[RESULTS] 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 ) .