Two‐To‐One Conduction Between the Inside and Outside of the Left Atrial Posterior Wall During Atrial Tachycardia: What Is the Mechanism?

A 54-year-old man was referred for a second procedure for recurrent atrial fibrillation (AF). He had undergone pulmonary vein (PV) isolation for AF 2 years prior. The baseline rhythm was AF. After defibrillation, left atrial (LA) mapping was performed using a three-dimensional electro-anatomical mapping (3D-EAM) system (CARTO™) and an OCTARAY mapping catheter (Biosense Webster Inc., Diamond Bar, CA, USA). No PV reconnections were observed. LA posterior wall (LAPW) box isolation, consisting of both LAPW roof and floor lines, was performed using an irrigated-tip ablation catheter (Thermocool SF, Biosense Webster Inc., Diamond Bar, CA, USA) (Figure 1A). Atrial tachycardia (AT) then occurred (Figure 2A). During AT, the tachycardia cycle length (TCL) inside the LAPW was 284 ms, whereas that outside the LAPW was 568 ms. The 3D-EAM was created focusing on the outside the LAPW (Figure 1B,C) and on the inside the LAPW (Figure 1D). Outside the LAPW, activation originated from the anterior LA, just anterior to the LAPW roof line (Figure 1B) (the distance between the roof line and the earliest activation site outside the LAPW: 14 mm), and was blocked at the LAPW floor line (Figure 1C). Inside the LAPW, the activation originated from within the LAPW (Figure 1D) (the distance between the roof line and the earliest activation site inside the LAPW: 16 mm). Overdrive pacing was performed using a mapping catheter positioned inside the LAPW (Figures 1D and 2B). During overdrive pacing, the electrocardiogram (EGM) recorded inside the LAPW was accelerated to the pacing cycle length (PCL) of 270 ms (black arrowheads in Figure 2B). In contrast, the EGM recorded outside the LAPW was accelerated to twice the PCL (540 ms), demonstrating 2:1 conduction (blanked arrowheads in Figure 2B). The post-pacing interval-minus-TCL was 16 ms (Figures 1D and 2B). The radiofrequency catheter ablation (RFCA) at this site terminated AT (Figure 1D). What is the mechanism of the 2:1 conduction between the inside and outside of the LAPW during AT? Pacing was performed from the distal coronary sinus (CS) (Figure 3A). Delayed EGMs were recorded by the catheters positioned inside the LAPW (black arrowheads in Figure 3A). Pacing performed within the LAPW captured the inside and outsaid the LAPW (Figure 3B,C). In detail, the EGMs recorded inside the LAPW were captured immediately after the pacing artifact (solid arrows and arrowheads in Figure 3B,C), whereas the EGMs recorded outside the LAPW were captured with a conduction delay (a stimulus-P-wave interval of 340 ms; blanked arrowheads and dashed arrows in Figure 3B). Incremental pacing gradually increased the stimulus-P-wave interval, up to 440 ms with the PCL of 330 ms (blanked arrowheads and dashed arrows in Figure 3C). RFCA was performed to target the EGMs recorded in the LAPW, resulting in LAPW box isolation, with the endpoint being non-capture of pacing within the LAPW. Our case provides further insight into the septopulmonary bundle's electrophysiological properties. Recordings were not performed in the right atrium to evaluate the potential of a bi-atrial circuit. However, based on the 3D-EAM showing the centrifugal pattern, and the entrainment mapping, the mechanism was micro-reentry localized at the LAPW. The EGMs recorded outside the LAPW during AT were a result of 2:1 conduction from the LAPW via the epicardial bridge, presumably due to the conduction through the septopulmonary bundle, rather than the residual endocardial conduction of incomplete roof line based on the following findings. First, during distal CS pacing, the EGMs recorded inside the LAPW were delayed with the isoelectric line (Figure 3A). Second, pacing performed within the LAPW captured the inside and outsaid the LAPW (Figure 3B,C). Third, the distance between the roof line and the ablation site inside the LAPW was 17 mm (Figure 1D). Finally, LAPW box isolation was achieved by RFCA targeting the residual EGMs inside the LAPW without the need for additional RFCA on the roof and floor lines. The stimulus P-wave interval gradually increased with incremental pacing performed inside the LAPW, with a similar atrial activation sequence (Figure 3B,C). These findings suggest the potential decremental nature of the septopulmonary bundle, rather than its longitudinal dissociation. The presence of an epicardial connection of the LA and its role in AT have been reported [1, 2]. These papers described roof-dependent AT, in which epicardial connections were involved as a part of the AT circuit. In this respect, our case is notable in that the presence of the septopulmonary bundle manifested without involvement in the AT circuit. In addition, we demonstrated the decremental properties of the septopulmonary bundle, which have rarely been reported. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.