Dose-response study of epidural ropivacaine for labor analgesia

Background: Ropivacaine has been introduced for use in epidural analgesia i n labor. However, there have been few formal dose-response studies of ropiv acaine in this setting. Methods: The authors performed a prospective, randomized, double-blind stud y examining the effectiveness of five different doses of ropivacaine (10, 2 0, 30, 40, and 50 mg) administered epidurally in a volume of 10 ml to estab lish analgesia in 66 parturients who were in active labor with cervical dil atation less than 4 cm. A dose was considered effective when the visual ana log scale pain score decreased by 50% or more from baseline. Results: A sigmoid dose-response curve and a probit log dose-response plot (linear regression coefficient, r = 0.84; coefficient of determination, r(2 ) = 0.71) were obtained. The ED50 (median effective dose) obtained based on the maximum Likelihood estimation was 18.4 mg (95% confidence interval, 23 .4-25.4 mg). Time to onset of analgesia, duration of analgesia, time to two -segment regression of sensory block level, and incidence of motor block we re not affected by the dosage of ropivacaine administered (P = 0.93, 0.12, 0.55, and 0.39, respectively). However, the upper level of sensory block wa s dose-related (P < 0.01). Conclusive in a traditional dose-response study, the ED50 of ropivacaine re quired to initiate epidural analgesia in early labor was found to be 18.4 m g (95% confidence interval, 13.4-25.4 mg).