesthetics in the in the first stage of labor. [3,4] The minimum local analgesic concentration (MLAC) has been defined as the median effective local analgesic concentration (EC50). The aim of this study was to determine the relative analgesic potencies of bupivacaine and ropivacaine by determination of their respective MLAC. METHODS: After institutional ethical approval, 41 women who requested epidural analgesia at less than 7 cm cervical dilatation and who had not received opioid were enrolled. Patients were allocated to either of two groups in this double-blinded, randomized, prospective study. Twenty patients received bupivacaine and 21 patients received ropivacaine. After placing a lumbar epidural catheter, 20 mL of the study solution was given over 5 minutes. The test dose was omitted for the purposes of the study. The concentration was determined by the response of the previous patient to a higher or lower concentration using up-down sequential allocation. Efficacy of analgesia was assessed using 100 mm visual analogue pain scores (VAPS) at 5 min intervals for the first 30 min following bolus injection. Three outcomes were considered: Effective: VAPS 10 mm or less during contractions within 30 minutes; directed a 0.01% wt/vol decrement for the next patient. Ineffective: VAPS greater than 10 mm due to pain which responds to rescue with 12 mL 0.25% wt/vol of the same local anesthetic; directed a 0.01% wt/vol increment for the next patient. Reject: VAPS greater than 10 mm due to pain which fails to respond to rescue; directed a repeat concentration for the next patient. In addition, motor function was assessed at 15 min intervals with a modified Bromage scale. RESULTS: Two patients were rejected in the bupivacaine group and 3 were rejected in the ropivacaine group, leaving 18 in each group for analysis. MLAC (95% CI) was estimated using the up-down formula of Dixon and Massey [5] and by probit regression analysis as a back-up sensitivity test. The results are shown in Table 1.Table 1Ropivacaine is significantly less potent than bupivacaine with a potency ratio of 0.57 (95% CI 0.40-0.74). In each of the groups, 1 patient had a modified Bromage score of 1. None of the remaining patients showed any evidence of motor block. DISCUSSION: It is clear from this research that ropivacaine is significantly less potent than bupivacaine in terms of analgesic efficacy for the parturients in this study. No difference in motor effects was observed. These data suggest that the therapeutic index of ropivacaine must be reevaluated.