Does the Presence of an Isoelectric Period Predict the Mechanism of a Post-Ablation Atrial Arrhythmia?

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EP LAB DIGEST. 2024;24(11):6.

Bradley P Knight, MD, FACC, FHRS

Dear Readers,

Organized atrial arrhythmias have been categorized as atrial flutter (AFL) or atrial tachycardia (AT) based on their rate and/or mechanism. Focal atrial tachycardias (FATs) have been defined as having a rate below 250 beats per minute (bpm) with a microrentrant or automatic mechanism, in contrast to AFLs, which have rates over 250 bpm and are due to macroreentry. FATs are also described as having discrete P waves with an isoelectric period (IEP) between each P wave (P-wave tachycardias) in contrast to a constant regular flutter wave appearance (F-wave tachycardias).¹ The presence of an IEP is an indication that during that period of time, little if any part of the atria is being activated. The analogy during ventricular tachycardia is the diastolic interval. Although previously the presence of an IEP likely did indicate a focal mechanism, the advent of catheter and surgical ablation for atrial fibrillation has led to more scar-based atrial arrhythmias. So, how well does the presence of an IEP in the modern era correlate with the mechanism of an atrial arrhythmia?

In 2023, an interesting paper was published by Nakatani and colleagues that used 3-dimensional (3D) mapping data to explore the IEP.² Their hypothesis was that the isoelectric intervals on the surface electrocardiogram (ECG) during ATs correspond to periods of relatively small activated atrial surface area and that this phenomenon can arise from complex activation patterns. To test this hypothesis, the authors quantified the “minimum activated area,” which was defined as the area activated within a 50-millisecond period when atrial activation was at a minimum using the Rhythmia mapping system (Boston Scientific). High-density activation maps of 126 ATs were studied in patients with ATs after prior atrial ablation. Isoelectric intervals were defined as the absence of surface ECG activity for ≥80 milliseconds simultaneously in all 12 leads. AT mechanisms were classified into 3 types: (1) macroreentrant ATs, which included perimitral, roof-dependent, and peri-tricuspid ATs; (2) localized reentrant ATs, defined as reentrant ATs including 2 or fewer atrial walls in the circuit; and (3) focal ATs, identified by centrifugal activation from a focal site. They then assessed the impact of the activated atrial surface on the presence of IEPs by applying an activation search algorithm to the voltage map and measuring the minimum activated area throughout the AT cycle, defined as the smallest activated area within a 50-millisecond period, by using signal processing algorithms (Lumipoint, Boston Scientific). Other mapping systems can display similar histograms for local activation times. 

Among the 126 ATs, 45% had a P-wave morphology with a clear IEP. Surprisingly, 40% of these P-wave ATs were due to large macroreentrant circuits. Equally interesting were their findings that 30% of the F-wave ATs were due to localized reentry and that 3% were focal. Predictors of an AT having an IEP were a large area of very low voltage consistent with a large amount of scar, a small minimum area being activated throughout the AT cycle, slow conduction within the circuit, and few areas of delayed conduction outside the circuit. The video examples included in the publication online are worth watching.

It is interesting that the word “isoelectric” does not appear in either the recent 2023 AF Guideline³ or the 2024 Consensus Statement.⁴ This recent study by Nakatani and colleagues using ultra-high-density mapping of post-ablation atrial arrhythmias has taught us that over one-third of patients who present with an AT that demonstrates an IEP actually have a macroreentrant mechanism, and that many localized atrial arrhythmias do not have an IEP. This is an important lesson to consider when mapping these patients in the cardiac electrophysiology laboratory. 

Disclosures: Dr Knight has served as a paid consultant to Medtronic and was an investigator in the PULSED AF trial. In addition, he has served as a consultant, speaker, investigator, and/or has received EP fellowship grant support from Abbott, AltaThera, AtriCure, Baylis Medical, Biosense Webster, Biotronik, Boston Scientific, CVRx, Philips, and Sanofi; he has no equity or ownership in any of these companies.

References

1. Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Circulation. 2016;133(14):e471-505. doi:10.1161/CIR.0000000000000310

2. Nakatani Y, Takigawa M, Ramirez D, et al. Electrophysiologic determinants of isoelectric intervals on surface electrocardiograms during atrial tachycardia insights from high-density mapping. JACC Clin Electrophysiol. 2023;9(10):2054-2066. doi:10.1016/j.jacep.2023.06.018

3. Joglar JA, Chung MK, Armbruster AL, et al. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. J Am Coll Cardiol. 2024;83(1):109-279. doi:10.1016/j.jacc.2023.08.017

4. Tzeis S, Gerstenfeld EP, Kalman J, et al. European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Latin American Heart Rhythm Society expert consensus statement on catheter and surgical ablation of atrial fibrillation. Europace. 2024;26(4):euae043. doi:10.1093/europace/euae043