Atypical Atrial Flutter: Strategies for Successful Circuit Identification and Ablation

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EP LAB DIGEST. 2024;24(6):1,8-11.

Background
Atypical atrial flutter (AAFL) encompasses all macro-reentry electrical circuits in the atrium that are not dependent on the cavotricuspid isthmus (CTI). While AAFL most commonly occurs in patients with previous ablation, cardiac surgery, or structural procedures, they can also arise de novo in the setting of atrial myopathic disease as well.^1-5 The actual incidence of AAFL seems to be underreported; however, some studies have shown an incidence of 5% after pulmonary vein isolation (PVI), up to 25% in those treated with additional linear lesions, and up to 14% after Maze procedures.^1-6 With increasing use of ablative and surgical therapies that introduce atrial scar, the burden of AAFL in the population will continue to increase. 

Diagnosis    
Prior cardiac surgery or ablation along with electrocardiogram (ECG) findings inconsistent with CTI flutter strongly suggest atypical flutter. At times, even typical-appearing flutter can involve atypical circuits in patients with prior cardiac interventions (Figure 1). Patients with prior ablation or cardiac surgery, or severely abnormal atrial dimensions, often must undergo electrophysiology study (EPS) to truly characterize a flutter circuit. 

Management    
Restoration of sinus rhythm in the setting of AAFL consists of noninvasive cardioversion and consideration of antiarrhythmic drugs or ablation. Given the possibility of eliminating the causative circuit with ablation, this is often the primary modality in managing AAFL. Understanding AAFL circuit anatomy is critical to providing a safe and effective ablation. The common sites in the right atrium (RA) include cannulation sites, prior patch closure of the intra-atrial septum, or lateral wall in progressive cardiomyopathies. The left atrium (LA) most commonly has circuits using the mitral valve (MV), PVs, posterior wall, or roof.^6-10 These arrhythmias have traditionally thought to be difficult for finding durable success with ablation, but current technology has increased the acute efficacy of AAFL ablation to greater than 95%.^9,11

Strategies for Successful Ablation

Basic EPS
At the outset of an ablation procedure for AAFL, basic EPS catheters and maneuvers can provide a host of data to inform ablative approaches. Placement of a coronary sinus (CS) catheter and other atrial sampling catheters will reveal an atrial activation pattern often suggestive of the AAFL circuit. Eccentric or “flat activation” with multiple CS electrodes activated simultaneously often support a LA circuit possibly involving the mitral annulus (MA) or LA roof, whereas concentric activation can suggest a septal or right-sided activation.¹² Basic overdrive maneuvers from the proximal or distal CS can confirm a relative relationship to the atrial flutter, with shorter post-pacing intervals (PPI) suggestive of proximity to the circuit.¹² Approximating the anatomical circuit is critical to considerations of immediate RA mapping or proceeding to transseptal puncture and LA mapping.

High Density (HD) Mapping
Mapping systems continue to develop catheters that collect many accurate points of local activation time (LAT) rapidly, allowing rapid characterization of AAFL circuits. Recording of LAT points that encompass the entire AAFL cycle length is critical to characterizing a flutter circuit, and lack thereof suggests an alternative atrial chamber as the active circuit. It also allows characterization of scar, incomplete prior ablation lines, areas of long slow activation, lines of block, or isochronal crowding, which are all useful for circuit characterization. Mapping can rapidly collect a high density of points, but it is up to the EP team to adjudicate accuracy to appropriately interpret AAFL boundaries critical to successful ablation. Rodriguez Font et al showed that around 3000 points of activation could be obtained in around 12 minutes. Prima facie interpretation of color-coded LAT can be deceptively simple, but it is important to note that areas where colors that seem out of order may represent multiple anatomical or annotation nuances.¹³ One possibility is that nonsequential activation may represent an area of functional block or perhaps an epicardial conduction over the septo-pulmonary bundle or vein of Marshall. Annotation or sampling settings can also provide pitfalls in HD activation mapping. Setting voltage filters too high will falsely create areas of scar and eliminate LAT points, change the interpretation of the arrhythmia, and limit annotation of the entire cycle length. Adjusting the voltage cutoffs down to include more areas and rechecking the signal to confirm their legitimacy is a useful strategy when the initial map is difficult to interpret.^11,14 Another common pitfall of HD activation mapping is the inappropriate annotation of far-field ventricular signal near the tricuspid or MV annulus, and points should be adjudicated for accuracy.¹⁴ Lastly, it is prudent to double-check contact with the mapping or ablation catheter in areas of scar to confirm whether it is real or falsely annotated due to poor tissue contact. 

