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Radial arterial catheterization in pediatric patients is occasionally difficult despite ultrasound guidance. We therefore assessed the factors affecting catheterization and tested an intervention designed to improve its success.For initial assessment, we performed multiple logistic regression analyses using 102 pediatric patients. Dependent variables included first-attempt and overall success or failure; independent variables were systolic blood pressure, weight, ASA physical status, trisomy 21, arterial diameter, and subcutaneous depth of the radial artery (<2, 2–4, ≥4 mm). The effect of subcutaneous arterial depth on cannulation success was assessed using Kaplan-Meier curves with log-rank and Dunn tests. We then assessed catheterization success in 60 patients who were randomized to no treatment or subcutaneous saline injection, as necessary, to increase the subcutaneous arterial depth from <2 to 2 to 4 mm.Subcutaneous arterial depth of 2 to 4 mm was derived as a significant independent predictor of initial and overall success from the multiple logistic regression analyses. The 2 to 4 mm group had a significantly shorter catheterization time compared with the other 2 groups in the log-rank test (2–4 vs <2 mm group; P = 0.01, 2–4 vs ≥4 mm group; P < 0.001), and higher success rate in the first attempt (<2 [43.8%] vs 2–4 mm [76.9%], P = 0.02; 2–4 [76.9%] vs ≥4.0 mm [19.4%], P < 0.001), and the overall attempt (<2 [62.5%] vs 2–4 mm [89.7%], P = 0.04; 2–4 [89.7%] vs ≥4.0 mm [51.6%], P = 0.002). Injecting subcutaneous saline to bring arterial depth from <2 mm to 2 to 4 mm significantly shortened catheterization time (P = 0.002), and improved the success rate in the first-attempt (saline injection [85.0%] vs <2 mm [30.0%], P < 0.001), and the overall attempt (saline injection [90.0%] vs <2 mm [55.0%], P = 0.02).Ultrasound-guided radial artery catheterization in pediatric patients was fastest and most reliable when the artery was 2 to 4 mm below the skin surface. For arteries located <2 mm below the skin surface, increasing the depth to 2 to 4 mm by subcutaneous saline injection reduced catheterization time and improved the success rate.