Novel High-Density Mapping Techniques for Atypical Atrial Flutter Characterization

EP Lab Digest. 2023;23(3):1,8-10.

Microreentrant atrial tachycardias (AT) are defined as atrial arrhythmias with greater than 85% of cycle length encompassed in a small area (<2-3 cm).¹ Activation mapping can readily distinguish macroreentry from the centrifugal activation of focal arrhythmias. However, optimal mapping of microreentrant AT circuits can be challenging, particularly in regions of scar. The small size of microreentrant flutter circuits in conjunction with poor spatial and directional resolution of conventional mapping techniques poses challenges to localizing and defining microreentrant circuits.

Case Presentation

A 73-year-old woman presented with recurrent paroxysmal atrial fibrillation (AF) and atrial flutter (AFL) despite amiodarone. She underwent prior catheter ablation with pulmonary vein isolation (PVI) and cavotricuspid isthmus ablation with radiofrequency ablation in 2019. She maintained sinus rhythm for over 2 years prior to developing recurrence. She underwent cardioversion for symptomatic atypical AFL several months prior to presentation (Figure 1). Other past medical history included tachyarrhythmia-induced cardiomyopathy with left ventricular ejection fraction (LVEF) of 30%, subsequently recovered to 55%. Repeat catheter ablation was recommended. Transthoracic echocardiogram revealed a LVEF 55%-60% with a left atrial (LA) volume index of 31.23 mL/m². No significant valvular disease was noted.

At baseline, mapping of the LA during sinus rhythm was consistent with antral isolation of the left-sided PVs. Reconnection of the right upper and right lower PVs was noted. Wide area circumferential ablation was performed around the right-sided veins, with demonstration of entrance and exit block from each vein and no evidence of dormant conduction with intravenous adenosine testing. Following reisolation of the right-sided PVs, atrial programmed stimulation easily induced AFL. Coronary sinus (CS) activation was regular with proximal to distal CS activation and cycle length of 330 milliseconds (Figure 2).

Prior to mapping, differential entrainment revealed postpacing interval minus tachycardia cycle length (TCL) values of 163, 151, and 86 milliseconds from the proximal CS, distal CS, and the base of the left atrial appendage (LAA), respectively. Activation mapping of the LA, and ultimately, the right atrium, was performed; however, the entire TCL was not mapped despite high-density activation mapping of both atria (Figure 3). Analysis of the obtained map revealed an area of scar in the anteroseptal LA. While a conventional area of isochronal crowding or deceleration was not appreciated in this region, several isochrones were appreciated within a small surface area.

When the Advisor HD Grid Mapping Catheter, Sensor Enabled (Abbott) was initially repositioned in this region, activation initially appeared unrevealing (Figure 4). However, by transitioning from bipolar mapping to omnipolar mapping (EnSite Omnipolar Technology, Abbott), continuous activation in this region was noted with adequate signal clarity to discriminate local activation from noise. Near-field electrograms (EGMs) showing continuous activity was noted on the mapping catheter spanning 307 milliseconds of a total TCL of 330 milliseconds (Figure 5). By decreasing the scar threshold from <.1 mV to <.03 mV, a clear isthmus between 2 islands of scar could be identified (Figure 6). Interestingly, when omnipolar vectors are superimposed on this map, circular activity could be identified in this region. Ablation performed at this site at 40W resulted in slowing of the atrial cycle length followed by termination in 5.6 seconds (Figure 7).

Video 1

At the completion of the study, there was no inducible AFL with burst pacing and premature extrastimuli from the CS, with and without isoproterenol. The patient has since maintained sinus rhythm without evidence of recurrent AF or AFL.

Discussion

Using conventional bipolar EGM acquisition and maximal dV/dT annotation, a reentrant circuit could not be defined. A broad region of scar was appreciated on the anteroseptal LA wall. Interrogation of this region using omnipolar mapping revealed fragmented, continuous activity in this region spanning 93% of the TCL. By decreasing the scar voltage threshold, a clear critical isthmus site was identified (Figure 8). Ablation in this area eliminated the tachycardia.

Video 2

This case is illustrative of 2 key considerations during mapping of microreentrant AT: low voltage (scar) mapping and wavefront directionality.

Prior studies have demonstrated that bipolar voltage amplitude cutoffs in healthy atria appear similar at <.5 mV across multiple interelectrode spacing orientations.¹ Smaller interelectrode spacing allows for more accurate quantification of scar area, with improved discrimination between scar and adjacent tissue.² While a scar threshold of <.5 mV may accurately distinguish scar from healthy atrial tissue, it is of paramount importance to recognize that microreentrant AT often occurs in areas adjacent to prior ablation lesions or near patchy areas of scar identified by electroanatomical mapping.³ In this setting, it is crucial to annotate every distinguishable atrial potential to accurately identify channels within areas of scar.⁴

To accurately annotate a potential, it must first be recorded. Grid-shaped catheters offer unique benefits. The Advisor HD Grid Mapping Catheter, Sensor Enabled consists of an array of 16 electrodes in a 4x4 square lattice configuration with 3-mm equidistant electrode spacing (Figure 9). Two reference electrodes are located on the 8 French catheter shaft. As interelectrode spacing decreases with modern mapping catheters, it is critical to recognize that the ability to discriminate between bipolar voltage peak values becomes increasingly subject to the direction of the incident wavefront being recorded. This is demonstrated in Figure 4, in which the C2-C3 bipole appears nearly isoelectric; however, high-frequency, low-voltage activity is appreciable on the orthogonal bipole of C2-D2.

Conventional mapping relies on time annotation and numerous acquisitions, often of the entire cardiac chamber, in relation to a constant reference point. Omnipolar mapping takes advantage of the mapping catheter’s 36 orthogonal bipolar pairs to create 3-electrode groups called “cliques”. By combining unipolar EGMs from 3-adjacent electrodes, local propagation can ultimately be treated as a traveling wave with a single direction, speed, and amplitude.⁵ This technique allows for instantaneous derivation of a wavefront’s direction, but also allows for recording orientations outside of the typical linear bipolar electrode orientation or orthogonal electrode orientation of conventional bipolar mapping with grid-shaped catheters. By orienting the recording directionality in a more parallel orientation with the propagating wavefront, higher amplitude signals may be recorded with improved signal-to-noise characteristics, as demonstrated in Figure 5.

Summary

Conventional atrial amplitude thresholds for scar discrimination may be inadequate to identify isolated channels between areas of scar. Bipolar EGM acquisition may be limited by wavefront directionality, even when appropriately positioned over the critical isthmus. In these situations, omnipolar activation may provide clues toward microreentrant flutter propagation. Omnipolar mapping may facilitate high-frequency, low-amplitude atrial EGM identification at supra-nominal gain settings with adequate signal-to-noise characteristics. The use of lower atrial amplitude scar thresholds in conjunction with omnipolar signal acquisition may elucidate microreentrant AT propagation in regions of prior ablation, surgical intervention, or scar. 

Acknowledgements. The authors would like to thank Ashley Zilinskas, BS, and Scott Lovejoy, BS, for their exceptional mapping support and for assistance with figure creation.

Contact the authors on Twitter: @AjayPMD, @jaykoneru, @KennethEllenbo1

Disclosures: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. They have no conflicts of interest to report regarding the content herein. Outside the submitted work, Dr Ellenbogen reports grants for research from Biosense Webster, Boston Scientific, and Medtronic; lectures and presentations for Abbott, Biosense Webster, Boston Scientific, and Medtronic.

References

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