Transvenous ablation of AF is currently performed either by multiple sequential radiofrequency (RF) applications aiming at electrical disconnection of PVs from the LA (segmental PV ablation, or PVA) (1, 2, 3) or by PV linear encircling (LA circumferential ablation, or LACA) (4, 5, 6). In the attempt to better prevent recurrences of AF, circumferential PV ablation has been further modified by the addition of a set of linear lesions deployed between the encircled PVs or by joining them with fixed anatomical barriers, such as the mitral annulus (7,8). The rationale for this approach is represented by substrate modification, which is believed to be effective mostly in patients with long-lasting AF due to extensive atrial remodeling and extra-PV triggers (9). Both segmental PVA and LACA have been reported to carry the risk for the development of left ATs of new onset (10,11).

Depending on the pathophysiological mechanism, ATs can be distinguished as focal ATs or reentrant ATs. Focal ATs are characterized by atrial activation originating from a small area spreading through the remaining atrium. The mechanisms of focal AT include automaticity, triggered activity, and small reentrant circuits. Macroreentrant AT is due to re-entry around a fixed or functional central obstacle (12).

The incidence of AT following radiofrequency (RF) ablation of AF ranges from 1% to 25% (13) and varies considerably according to the ablation technique. It is more frequently observed in patients undergoing LACA (13,14) than in patients undergoing segmental PVA (15). Moreover, the two approaches are associated with different specific risks for focal or macroreentrant AT. In fact, macroreentrant ATs are more commonly seen following circumferential ablation (7,16,17), while focal ATs more frequently complicate ostial or antral PVA (15,18) (Table 18.1).

Macroreentrant AT is identified during electrophysiologic (EP) study as tachycardias showing a continuous sequence of atrial activation, a range of activation time covering almost the full interval of the tachycardia CL, and concealed entrainment by pacing at sites within the presumed reentrant circuit. Regular AT occurring after AF ablation has been shown to derive from iatrogenic obstacles created by ablation lines encircling the PVs (16,19) or connecting them with contralateral veins or fixed anatomical barriers. Deployment of ablation lines may offer an ideal substrate for reentry through conduction gaps.

In a prospective randomized study comparing circumferential PVA with a modified approach including ablation lines on the posterior wall and the mitral isthmus, Pappone et al. (11) showed that the addition of linear lesions within the LA connecting encircling lesions of lateral and septal PVs prevents postablation AT. Indeed, AT occurred in 10% of patients treated with the conventional ablation protocol, and in 4% in the modified approach group. Of note, all patients in which completeness of PV encircling was achieved (80%) remained free from AT during the follow-up, whereas multiple gaps were documented in patients who underwent a repeated procedure for incessant AT. This observation supports the concept that completeness of ablation lesions is crucial for preventing macroreentry. It remains to be established whether completion of ablation lines is easily achievable and should be systematically validated.

In another study, Ernst et al. investigated the role of LA linear lesions (20). Using four prespecified ablation designs, the authors could prove that complete linear lesions are very difficult to achieve, and that the presence of gaps across the ablation lines strongly correlates with the occurrence of clinical AT. Their study also showed that even if completion of ablation lines is achieved during the index procedure, conduction gaps may later emerge as a result of the tissue healing process.

Despite the adoption of complex ablation designs and the wider use of irrigated-tip catheters to improve lesion deployment, the incidence of AT complicating LACA remains high. Furthermore, bridging encircling lines with anatomic structures or other ablation lines has proven to be challenging and presents safety issues. In their series of 100 patients undergoing PV isolation and linear ablation of the cavotricuspid and mitral isthmuses, Jaïs et al. (21) reported 14 ATs during the follow-up. Recovered or incomplete mitral isthmus line accounted for 5 (36%) of the new-onset ATs. Noteworthy, bidirectional isthmus block was achieved in 92% of patients and was carefully validated in all patients. It should also be emphasized that endocardial RF ablation resulted in complete mitral isthmus block in only 32% of patients, the remaining 68% requiring an epicardial approach through the coronary sinus. In another study including 349 consecutive patients who underwent LACA for paroxysmal or persistent AF (8), 8% of patients had clinical macroreentrant AT at follow-up and underwent a second procedure. In this study, encircling of the PVs was implemented by further ablation lines in the posterior LA and the mitral annulus.