RESUMO
BACKGROUND: The minimum local analgesic concentration has been defined as the median effective local analgesic concentration (EC50) in a 20-ml volume for epidural analgesia in the first stage of labor. The aim of this study was to assess the relative analgesic potencies of epidural levobupivacaine and ropivacaine by determination of their respective minimum local analgesic concentrations. METHODS: Parturients at 7 cm of cervical dilation or less who requested epidural analgesia were allocated to one of two groups in this double-blind, randomized, prospective study. After lumbar epidural catheter placement, 20 ml of the test solution was given: levobupivacaine (n = 35) or ropivacaine (n = 35). The concentration of local anesthetic was determined by the response of the previous patient in that group to a higher or lower concentration using up-down sequential allocation. Analgesic efficacy was assessed using 100-mm visual analog pain scale scores, with 10 mm or less within 30 min defined as effective. An effective result directed a 0.01% wt/vol decrement for the next patient. An ineffective result directed a 0.01% wt/vol increment. RESULTS: Of 105 women enrolled, 35 were excluded, leaving 70 for analysis. The minimum local analgesic concentration of levobupivacaine was 0.087% wt/vol (95% CI, 0.081-0.094%), and the minimum local analgesic concentration of ropivacaine was 0.089% wt/vol (95% CI, 0.075-0.103%). Levobupivacaine and ropivacaine were of similar potency with a ropivacaine:levobupivacaine potency ratio of 0.98 (95% CI, 0.80-1.20). No difference in motor effects was observed. CONCLUSIONS: This study demonstrated that levobupivacaine and ropivacaine are of similar potency for epidural analgesia in the first stage of labor.
