Unidirectional Conduction at the Left Atrial Roof Observed in the Chronic Phase After Single‐Ring Isolation for Long‐Standing Persistent Atrial Fibrillation

Single-ring isolation (SRI) was achieved, but residual unidirectional conduction remained along the left atrial roof. After SRI, repetitive activity from the left superior pulmonary vein (LSPV) triggered atrial fibrillation. Additional ablation at the unidirectional conduction gap site successfully achieved bidirectional conduction block. During the second session, similar unidirectional conduction persisted. Ablation of the roof gap during LSPV pacing restored sinus rhythm. Catheter ablation for atrial fibrillation (AF) is commonly performed using pulmonary vein isolation (PVI). A single-ring isolation (SRI) technique, which incorporates posterior left atrial (PLA) isolation, has also been introduced 1. Nonetheless, PLA reconnection after SRI is not uncommon and may contribute to AF recurrence 1. We report a rare case in which chronic post-SRI left atrial (LA) roof reconnection manifested as unidirectional conduction, serving as the mechanism of recurrent AF. A 62-year-old man underwent his first ablation for long-standing persistent AF. SRI was performed using a radiofrequency (RF) catheter (NaviStar ThermoCool SmartTouch, Biosense Webster) with CARTO guidance (Figure 1A). Circular mapping catheters were positioned in both superior pulmonary veins (PVs), and an octapolar catheter was placed in the coronary sinus (CS). During single-ring ablation, both PV potentials disappeared simultaneously, achieving SRI. However, rapid ectopic beats from the left superior PV (LSPV) propagated through the LA and right superior PV (RSPV), initiating AF. After ectopic beats terminated, no activation was seen in either superior PV during AF, indicating a conduction gap permitting unidirectional conduction between the PV/PLA region and the remaining atrium (Figure 1B). Mapping during LSPV pacing identified LA roof gaps, which were subsequently ablated using RF energy delivered at 35 W with a contact force of 10–20 g for 30 s (Figure 1C). Adenosine confirmed no dormant conduction. Ten months later, the patient experienced recurrent paroxysmal AF and underwent repeat ablation. Mapping catheters were positioned as before. During sinus rhythm, no electrical activity was recorded in any PV or the PLA, confirming SRI entry block. However, LSPV ectopic beats conducted to the CS and LA, appearing as premature atrial contractions (Figure 2A). CS pacing failed to capture the PVs or PLA, confirming entry block (Figure 2B). LSPV pacing captured the RSPV, CS, and right atrium in a 1:1 fashion, demonstrating persistent unidirectional conduction to the LA (Figure 2C). Pacing from the RSPV and PLA also conducted to the atrium outside the SRI region. Spontaneous AF was again triggered by LSPV ectopy with the same pattern (Figure 3A). Detailed mapping along the previous SRI line during LSPV pacing revealed an LA roof gap at an almost identical site to that observed in the initial session, which was subsequently ablated (Figure 3B), achieving exit block shortly thereafter (Figure 3C). Although rapid firing persisted within the LSPV and reached the RSPV, it no longer conducted beyond the SRI, allowing sinus rhythm to continue. After confirming bidirectional block of the SRI line, the procedure was completed. The patient has remained free of atrial tachyarrhythmias. The key finding is the persistence of unidirectional conduction along the LA roof in both acute and chronic phases after SRI. The significance of exit block in PVI has been debated. Gerstenfeld et al. demonstrated PV exit block in approximately half of cases with confirmed entrance block and proposed exit block as an endpoint 2. Conversely, Duytschaever et al. found unidirectional PV-LA conduction in only 0.6% of RF-PVI cases, supporting entry block alone as adequate 3. Andrade et al. showed that, in cryoballoon PVI, entrance block reliably follows exit block, making the entrance block sufficient 4. Collectively, these findings, together with the source–sink mismatch from the narrow PV sleeve to the larger left atrium, indicate that unidirectional PV-LA conduction is rare during PVI. In this case, unidirectional conduction occurred within the LA roof rather than at the PV-LA junction, distinguishing it from prior reports. Nogami et al. reported unidirectional conduction across a surgically created isolation line in the left atrium 5. However, because the lesion was made by surgical incision and cryoablation and assessed using only a single esophageal electrode, the precise site of the conduction gap could not be determined. To our knowledge, this is the first clear demonstration of unidirectional conduction across catheter-created SRI lesions. Although rare, this phenomenon shows that conduction to the LA can persist after SRI despite confirmed entry block, and exit block should therefore also be evaluated using pacing maneuvers. A plausible mechanism involves the septopulmonary bundle, which courses across the epicardial mid LA roof. This region is relatively thick and may contain adipose tissue, potentially limiting transmural lesion formation and allowing residual gaps to persist. SRI is also known to be susceptible to reconduction, particularly at the LA roof, partly due to epicardial fiber contributions. These factors likely contribute to the phenomenon observed in this case and highlight the need for careful lesion durability assessment when performing SRI. In addition, conduction across a narrow residual pathway at the LA roof may become direction-dependent due to the interaction between incomplete transmural lesions, epicardial fiber orientation, and local source–sink relationships. Such direction-dependent conduction may allow propagation in one direction while suppressing conduction in the opposite direction, resulting in functional unidirectional block despite apparent electrical isolation. This case demonstrates that unidirectional conduction may persist after SRI in both acute and chronic phases. Even when entry block is confirmed, exit block should also be assessed to ensure complete isolation. The authors have nothing to report. The authors have nothing to report. This study was approved by the Yokohama City Bay Hospital Ethics Committee. Written informed consent was obtained from the patient. The authors declare no conflicts of interest. All data used in this report are available upon reasonable request.