Caudal block is the world-widely used technique for an effective postoperative analgesia in children undergoing OPD-based infra-umbilical surgery. Because the caudal block is usually performed with a single-shot, proper dosage is required to achieve sufficient analgesic levels. Several clinical researches have been performed to assess the drug spread levels using mathematical equation, X-ray evaluation, and ultrasonography during caudal block in children. In our previous study using fluoroscopy, we obtained the results that the established weight-based doses provided adequate block levels for each type of surgery.

Ultrasonography is precise tool to evaluate the spinal structures and can provide estimated information in real time during caudal or epidural block in children.

The aim of this study is to evaluate the drug spread level with a weight-based doses using ultrasonography during caudal block in children and assess the reliability of the real time ultrasonography to determine the drug spread levels. After general anesthesia, the patient will be placed in lateral decubitus position for caudal block. Total 1.5 ml/kg of 0.15% ropivacaine will be injected into the caudal space. The spread level will be step-wise evaluated using ultrasonography at the injected drug dose of 0.5 ml/kg, 1.0 ml/kg, 1.25 ml/kg, and 1.5 ml/kg.

Inclusion Criteria:

• children who needed caudal block for postoperative analgesia (age 3 months ~ 5 years old)
• Consent from one of parents 3. ASA physical status I or II

Exclusion Criteria:

• bleeding tendency
• spinal anomaly
• infection focus on back
• allergic reaction to local anesthetics
• foreigner of illiteracy who cannot read consent form

3 months old ~ 5 years old children undergoing minor infra-umbilical urological surgery