Spontaneous Mahaim Fiber Automaticity as a Rare Cause of PVC-Like Complexes

Abstract

Background

Mahaim fibers are rare types of accessory pathways with atrioventricular node–like decremental conduction properties. They present a significant challenge in terms of diagnosis and treatment.

Case Summary

We present the case of a patient with bouts of left bundle branch block–morphology wide complex tachycardia, ultimately diagnosed as atrioventricular reciprocating tachycardia with antegrade conduction over an atriofascicular accessory pathway. We demonstrate a systematic approach to diagnosis of left bundle branch block–morphology tachycardia along with several classic features of Mahaim fibers. These include emergence of an “M potential” with decremental conduction during incremental atrial pacing, early right ventricular apical activation during tachycardia, and radiofrequency-induced automaticity. We also demonstrate the rare finding of spontaneous Mahaim automaticity, captured as premature ventricular contraction–like beats on electrocardiogram.

Discussion

Prior accounts of spontaneous automaticity in atriofascicular pathways are exceptionally rare. In the literature, automaticity is most often described after radiofrequency ablation.

Take-Home Message

The case exemplifies the unusual electrophysiological property of this rare entity and uniquely illustrates Mahaim spontaneous automaticity masquerading as right ventricular apical ectopy.

Graphical Abstract

History of Presentation

A 21-year-old man presented to the emergency department with bouts of abrupt onset and offset rapid palpitations. Initial electrocardiogram (ECG) demonstrated sinus rhythm with frequent wide complex beats, deemed to be most consistent with premature ventricular contractions (PVCs) (Figure 1C). While in the department, his palpitations recurred, and a left bundle branch block (LBBB)–morphology wide complex tachycardia (WCT) at 202 beats/min was captured on ECG. The QRS duration was ∼130 ms. Late precordial transition and left axis deviation were observed. However, variation of the transition between leads V5 and V6 in addition to slight deviation in the frontal axis (lead II altering between negative and isoelectric) were noted during the same tachycardia (Figures 1A and 1B). Termination of the tachycardia occurred with administration of adenosine.

Figure 1.

ECGs During Wide-Complex Tachycardia and Sinus Rhythm

(A and B) 12-lead ECGs taken minutes apart at index presentation, with change in precordial transition and frontal axis plane noted (dashed red boxes) and brief change in frontal axis on lead II rhythm strip (solid red box). (C) 12-lead ECG showing LBBB morphology PVCs with late precordial transition. ECG = electrocardiogram; LBBB = left bundle branch block; PVC = premature ventricular contraction.

Past Medical History

The patient's medical history was noncontributory. In particular, there was no personal or family history of cardiac disease. Recent echocardiography did not reveal any evidence of structural cardiac disease. The patient did not take any regular prescribed or over-the-counter medications, and he did not use illicit substances.

Differential Diagnosis

We list below the initial differential diagnosis for LBBB-morphology tachycardia, along with our diagnostic reasoning relevant to each differential for this case:

• 1.Myocardial ventricular tachycardia (VT): The ECG in Figure 1A demonstrates negative precordial concordance, which is a feature suggestive of VT, however this was not present on Figure 1B. Ventriculoatrial (VA) dissociation, a pathognomic feature hallmark of VT, was not noted on any ECGs (however this is often not visible with VTs at faster rates, and it is not visible during VTs with retrograde VA conduction). VT was considered by us to be a likely mechanism for the tachycardia, as the patient's 12-lead ECGs demonstrated frequent wide complex ectopic beats, not preceded by P waves, with similar morphology to the clinical WCT (see Figure 1C). These had the appearance of frequent PVCs, which are often seen in the setting of VT, particularly VT with a focal mechanism. The morphology of the WCT localized to the exit region of the right ventricular (RV) apex. PVCs and focal VT from RV apex/moderator band or related RV papillary muscle was considered as a likely diagnosis for the recurrent sustained WCT and frequent wide complex ectopic beats.
• 2.Supraventricular tachycardia (SVT) with fixed or functional LBBB: The absence of LBBB on baseline ECG immediately excludes SVT with fixed bundle branch block. Rate-related LBBB during SVT was considered a possibility, however the frequent wide complex beats were not preceded by P waves, therefore they were not felt to be explained by bundle branch block aberration. Adenosine sensitivity of WCTs can assist in differentiation of SVT and VT. The drug's negatively dromotropic action terminates tachycardias, which are atrioventricular node (AVN) dependent. However, important caveats exist. Adenosine exerts extranodal effects and may terminate cyclic AMP-dependent VT.¹ Adenosine-sensitive VTs most commonly originate in the left ventricular and RV outflow tracts but have been documented in other locations, which include the apical RV and left ventricular septum.²
• 3.Atriofascicular antidromic tachycardia (Mahaim fiber tachycardia): Also a possibility, but not one that was strongly considered by us in advance of the procedure given the frequent wide complex beats.
• 4.SVT with a bystander atriofascicular accessory pathway: Again, also a possibility, but not one that was strongly considered by us in advance of the procedure given the frequent wide complex beats.
• 5.Bundle branch re-entry VT: This would be unlikely in the absence of structural heart disease or conduction system disease.³
• 6.Ventricular-paced rhythm or pacemaker-mediated tachycardia: Not relevant to this case.

