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.

Forty-four children scheduled for hypospadias randomize into 2 groups. Group E (n=22) (0.2% ropivacaine 0.7 ml／kg and S-ketamine 0.5 mg/kg), group C(n=22)( 0.2%ropivacaine 0.7ml／kg and equivalent saline) intraoperative and postoperative hemodynamics will be recorded. Postoperative pain is assessed using an established 6-item FLACC score at 1h,3h,6h,12h,24h,48h, or the time when children complain of pain after surgery. A score of more15μg/kg is administered.Investigators propose to compare the duration of caudal analgesia provided by plain ropivacaine and by a mixture of ropivacaine and S-ketamine.

Inclusion Criteria:

• informed parent consent, ASA I or II children, between 3months and 18 years of age,and weighing <28kg, scheduled for elective hypospadias surgery with general anesthesia will be recruited.

Exclusion Criteria:

• Patients who have congenital abnormalities of lower spine and meninges.
• Patients with hypersensitivity to any local anesthetics
• Children with coagulation disorders
• Presence of Infections at puncture sites
• Preexisting neurological disease
• Refusal to consent by parent/guardian

The Patients allocated to study groups using computer generates random list & group assignment is sealed in sequentially numbered opaque envelopes that open after induction of anesthesia.