Coexistent “Classic” Atrioventricular and “Mahaim” Atriofascicular Pathways in Pediatric Patients with a Structurally Normal Heart

Introduction

Multiple accessory pathways are found in 9-10% of the pediatric population¹ and are mostly associated with structural heart disease,² especially with Ebstein’s anomaly and cardiomyopathies. The presence of multiple accessory pathways carries a higher risk of developing paroxysmal supraventricular tachycardias³ including antidromic atrioventricular reciprocating tachycardia (AVRT). Having multiple pathways also confers a higher risk of rapid anterograde conduction during atrial fibrillation,⁴ which can lead to ventricular fibrillation. The most common type of accessory pathway (classic Wolff-Parkinson-White [WPW] syndrome) represents a small myocardial band connecting the atrial and ventricular myocardium, bypassing the normal atrioventricular (AV) conduction system.⁵ There are less common variants of accessory pathways that involve specialized conduction fibers, often called Mahaim fibers, in addition to the normal AV conduction system, and they can exhibit decremental conduction properties.⁶ These accessory pathways are most commonly located in the right atrium along the lateral aspect of the tricuspid valve annulus, and incorporate an AV node-like structure⁷ and long fibers which insert into the distal right bundle branch or adjacent myocardium (atriofascicular pathways). 

In this article, we present two cases of multiple accessory pathways of different characteristics (“classic” and “Mahaim” type) in pediatric patients without any structural heart disease.

Case Description

The first patient is a 13-year-old female with complaints of frequent and very symptomatic palpitations. Echocardiography showed no structural heart disease, and baseline electrocardiogram (ECG) demonstrated normal sinus rhythm with a mild degree of ventricular preexcitation (Figure 1). The patient underwent an electrophysiological study under general anesthesia. Right ventricular pacing showed eccentric retrograde conduction with distal to proximal coronary sinus (CS) activation. Incremental atrial pacing increased the amount of ventricular preexcitation, thereby demonstrating a left-sided accessory pathway pattern. Pacing also showed a long accessory pathway antegrade effective refractory period (ERP) of 560 milliseconds (ms). Programmed stimulation easily induced a narrow complex tachycardia with the same eccentric retrograde activation as right ventricular pacing. After some narrow beats, the QRS widened with a left bundle branch block (LBBB) pattern (Figure 2). The analysis of the HV interval during narrow and wide QRS phases of tachycardia showed marked shortening of the HV interval from 40 ms to 10 ms during wide QRS complexes compared to the narrow complex beats, which ruled out a rate-dependent LBBB, and was consistent with bystander preexcitation via a second accessory pathway with a right-sided location. The pacing maneuver, His refractory PVC resetting the tachycardia with the same retrograde activation sequence, proved the tachycardia to be an orthodromic AVRT with bystander preexcitation. Transseptal puncture was performed with fluoroscopic guidance and intracardiac pressure monitoring. The atrial insertion of the left-sided accessory pathway was mapped during tachycardia. Ablation with an irrigated radiofrequency (RF) catheter at the inferoposterior aspect of the mitral annulus terminated the tachycardia in the retrograde limb of the circuit, and rendered it noninducible. After ablating this left-sided accessory pathway, the retrograde activation became concentric and decremental. Programmed atrial stimulation revealed progressive preexcitation with a different morphology, and progressive lengthening of the stimulus-delta interval and shortening of the HV interval. This was consistent with the presence of a second decremental anterograde pathway. Pacing from the lateral right atrium showed the same preexcited QRS morphology (Figure 3) as the one with wide complex orthodromic tachycardia. During atrial pacing, mapping along the lateral tricuspid annulus revealed a low amplitude, sharp intracardiac electrogram potential consistent with a Mahaim potential (Figure 4). Ablation at this site immediately blocked the accessory pathway conduction (Figure 5), and no preexcitation could be demonstrated with programmed stimulation after the ablation.