Investigations and Management

Serum electrolytes and thyroid function tests were within normal reference ranges. Repeat echocardiography confirmed a structurally normal heart. No antegradely conducting accessory pathway was elicited by adenosine challenge. Flecainide and bisoprolol were commenced before initial discharge.

The patient returned for electrophysiology study. Flecainide and bisoprolol were withheld for 5 days prior to the procedure. The presenting rhythm was sinus with frequent wide complex beats, as noted previously. Nonsustained WCT occurred during RV catheter placement, with the same morphology as the clinical tachycardia. The tachycardia was noted to have identical morphology to the frequently occurring wide complex beats. It spontaneously terminated. Baseline conduction intervals were normal. There was no evidence of dual AVN physiology. VA conduction was central and decremental. Based on our prior diagnostic thought process as outlined above, we felt the patient was having a focal VT and frequent PVCs from the same RV source, and we therefore next elected to map the PVCs.

Activation mapping using an ST DF ablation catheter (ThermoCool SmartTouch, Johnson & Johnson MedTech) was performed. The earliest activation was localized to the junction of the RV free wall and the moderator band, close to the RV apex. At this location, bipolar signal was 20 ms pre-QRS onset with a corresponding negative unipolar signal (Figure 2A). This anatomical location is illustrated in Video 1, where the green tip of the ablation catheter is highlighted at the earliest activation point. After delivery of radiofrequency ablation (RFA) at this location, the wide complex beats became quiescent. Isuprel infusion at 3 μg/min was then started. With this, the same WCT again initiated, with cycle length 360 ms. At this point, diagnostic catheters in the atrium, coronary sinus, and His locations were in place, and an A:V relationship of 1:1 and concentric coronary sinus activation during WCT were noted. The earliest ventricular signal was on the RV apical catheter (Figure 2B). RV apical activation was earlier than the His V electrogram (radiofrequency: 1-2) during tachycardia, compared with sinus rhythm (Figure 2B).The short HV interval observed during tachycardia excludes SVT with functional LBBB. Entrainment from the RV apex revealed a short postpacing interval (70 ms) with a VAV response, consistent with atrioventricular reciprocating tachycardia. The tachycardia spontaneously terminated on a V. These observations were consistent with an atriofascicular (Mahaim) fiber as the mechanism for tachycardia. The “PVC-like” wide complex beats were spontaneous automatic ectopic beats originating from the Mahaim fiber.

Figure 2.

Electrophysiology Study

(A) CARTO map with accompanying EGM; the earliest site of ventricular activation close to the RV apex during tachycardia is demarcated by a red dot. (B) EGM displaying sinus rhythm without pre-excitation to the right of the panel. The His signal (orange dashed circle) is earlier than the RV apex. To the left of the panel, during WCT, the RV signal becomes earlier than the His, and is at the onset of the QRS. EGM = electrogram; RV = right ventricular; WCT = wide-complex tachycardia.

