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  • However during pacing at the most


    However, during pacing at the most proximal CS electrodes (CS 9–10), although the conduction time to the CS 1–2 was prolonged, the septal to lateral activation sequence (CS 5–6 to CS 3–4 to CS 1–2) indicated the presence of residual conduction on the MI line (Fig. 3B). Detailed mapping on the MI line performed to localize a gap during pacing from the proximal CS (CS 7–8) revealed a fragmented potential between the CS 5–6 and CS 7–8 electrodes, demonstrating delayed conduction through the MI line. Supplemental RF application at this Rapalink-1 site resulted in a sudden change of the activation sequence on the CS catheter. The conduction time from the proximal CS (CS 7–8) to the distal CS (CS 1–2) was then prolonged from 160 to 215ms, and separated double potentials could be recorded on the mapping catheter positioned along the MI line (Fig. 4A). Finally, the differential pacing maneuver confirmed the establishment of the bidirectional block on the MI line. During pacing from CS 7–8, the delay from the pacing artifact to the atrial potential on the ablation catheter was 215ms (Fig. 4A). Changing the pacing site to the proximal CS (CS 9–10) resulted in a shorter perimitral activation time (Fig. 4B), thus suggesting the establishment of a complete bidirectional conduction block on the MI line. The activation time from CS 1–2 to CS 9–10 during pacing from CS 1–2 remained 180ms, as during the unidirectional conduction block. The LA–PV reconnection was detected in three PVs (the left and right superior PVs and the right inferior PV), which were successfully re-isolated. Following these procedures, no atrial arrhythmia could be induced by atrial pacing. The patient had no recurrence of AT during the subsequent 6-month observation period.
    Discussion Ablation along the MI line, that is, from the left inferior PV to the mitral annulus has been performed to treat patients with persistent AF and perimitral AT. The endpoint of this procedure was defined as the establishment of a bidirectional block on the line [6]. For CTI ablation, the fundamental methods of assessing bidirectional block have been described in previous reports [5,7]. Shah et al. proposed the criteria for achieving a bidirectional block for CTI as follows: (1) the presence of widely separated local double potentials along the length of the ablation line during CS pacing, (2) mapping the activation detour during pacing from either side of the line, and (3) differential pacing to distinguish slow conduction across the line from complete block [8]. These criteria can also be adapted for confirming a bidirectional block on the MI line [7]. Although a unidirectional conduction block on the CTI during ablation was described in patients with typical atrial flutter [5,6], macronutrients was rarely seen on the MI during ablation. Matsuo et al. previously demonstrated a unidirectional conduction block on the MI with unidirectional conduction in a clockwise direction during ablation for perimitral AT [9]. In the present case, we provide the first report of a unidirectional conduction block with residual unidirectional conduction in a counterclockwise direction on the MI. The mechanisms underlying the unidirectional conduction block in the linear ablation region remain unknown. In patients with typical atrial flutter, Matsushita et al. reported that an abrupt change of fiber orientation in cases of incomplete linear lesion by RF application may result in some degree of conduction disturbance on the line [10]. Cabrera et al. also described how heterogeneities in the refractoriness between fibers with different orientations at the CTI may create a unidirectional conduction block through ablation [11]. Anisotropic wave propagation at the MI in association with different fibers, however, has not been investigated. In the present case, the change of the electrical orientation on the MI line because of prior ablation, including RF application to the PV antrum and the LA around the MI and CS, may have provided a substrate for unidirectional conduction block on the MI line.