The second patient is a 13-year-old female with a history of frequent palpitations and supraventricular tachycardia which could be terminated with intravenous adenosine. Baseline ECG showed normal sinus rhythm with ventricular preexcitation and a pattern consistent with an inferoseptal location of the accessory pathway (Figure 6). Echocardiography, labs, and family history were unremarkable. The patient underwent an electrophysiological study under general anesthesia. With incremental atrial pacing, the antegrade ERP of the accessory pathway was 260 ms, which was shorter than the antegrade ERP of the AV node. Mapping of the inferoseptal region during continuous atrial pacing showed the earliest local ventricular activation was just inside the CS ostium. Irrigated RF delivery at this site blocked accessory pathway conduction, which resulted in a narrow QRS and normal HV interval. However, immediately after the ablation, a nonsustained slow rhythm with wide QRS and a cycle length of 876 ms emerged with an HV interval of 0 ms and the QRS morphology of a LBBB pattern (Figure 7). After this rhythm subsided, incremental atrial pacing was performed from the lateral right atrium, which showed progressive preexcitation with decremental conduction. Specifically, progressive widening of the QRS with HV shortening and prolongation of the stimulus-QRS interval was observed (Figure 8). This confirmed the presence of a second accessory pathway with decremental conduction properties. The preexcited QRS morphology was the same as that observed during the spontaneous wide QRS rhythm which occurred after ablation. This finding was consistent with enhanced automaticity from the decremental accessory pathway.⁸ The mapping of the tricuspid annulus during continuous atrial pacing from the lateral right atrium identified the sharp “His-like” potential on the lateral aspect of the annulus, sometimes referred to as an M potential (Figure 9). Ablation at this site resulted in immediate elimination of preexcitation (Figure 10).

Discussion

The ECG manifestation of ventricular preexcitation via an accessory pathway represents the fusion of two or more wavefronts propagating from the atria to the ventricles.⁹ Most accessory pathways are characterized by non-decremental conduction properties in contrast to the AV node which exhibits decremental conduction. With non-decremental accessory pathway conduction, part of the ventricles is activated by the accessory pathway, thereby generating the characteristic delta wave ECG pattern before the rest of the ventricles are depolarized by fast propagation from the normal AV conduction system. The morphology of the delta wave and its prominence depends on multiple factors including the accessory pathway location and the balance between accessory pathway and AV nodal conduction, which is influenced by the sympathetic and parasympathetic tone.¹⁰

Several algorithms have been developed for ECG identification of the accessory pathway location.^11-14 These algorithms are based on delta wave polarity or the main vector of the QRS complex, and they localize the accessory pathways with variable accuracy. The specificity of the ECG features in localization of the accessory pathway is diminished¹⁵ by the presence of multiple pathways conducting in an antegrade direction. In addition, the presence of decremental accessory pathways can be very difficult to discern from the ECG, since in the resting state, the difference between the accessory pathway and the AV nodal conduction properties can be quite small.¹⁶ This may result in little or no preexcitation on the ECG. The identification of a decremental anterograde pathway becomes even more difficult in the presence of another “classic” atrioventricular accessory pathway with manifest preexcitation on the ECG. On one hand, the prominent delta wave by the classic accessory pathway can obscure the subtle preexcitation by the decremental accessory pathway. However, even this subtle preexcitation can influence the morphology of the preexcited QRS complex, further complicating the identification of the accessory pathway location by the ECG.

Atriofascicular pathways are known to incorporate an AV node-like structure that is responsible for decremental antegrade conduction.⁸ Pacemaker cells are identified in these pathways.¹⁷ An accelerated automatic rhythm is sometimes seen during ablation of these pathways.¹⁸ Spontaneous automaticity of the atriofascicular pathways is rare but described in the literature.¹⁹ 

Summary

Multiple accessory pathways with classic preexcitation and its variants can exist in patients without structural heart disease. This can lead to confounding ECG features and complicated ablation procedures. Multiple accessory pathways also carry a higher risk of malignant arrhythmias. It is important to be vigilant during the ablation procedure and conduct a thorough EP study both before and after ablating the evident accessory pathway. This may help identify the uncommon and less evident accessory pathways that otherwise may have been missed. 

Disclosures: Dr. Papiashvili has no conflicts of interest to report regarding the content herein.

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