Functional Substrate Mapping Under Sinus Rhythm Predicts Critical Isthmus of Atrial Tachycardia

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EP LAB DIGEST. 2023;23(5):24-27.

Atrial tachycardia (AT) is a common rhythm disorder reported either after previous cardiac surgery or ablation procedures or in patients with de novo atrial scar. Additionally, atrial incisions after several surgeries may create an arrhythmogenic substrate to maintain scar-related ATs. Activation mapping during AT as well as entrainment maneuvers are the standard care to understand the mechanism of tachycardia and tailor an ablative approach. Recent advances in multipolar catheter technology, such as omnipolar technology, better allow for evaluation of critical sites relevant to tachycardia mechanism. However, data regarding the characteristics of abnormal substrate under sinus rhythm to predict critical isthmus (CI) of reentrant ATs is scant. Noninducibility during the procedure, or termination or degeneration of clinical ATs during anesthesia induction or pacing maneuvers, is not uncommon and can jeopardize procedural success, which warrants baseline evaluation of atrial substrate to predict potential sites harboring CI of reentrant ATs.

Voltage mapping has long been used to depict potential arrhythmogenic sites as a surrogate of underlying atrial substrate.^1,2 Although the association of atrial low-voltage area with AT formation is well known, a reliable method to identify areas responsible for reentry circuits has yet to be defined. Recently, novel functional mapping strategies such as decrement evoked potential (DEEP) mapping or isochronal late activation mapping (ILAM) were defined to predict the critical sites participating in ventricular reentrant tachycardias.^3,4 Similarly, several studies reported the utility of functional substrate mapping (FSM), aiming to elucidate conduction slowing shown by isochronal crowding during atrial late activation mapping to predict CI of reentry.^5-7

In this case study, we describe 3 cases of FSM characteristics during sinus/paced rhythm and their relation to CI of reentrant ATs in patients with a history of left ATs.

Case Presentation #1

A 76-year-old woman with a history of radiofrequency (RF) ablation for paroxysmal atrial fibrillation (AF) underwent a redo procedure using 3-dimensional (3D) mapping (Carto, Biosense Webster, Inc, a Johnson & Johnson company). Voltage mapping using a multispline catheter (Pentaray, Biosense Webster) under sinus rhythm revealed localized atrial scar on the anterior wall close to the mitral annulus with isolated pulmonary veins (PVs) (Figure 1A). Before induction of AT, atrial late activation mapping was performed to determine deceleration zones (DZ). ILAM revealed isochronal crowding inside the low-voltage zone on the anterior wall with continuous fragmented electrograms (EGMs) (Figure 1B, Video 1). ILAM was performed during sinus rhythm by manually annotating the offset of the local bipolar atrial EGM. The atrial activation window was between the onset of the earliest left atrial (LA) local EGM and total activation was divided into 8 equally distributed isochrones. Programmed atrial stimulation from the low-voltage area revealed localized reentry (tachycardia cycle length [TCL]: 250 milliseconds) with continuous EGMs during AT with the same location detected during sinus rhythm (Figure 1C). Ablation from this site terminated the tachycardia (Figure 1D). At 8-month follow-up, the patient was free of any AT.

Video 1

Video 1. Atrial late activation propagation under sinus rhythm.

Case Presentation #2

A 71-year-old woman with a history of previous cryoballoon ablation for paroxysmal AF underwent a redo procedure due to AT episodes detected by 24-hour Holter recording. Using 3D mapping (Carto) with a multispline catheter (Pentaray), voltage mapping under sinus rhythm revealed extensive low-voltage areas on the whole LA with isolated PVs (Figure 2A). ILAM revealed 2 DZs; the first with latest activation on the anterior wall and the second close to the ridge region on the roof with continuous fragmented EGMs (Figure 2B). A localized reentrant AT (TCL: 260 milliseconds) was induced and successfully terminated after single RF application (Figure 2C, Video 2). Another localized reentrant AT was later induced by programmed atrial stimulation (TCL: 300 milliseconds) corresponding to the second DZ, which was terminated after RF application (Figure 2D). Despite the extensive atrial substrate, only focal ablation at the isochronal crowding sites was performed (Figure 2E). At 18-month follow-up, the patient was free of any AT.

Video 2

Video 2. Termination of AT with single RF application.

Case Presentation #3

A 70-year-old man with a history of persistent AF underwent RF ablation with 3D mapping (EnSite X, Abbott). Initial voltage mapping using a multipolar grid catheter (Advisor HD Grid, Sensor Enabled, Abbott) demonstrated low-voltage zones on the anterior wall (Figure 3A). Atrial late activation mapping under distal coronary sinus (CS) pacing potential mapping revealed DZ on the anterior wall with continuous fragmented EGMs (Figure 3B, Video 3). After PV isolation, programmed stimulation induced localized reentry on the anterior wall with characteristic continuous fragmented EGMs during AT at the same location detected during ILAM under CS pacing (Figures 3C and 4). Ablation at this site terminated the tachycardia and an anterior mitral line was created from the mitral annulus to right superior PV, including the low-voltage zone with DZ. At 2-month follow-up, the patient was free of any AT.