Atrial pacing at 400 ms during sinus rhythm reproduced an identical LBBB-morphology pattern (Figure 3A). The tricuspid annulus was mapped during atrial pacing with identification of a discrete, sharp potential located between the A and V electrograms at a 7 o'clock position on the tricuspid annulus (TA) (Figure 3A). Decremental conduction was elicited with reduction of the A-TA potential interval, explaining reduction in the atrioventricular interval (Figure 3B). RFA on the TA caused loss of conduction over the Mahaim fiber (Figures 3C and 3D). Only conduction over the AVN could be seen with atrial pacing after ablation. Video 1 also illustrates the 2 ablation lesion sets (firstly delivered at the junction of the RV free wall and the moderator band, which would be the ventricular insertion of the Mahaim fiber, and next at inferolateral tricuspid annulus where the “M” potential was mapped).

Figure 3.

Identification of Mahaim Potential and Ablation

(A) EGMs showing atrial pacing (cycle length: 400 ms) during sinus rhythm, reproducing an LBBB pattern. Mapping of the TA identifies Mahaim potential at a 7 o'clock position on the TA seen on the radiofrequency catheter EGM (green dashed circle). (B) Decremental conduction properties elicited with reduction of the A-TA potential interval explaining reduction in the atrioventricular interval. (C) Ablation on TA at 7 o'clock performed during atrial pacing showing loss of conduction over the Mahaim fiber with narrowing of the QRS. (D) CARTO map displaying location of lesions delivered at 7 o'clock on the TA in RAO and LAO views. EGM = electrogram; LAO = left anterior oblique; LBBB = left bundle branch block; RAO = right anterior oblique; TA = tricuspid annulus.

Outcome and Follow-Up

There were no procedure-related complications. The patient was discharged home on the same day and has remained free from recurrent symptoms after more than 1 year of follow-up.

Discussion

“Mahaim fiber” is an umbrella term that describes atriofascicular or nodoventricular accessory pathways with unique decremental conduction properties. The majority of these fibers arise in the anterior and anterolateral right atrium, bridge the TA, and insert directly into or adjacent to the right bundle branch block, earning the term “atriofascicular” pathways.⁴ These pathways arise from islets of tissue resembling AVN tissue histologically and physiologically, known as “nodes of Kent.”⁵ Although exceptions exist, these pathways classically conduct exclusively in an anterograde direction.⁶ True nodoventricular pathways, originally described by Mahaim and Winston,⁷ have similar properties, but they are extremely rare and may conduct retrogradely.

As witnessed in this case, Mahaim fibers may manifest with spontaneous or postablation automaticity, although pre-excitation is often absent in sinus rhythm. However, spontaneous automaticity is rare, making this case unusual.^8,9 Sternick et al¹⁰ describe the stimulation of Mahaim automaticity on initiation of RFA at both the site of the proximal Mahaim potential on the TA and its distal insertion in the vast majority of patients in their series. This automaticity is heat induced, akin to the junctional rhythm that occurs during slow-pathway modification of AVN re-entry tachycardia.¹¹

Another characteristic feature of Mahaim tachycardias, as seen in this case, is variation in axis. This phenomenon is dictated by the conduction in the right bundle branch. When retrograde conduction occurs via the right bundle branch, ventricular activation spreads rapidly via the left anterior fascicle. In contrast, when the right bundle branch is blocked, there is slow intramyocardial conduction via the RV free wall. With complete retrograde right bundle branch block, significant prolongation of the tachycardia cycle length coupled with a more superior axis are seen.⁵

In conclusion, this case illustrates many of the typical characteristics of the Mahaim fiber, an unusual type of accessory pathway. Spontaneous Mahaim automaticity, which is rare, may mimic RV apical ectopy, making this entity an important differential diagnosis to consider when tackling LBBB morphology tachycardias.

Funding Support and Author Disclosures

The authors have reported that they have no relationships relevant to the contents of this paper to disclose.

Take-Home Messages

• •Mahaim fiber (atriofascicular) accessory pathways are a rare cause of LBBB-morphology tachycardia.
• •Mahaim fiber tachycardias present a diagnostic challenge given their unusual and variable electrophysiological properties.
• •Spontaneous Mahaim fiber automaticity is a rare finding, which may mimic RV apical ectopy.

Appendix

Video 1

Intracardiac Echocardiography Image and CARTO Map Identifying the Site of Earliest Ventricular Endocardial Activation, With the Catheter Positioned at the Junction of the Moderator Band and RV Free Wall, Close to the Apex