BACKGROUND Verification of bidirectional block is important for mitral isthmus (MI) ablation. However, recurrent perimitral reentry exists despite apparently MI block. OBJECTIVE The purpose of this study was to identify and investigate the characteristics of the ridge breakthrough despite apparent bidirectional MI block and related reentry. METHODS In 60 patients undergoing MI ablation and achieving the criteria of bidirectional block when assessed on the line, the pattern under differential pacing was reassessed at the ridge away from the line to check whether a breakthrough existed. Also, activation and entrainment mapping was performed in 7 patients with ridge related reentry (RRR) to investigate its possible mechanism. RESULTS A ridge breakthrough was found in 7 of 60 patients (11.7%) apparently fulfilling the criteria of bidirectional block. The delay from pacing artifact during distal coronary sinus pacing was shorter at the ridge than on the line (95.6 +/- 11.7 ms vs 130.9 +/- 15.3 ms; P < .001). In 7 patients with RRR, the left atrial endocardial activation time accounted for 58.5% +/- 3.2% of the tachycardia cycle length (TCL) and wide double potential could be recorded on the line. The post-pacing interval (PPI) - TCL after entrainment at the ridge was shorter than that on the line (11.4 +/- 3.9 ms vs 34.3 +/- 6.6 ms; P < .001), and in 2 patients in whom entrainment from the coronary sinus was possible, the PPI TCL was 15 and 18 ms, respectively. CONCLUSION Apparent bidirectional MI block despite a ridge breakthrough is not uncommon and may lead to RRR, while the line is not part of the reentry. (C) 2016 Published by Elsevier Inc. on behalf of Heart Rhythm Society.