Video 3

Video 3. Atrial late activation propagation under CS distal pacing.

Discussion

Our findings demonstrate that FSM can provide valuable data for the prediction of CI sites of left-sided ATs, especially in patients with underlying atrial low-voltage areas. In addition to the DZs, continuous fragmented EGM morphology corresponded well to the CI of localized reentry as well as a successful ablation site.

Several recent studies demonstrated the utility of late potential-based FSM to predict the CI of reentry in patients with ventricular tachycardia.^3,8 Specifically, DZs of ILAM were well correlated with the CI of reentrant circuit. However, data regarding the utility of such an approach in patients with AT are scarce. Correlation of CI of atypical left AT with isochronal crowding on ILAM was shown by Kapur.⁷ In addition, in a recent multicenter retrospective analysis of high-density LA mapping, Woods et al⁵ demonstrated that DZs under sinus/paced rhythm colocalized with CI of localized reentry during AT. Moreover, Tsai et al⁶ demonstrated the utility of atrial ILAM to predict CI sites in patients with left macroreentrant ATs. Similar to these studies, our findings corroborated the relevance of atrial late activation-based FSM to predict the CI of reentry. Therefore, careful evaluation of DZs under sinus/paced rhythm with EGM characteristics could provide valuable data regarding the critical site of reentry.

Although endocardial bipolar voltage mapping has long been used to obtain data regarding the anatomical sites that may contribute to the AT, its ability to identify critical areas having a role in reentry is limited. In a recent report, Ramirez et al² reported that the association of voltage with conduction velocity during reentrant ATs was modest. Therefore, a voltage-based ablation approach may lead to extensive ablation in cases of high atrial scar burden. In addition, characteristic EGMs having a role in the CI of localized reentry might be overlooked when the presence of such EGMs is inside dense scar zones according to the default values of voltage thresholds considered. Therefore, evaluation of functionality of this substrate beyond voltage data is needed to predict the CI of reentrant circuit. Moreover, EGM characteristics may also give important data on continuous fragmented morphology, located in the DZ of ILAM. The correlation of DZs with successful ablation sites indicates that a FSM-based approach may result in ablation by targeting only relevant sites having a role in CI besides targeting extensive scar tissue modification. Whether this approach may limit ablation burden necessitates further comparative studies.

The novelty of such an approach lies in that a FSM-based strategy may provide valuable data to predict potential sites harboring critical sites of AT in cases of atrial scar. Although ablation of clinical ATs is the main strategy in any patient group, recurrence after successful ablation is not uncommon, especially in patients with atrial scar due to prior ablation procedure or atrial surgery. Therefore, this approach mainly aims to evaluate the substrate during sinus/paced rhythm to uncover the potential sites that may cause recurrence. In cases #1 and #2, AT was the pattern of recurrence after AF ablation. Independent from the extent of scar, FSM revealed important clues to predict CI of reeentry beyond focusing on only non-PV triggers. In case #3 with persistent AF, programmed stimulation induced localized reentry on the anterior wall after PVI, which corresponded well to the DZ of ILAM during CS distal pacing. Despite the inconsistent findings of substrate-based ablation approaches in cases of atrial scar, recent findings highlighted the favorable outcomes of modification of low-voltage zones in persistent AF.^9,10 These findings may further refine voltage-based ablation strategies in targeting functional sites. Finally, as noninducibility of AT is still a concern during procedures, these sites might be targeted empirically to prevent recurrent ATs.

Despite promising data regarding the use of FSM, an optimal ablation strategy has not been defined, especially in the case of multiple DZs. In case #2, multiple DZs with characteristic EGMs predicted the CI of different inducible ATs. However, the activation map and FSM may not be congruent due to bystander slow conduction zones with DZs, which are not related to the clinical tachycardia. Although targeting these sites may decrease future recurrence, further ablations may lead to intra-atrial conduction slowing as well as restrictive physiology. Therefore, when bystander presence of such signals and DZs that are not colocalized with CI of inducible ATs, especially in cases of extensive low-voltage zones, is considered, further studies are needed to evaluate the optimal ablation approach.

Conclusion

Our findings regarding atrial FSM under sinus/paced rhythm provide proof of concept for prediction of the CI of ATs, especially in patients with underlying low-voltage zones. In addition to the DZs demonstrated by ILAM, continuous fragmented EGMs correlated with the successful ablation site of ATs; therefore, targeting these sites seems to be a reasonable approach in case of noninducibility. Further studies are needed to evaluate the validity of such a strategy as an empirical ablation target in patients with underlying atrial substrate. 

Disclosures: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Drs Yorgun and Aytemir report proctoring for Medtronic, Abbott, and Johnson & Johnson.

Connect with the author on Twitter at @hikmetyorgun.

References

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10. Huo Y, Gaspar T, Schönbauer R, et al. Low-voltage myocardium-guided ablation trial of persistent atrial fibrillation. NEJM Evid. 2022;1(11). doi:10.1056/EVIDoa2200141