In studies of atypical AFL with or without previous ablation for AF in the left atrium [1‐5], RF ablation has mainly been performed by two approaches: linear lesions joining two anatomic barriers or silent areas, or both [1, 12] or, second, an electrophysiological approach targeting a critical isthmus presenting with a zone of slow conduction shown to be part of the circuit by pacing maneuvers [3, 4, 6]. Application of RF at this critical isthmus had been performed successfully with RF applications shorter than those usually needed for linear ablation [4, 6].

AFL termination by ablating inside the CS has been previously reported [6, 7]. An atypical AFL circuit involving the CS and left atrial myocardium was described by Olgin et al. [7], with circumferential successful ablation within the distal CS. In the report of Della Bella et al. [6], two successful procedures were performed within the CS at a site where APs exhibited fragmented and prolonged mid-diastolic activity, and where concealed entrainment could be demonstrated.

To our knowledge, this is the first study to include a large number of patients with CS-dependent AFL who were ablated by targeting mid-diastolic APs inside the CS. The procedures were successful after a short RF application time.

Anatomic studies [13‐16] have shown that the proximal 4 cm of the CS is surrounded by a cuff of striated muscle, which is continuous with the RA myocardium at the CS ostium, and has multiple connections to the left atrial myocardium, and fewer connections to its distal portion. These muscular sleeves are extremely variable in number, location, and thickness. Platonov et al. [17] described a discrete inferior connection between the CS and left atrial myocardium at the level of the right inferior pulmonary vein. This inferior connection may be more prominent than the Bachmann bundle [17, 18].

The genesis of fractionated prolonged APs in the CS is attributable to anisotropic conduction due to its complex anatomy, in combination with conduction slowing during cycle shortening AFL. In our patients, APs were constantly prolonged in duration, spanning a large proportion of the AFL cycle length. The mid-diastolic AP location and AFL termination in a short time argue strongly in favor of a location representing a critical isthmus within the CS musculature. The site of application of RF in the CS was a median distance of 2 cm from the ostium, and a maximum distance of 4 cm, a zone that overlaps the anatomically described extension of the CS muscular cuff [14, 15].

In some patients, CTI ablation interrupted AFL, but the same arrhythmia with prolonged cycle length was subsequently reinduced despite the presence of complete CTI block criteria. In other cases, CTI ablation was ineffective, but AFL cycle length increased. However, RF inside the CS targeting mid-diastolic APs was successful for AFL termination and subsequent noninducibility. In one patient with both CTI and CS entrainments, CS ablation as a first attempt was successful in AFL interruption and noninducibility.

Olgin et al. [7] illustrated a case of atypical AFL, in which entrainment mapping suggested a circuit involving the CS exiting in the lateral left atrium, descending the interatrial septum, and reentering the CS. Spaced double potentials throughout the length of the CS were recorded. Successful ablation was accomplished by circumferential RF application within the distal CS. Conversely, APs inside the CS in our patients were always fragmented and polyphasic (no spaced double potentials were recorded), and the successful CS target was more proximal.

Several reports [19, 20] described lower loop reentry as a variant of isthmus-dependent AFLs. In this case, the reentrant circuit turns around the inferior vena cava (IVC), or the IVC and tricuspid annulus in a figure-8-double loop reentry, both types involving also the CS musculature. Thus, CS involvement might suggest a similarity with our patients. Nevertheless, collision front in lateral RA described in lower loop reentry and RF interruption of these AFLs at ICT without needing of adjunctive RF lesions are not in agreement with our observations, making this mechanism improbable in our series. More recently, the same group [21] described in eight patients referred for typical AFL ablation, an intra-isthmus reentry circuit confined to the septal CTI, circling the CS ostium, and showing a collision wave front at the RA wall; in these AFLs, successful ablation required septal CTI RF applications. In our series, no patient presented with these electrophysiological characteristics, and no patient was ablated at the septal CTI.

The correlation between AFL wave morphology and the underlying reentrant circuit is imperfect with infrequent but possible overlap between CTI and non CTI-dependent AFLs [22]. No typical and constant ECG pattern of CS-dependent AFL was identified in this study. Several patients had a “sawtooth” ECG pattern in the inferior leads and positive F wave in V1, while others presented with variable ECG patterns. The intracardiac activation pattern in our patients was also not useful for differentiating CS-dependent AFL from CTI-dependent AFL. The ability to identify patients with CS-dependent AFL before ablation was not possible using surface ECG and endocavitary atrial activation patterns.

Delivery RF energy into the CS is often necessary to achieve complete block in the left mitral isthmus during a left atrial procedure for AF [23, 24]. More recently, the CS has been a target for more extensive RF applications, including AF termination [25, 26]. Ablation of the left atrial isthmus is often completed inside the CS on a line between the left inferior pulmonary vein and mitral annulus [23, 27]. A case was recently reported of acute occlusion of the left circumflex coronary artery during left mitral isthmus linear ablation [28].

In our series, RF was applied in the first 4 cm of the CS, a zone where the wall is thick and arterial vessels are more distal. Additionally, the RF dose was generally low, contributing to the safety of ablation. In previous studies in humans and animals, ablation of epicardial located accessory pathways inside the CS was also shown to be relatively safe [29‐33].

This study has several limitations. It included a heterogeneous study population with a complex rhythm disturbance with a lack of detailed mapping and standardization of ablation therapy. Mapping used conventional electrophysiological criteria, entrainment mapping was not systematically and extensively performed at sites other than the CS, and no electroanatomic map was planned. On the other hand, a pragmatic clinical approach was used, which was successful in terms of acute and long-term results.

The results of this study strongly suggest that mid-diastolic APs in the CS should be evaluated systematically in patients with a “failed” CTI procedure for typical ECG AFL and in some patients with atypical AFL. This strategy may be help in avoiding an unnecessary transseptal approach for atrial left side ablation.