Preemptive analgesia can improve postoperative pain management. Ketamine may prevent central sensitization during surgery and result in preemptive analgesia. The purpose of this study is to examine the effectiveness of ketamine as a preemptive analgesic as previous studies have shown the involvement of N-methyl-D-Aspartate (NMDA) receptor in neuroplasticity.

After receiving consent inform from parents, 40 children scheduled for orthopedic surgeries will be randomized to one of two groups: epidural group and intravenous group, both will receive 1 mg kg-1 S(+)-ketamine. All patients will receive caudal block anesthesia with marcaine. Cardiovascular monitoring will be assessed during operation. Follow up will continue for 24 hours after caudal block. Duration of analgesia, first time of analgesic request and complications will be recorded by an orthopedic assistant that is blinded to study. Data will be analyzed statistically by Chi square, t test and nonparametric tests.

Inclusion Criteria:

• Children aged < = 12 years
• Children scheduled for elective orthopedic surgery with caudal block
• ASA score < = 3

Exclusion Criteria:

• Contraindication for caudal block such as vertebral defect or infection at the site of block
• Disagreement of parents
• Patient's age > 12 years
• ASA score > = 4