Caudal analgesia with ropivacaine is commonly used in sub-umbilical pediatric surgery. However, increasing the dosage of ropivacaine has not been found to prolong the action significantly while complications will be serious. Ketamine as an additive to caudal administration had been shown to prolong the duration of postoperative analgesia, while the analgesic effectiveness of S-ketamine, the S(+)-enantiomer of ketamine with less possibility to induce psychomotor disturbances, is not clear. In this prospective randomized double-blind clinical trial , the investigators aimed to study the effect of S-ketamine as additive on the duration of caudal analgesia.