AbstractBackgroundCaudal ketamine has been shown to provide an effective and prolonged post-operative analgesia with few adverse effects. However, the effect of caudal ketamine on the minimum local anesthetic concentration (MLAC) of ropivacaine for intra-operative analgesia is unclear.MethodsOne hundred and sixty-nine children were randomized to five groups: Group C (caudal ropivacaine only), Group K0.25(caudal ropivacaine plus 0.25 mg/kg ketamine), Group K0.5(caudal ropivacaine plus 0.5 mg/kg ketamine), Group K0.75(caudal ropivacaine plus 0.75 mg/kg ketamine), and Group K1.0(caudal ropivacaine plus 1.0 mg/kg ketamine). The primary outcome was the MLAC values of ropivacaine with/without ketamine for caudal block.ResultsThe MLAC values of ropivacaine were 0.128% (0.028%) in the control group, 0.112% (0.021%) in Group K0.25, 0.112% (0.018%) in Group K0.5, 0.110% (0.019%) in Group K0.75, and 0.110% (0.020%) in Group K1.0. There were no significant differences among the five groups for the MLAC values (p = 0.11). During the post-operative period the mean durations of analgesia were 270, 381, 430, 494, and 591 min in the control, K0.25, K0. 5, K0.75, and K1.0groups respectively, which shown that control group is significantly different from all ketamine groups. Also there were significant differences between K0.25and K0.75groups, and between K1.0groups and the other ketamine groups.ConclusionsAdding caudal ketamine to ropivacaine prolong the duration of post-operative analgesia; however, it does not decrease the MLAC of caudal ropivacaine for intra-operative analgesia in children.Clinical trial registrationChiCTR-TRC-13003492. Registered on 13 August 2013.