The primary objective of the study is to evaluate the vasodilative effect of subcutaneous nitroglycerin during ultrasound-guided radial artery cannulation in pediatric patients. The hypothesis of this study is that the subcutaneous nitroglycerin will increase the radial artery size and improve the first-attempt success rate of radial artery cannulation. This is a single-center, double-blind, randomized, placebo-controlled study comparing the effect of subcutaneous nitroglycerin and saline on radial artery cannulation in pediatric patients undergoing general anesthesia. Prior to the procedure, each patient will be randomized into either the control arm, saline, or study arm, nitroglycerin.

Inclusion Criteria:

• General anesthesia
• Arterial cannulation for hemodynamic monitoring, multiple blood sample

Exclusion Criteria:

• Hypersensitivity or contraindication to nitroglycerine
• Unstable vital sign, significant arrhythmia or hypotension, Shock
• High risk of peripheral ischemia
• Skin disease, infection, hematoma, recent cannulation at radial artery.
• Increased intracranial pressure, Intracranial hemorrhage
• Glaucoma
• Recent use of PDE5 inhibitors (<24 hours after sildenafil or vardenafil; <48 hours after tadalafil)