Identification of 2 Distinct Boundaries
Dual-loop tachycardias have historically been considered rare findings in AAFL ablation. Santucci et al propose in their work that all atrial flutters consist of dual loops.¹⁵ In this model, all AAFL arise between 2 critical nonconductive boundaries such as isolated PVs, anatomic boundaries such as the MV, or preexisting scar. Each of these critical boundaries contain >90% of the cycle length for the tachycardia. When LAT mapping is examined with each boundary en face, one boundary shows activation in a clockwise direction while the other will have a counterclockwise activation. HD mapping allows for the direct visual representation of the dual-loop circuit and the boundaries that facilitate the propagation of the arrhythmia. The creation of a line of block with ablation from one critical boundary to the other critical boundary will result in termination of the tachycardia. Creation of a line of block from a critical boundary to a noncritical boundary will not terminate the AAFL and will often simply change the cycle length due to incorporation of that new boundary into the circuit. For example, Figure 2 demonstrates counterclockwise rotation around the MA and clockwise rotation around the isolated right PVs. A line of block from the right upper PV to the MV annulus will terminate the tachycardia with noninducibility. If the same tachycardia was viewed in a single loop around either the right PV or MV alone, it may result in delivery of either a roof line or lateral MA line, respectively, resulting in a cycle length shift and failure to terminate the AAFL with the initial line. Complex AAFL circuits involving significant scar burden often require thorough understanding of the critical components of the circuit for successful ablation (Figure 3). Santucci et al applied this philosophic shift for AFL conceptualization and reported a 96% success rate in AAFL termination intraprocedurally.¹⁵ This high success rate would put AAFL ablation on par with typical AFL and atrioventricular nodal reentry ablation. 
 

Entrainment
Entrainment remains an essential tool to confirm findings of a HD activation map. There is often a misconception that entrainment will terminate or change the tachycardia; however, the reality is that termination due to overdrive pacing is rare. Rodriguez Font et al used HD mapping for tachycardias to identify the potential circuit, then performed entrainment along the circuit to recreate a low-density map marking only where PPI-TCL was <30 ms. This low-density entrainment map within the circuit saw termination of the tachycardia in only 2 of 277 entrainment attempts. In both terminations, the same tachycardia was easily re-inducible.¹³ In the current era of HD mapping for AAFL, entrainment is particularly useful in the setting of highly scarred atrial myocardium where the circuit may not be easily mappable due to the complex long and high fractionated nature of the activation of the local tissue (Figure 4). Somewhere between 10%-30% of complex atypical flutter circuits are not readily defined by mapping alone. When entrainment is added to HD mapping, some areas initially thought to be in the circuit by activation mapping can be revealed to be passive and not critical to the circuit continuing.^16,17

Our Approach to Flutter Ablation
1. Place the CS catheter and observe the activation pattern. If there is concentric activation, we perform entrainment from the CTI to rule out typical flutter.
2. We will typically perform ablation without HD mapping when the flutter is found to be CTI dependent by entrainment maneuvers.
3. If there is any question about left vs right atrial flutter based on CS activation, we perform right atrial mapping prior to transseptal access and left atrial mapping.
4. Evaluate the HD activation map to find the 2 critical boundaries where >90% of the cycle length can be observed, and create a design line between those 2 boundaries.
5. We perform entrainment in selected patients when the 2 critical boundaries are not clearly identified by activation mapping.
6. In cases where the activation mapping is not helpful due to a highly scarred atrium, we look at the voltage map at different lower cutoffs of scar annotation to identify possible islands of scar that correlate with CS activation and activation map. Entrainment is performed from multiple areas around the scar to confirm the circuit prior to ablation.
7. We perform differential pacing to confirm block across the line before and after a waiting period.

Conclusion
With increasing cardiac procedural intervention, the occurrence of AAFL will continue to present a significant and growing challenge in the population. Comprehensive understanding of AAFL circuits will allow increasingly high rates of success in ablative management of this arrhythmia. 

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest, and report no conflicts of interest regarding the content herein. 

References 

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12. Pascale P, Shah AJ, Roten L, et al. Pattern and timing of the coronary sinus activation to guide rapid diagnosis of atrial tachycardia after atrial fibrillation ablation. Circ Arrhythm Electrophysiol. 2013;6(3):481-490. doi:10.1161/CIRCEP.113.000182

13. Rodríguez Font E, Alonso-Martín C, Guerra JM, et al. From high-density mapping to low-density mapping: outlining the active circuit in complex atrial re-entrant tachycardias. JACC Clin Electrophysiol. 2020;6(5):523-532. doi:10.1016/j.jacep.2019.12.001 

14. Di Cori A, Mazzocchetti L, Parollo M, et al. Clinical impact of high-density mapping on the acute and long-term outcome of atypical atrial flutter ablations. J Interv Card Electrophysiol. 2024;67(1):43-51. doi:10.1007/s10840-022-01121-3 

15. Santucci PA, Bhirud A, Vasaiwala SC, Wilber DJ, Green A. Identification of 2 distinct boundaries distinguishes critical from noncritical isthmuses in ablating atypical atrial flutter. JACC Clin Electrophysiol. 2024;10(2):251-261. doi:10.1016/j.jacep.2023.09.024

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17. Strisciuglio T, Vandersickel N, Lorenzo G, et al. Prospective evaluation of entrainment mapping as an adjunct to new-generation high-density activation mapping systems of left atrial tachycardias. Heart Rhythm. 2020;17(2):211-219. doi:10.1016/j.hrthm.2019.09.014