Limitations of Using Endocardial Activation Criteria to Define Isthmus Block During Radiofrequency Ablation of Typical Atrial Flutter Kuan-Cheng Chang, Yu-Chin Lin, Jan-Yow Chen, Hsian-Tai Chou, Jui-Sung Hung China Medical College Hospital, Taichung, Taiwan Introduction: Demonstration of complete bi-directional isthmus block (CBIB) according to the criteria of changes of endocardial conduction properties during proximal coronary sinus (PCS) pacing and low lateral right atrial (LLRA) pacing is an essential end point for ablation of typical atrial flutter (AFL). This study showed several limitations by using the endocardial activation criteria. Methods: Twenty consecutive patients (16 males, 4 females, age 68 + 13 years, range 43 to 86 years) with isthmus-dependent AFL as confirmed by entrainment pacing who underwent RF ablation were studied. When the ablation was performed during sinus rhythm (SR), simultaneous LLRA pacing to assess the immediate effect of radiofrequency (RF) ablation on counterclockwise (CCW) isthmus conduction was initiated. After completion of each RF pulse, the clockwise (CW) isthmus conduction was evaluated by pacing PCS at the same cycle length (CL). When the ablation was performed during AFL, after restoration of SR, the pacing/ablation strategy was the same as that during SR. The transisthmus conduction time (TICT) in the CCW direction was measured from pacing spike to the PCS electrogram during LLRA pacing, and that in the CW direction was measured from pacing spike to the Halo 1 electrogram during PCS pacing. Results: All patients had CCW AFL (CL 225 + 29 ms) and 3 of them had both CCW and CW AFL (CL 212 + 6 ms). Spontaneous or induced atrial fibrillation was present in 5 of the 20 patients. In 6 of the 20 patients, transient unidirectional-like conduction block was observed before achieving CBIB during RF ablation. All the transient unidirectional-like conduction blocks were demonstrated in the CCW direction. The TICT was 105 + 11
