Relative Analgesic Potencies of Levobupivacaine and Ropivacaine for Caudal Anesthesia in Children Pablo Ingelmo, MD* Geoff Frawley, MD† Marinella Astuto, MD* Chris Duffy, MD† Susan Donath, PhD‡ Nicola Disma, MD§ Giuseppe Rosano, MD§ Roberto Fumagalli, MD* Antonio Gullo, MD§ BACKGROUND: Comparing relative potency of new local anesthetics, such as levobupivacaine and ropivacaine, by the minimum local analgesic concentration model has not been described for caudal anesthesia. Therefore, we performed a prospective, randomized, double-blind study to determine the minimum local analgesic concentrations of a caudal single shot of ropivacaine and levobupivacaine in children and to describe the upper dose-response curve. METHODS: We performed a two-stage prospective, randomized, double-blind study comparing the dose-response curves of caudal ropivacaine and levobupivacaine in children. In phase 1, 80 boys were randomized to receive either ropivacaine or levobupivacaine. In the second phase a further 32 patients were randomly allocated to receive caudal anesthesia with doses designed to delineate the upper dose- response range (the 50% effective dose [ED 50 ]–ED 95 range). RESULTS: There were no significant differences in ED 50 values for caudal ropivacaine and levobupivacaine. The ED 50 for levobupivacaine estimated from the Dixon Massey method was 0.069% (95% CI 0.056%– 0.082%) and for ropivacaine was 0.075% (95% CI 0.058%– 0.092%). Estimated by isotonic regression the ED 50 and ED 95 respectively of levobupivacaine were 0.068 (0.04 – 0.09) and 0.20% (95% CI 0.16%– 0.24%). For ropivacaine ED 50 and ED95 were 0.066 (0.033– 0.098) and 0.225% (95% CI 0.21%– 0.24%). CONCLUSIONS: In children receiving one minimum alveolar anesthetic concentration of sevoflurane, there were no significant differences in the ED 50 for caudal levobupivacaine and ropivacaine. The potency ratio at ED 50 was 0.92 and 0.89 at ED 95 , indicating that caudal levobupivacaine and ropivacaine have a similar potency. (Anesth Analg 2009;108:805–13) An efficient way to compare the potency of new long-acting local anesthetics such as levobupivacaine and ropivacaine in a clinical setting is to determine the minimum effective local anesthetic concentration (MLAC) required to produce adequate pain relief in 50% of patients (the Dixon Massey Up-Down sequential allo- cation technique). 1 Adult studies comparing the mini- mum local analgesic concentrations of levobupivacaine and ropivacaine have reported conflicting results. 2– 4 Stud- ies in obstetric patients based on up-down sequential allocation designs have suggested that ropivacaine is less potent than racemic bupivacaine. 4,5 However, some studies have determined that ropivacaine and bupivacaine are equipotent and that although their 50% effective dose (ED 50 ) may vary, the dose-response curves overlap at clinically relevant doses. 6 For epi- durally administered surgical anesthesia, ropivacaine and bupivacaine have essentially similar sensory and motor block profiles but with intrathecal studies sensory-motor separation is more marked. 6,7 To determine the dose-response relationship re- quires information on the ED 50 (MLAC), the slope of the dose-response curve and the maximal efficacy. Deng et al. 8 reported an ED 50 of ropivacaine, but the MLAC of caudal levobupivacaine has not been de- scribed in children and little is known about the upper dose-response curve of either drug. The MLAC method provides accurate estimation of the ED 50 for a certain response but does not allow for interpretation of the whole dose-response curve. Although imprecise ex- trapolation of data to provide the ED 95 dose using logistic regression analysis after probit transformation has been reported. It is important to have more detail about the upper dose-response range for pediatric From the *Department of Perioperative Medicine and Intensive, A.O. San Gerardo, Monza, Dipartimento di medicina sperimentale ambientale e biotecnologie mediche, Universita ` degli Studi Milano Bicocca, Milan, Italy; †Department of Paediatric Anesthesia and Pain Management, Royal Children’s Hospital. Melbourne Australia; ‡Clinical Epidemiology and Biostatistics Unit, Murdoch Children’s Research Institute, University of Melbourne Australia; and §Depart- ment of Anesthesia, A.O.U. Policlinico Catania, Universita ` degli Studi di Catania, Italy. Accepted for publication September 2, 2008. Supported by the Department of Perioperative Medicine and Intensive, A.O. San Gerardo and by the Department of Anesthesia, A.O.U. Policlinico Catania. Address correspondence and reprint requests to Dr. Geoff Frawley, Department of Paediatric Anesthesia and Pain Manage- ment, Royal Children’s Hospital, Melbourne Australia. Address e-mail to geoff.frawley@rch.org.au. Copyright © 2009 International Anesthesia Research Society DOI: 10.1213/ane.0b013e3181935aa5 Vol. 108, No. 3, March 2009 805