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 ) .