This prospective randomized controlled trial aims to compare basilic vein and cephalic vein approaches for midline catheter placement in pediatric patients undergoing general anesthesia. Participants aged 2 to 17 years who require midline catheter insertion will be randomly assigned 1:1 to either the basilic vein group or the cephalic vein group. The primary outcome is the clinical blood sampling success rate during the catheter dwell period, defined as successful aspiration of ≥2 mL of blood through the midline catheter at least once without additional venipuncture. Secondary outcomes include operator-rated difficulty, number of puncture attempts, first attempt cannulation success rate, vein diameter and depth, catheter dwell time, catheter-related complications, and patient satisfaction. A total of 96 participants (48 per arm) will be enrolled at Seoul National University Children's Hospital.

Inclusion Criteria:

• Age 2-17 years undergoing general anesthesia
• Midline catheter placement deemed necessary by attending anesthesiologist (≥1 of: expected ≥2 postop blood draws; expected blood loss ≥40% EBV; continuous antibiotic therapy; hospitalization ≥7 days; surgical team request)

Exclusion Criteria:

• Need for central venous catheter per surgical team
• Coagulopathy
• History of severe thrombosis
• Other investigator-determined ineligibility