A Fasciculoventricular Accessory Pathway Featuring Functional Decremental Conduction and QRS Variability

A 36-year-old woman was admitted for evaluation due to the presence of a pre-excited QRS complex on a surface ECG. The patient did not report experiencing palpitations or any episodes of tachycardia.

Her initial ECG demonstrated a shortened PR interval (100 ms) and a QRS complex with subtle features suggestive of a delta wave (Figure 1A). Pre-procedure transthoracic echocardiography revealed no structural abnormalities. Upon reviewing the patient’s previous ECG records, no arrhythmias were detected, except for a slightly shortened PR interval. The ECGs of other family members were normal. Additionally, the patient had no family history of sudden cardiac death.

To investigate further, a diagnostic electrophysiological study was performed. The baseline measurements revealed atrial-to-His (AH) and His-to-ventricle (HV) intervals of 52 and 33 ms, respectively (Figure 1B). Retrograde conduction was concentric and decremental. During atrial pacing from the coronary sinus and high right atrium with cycle lengths progressively decreasing from 500 to 300 ms, 1:1 antegrade atrioventricular (AV) conduction was maintained, and there was no increase in pre-excitation.

The HV interval, QRS morphology and QRS duration remained consistent with those observed during sinus rhythm. At a coupling interval of 290 ms, the AH interval was markedly prolonged, while the HV interval stayed unchanged (Figures 2A and 2B). Additionally, junctional extrasystoles were observed, characterised by QRS morphology and HV intervals indistinguishable from those recorded during sinus rhythm (Figure 2C).

Wenckebach’s block occurred at a paced atrial cycle length of 300 ms. Notably, atrial pacing at an AV Wenckebach conduction cycle length of 280 ms revealed distinct QRS morphologies accompanied by variable HV intervals (Figure 3). During both sinus rhythm and atrial pacing, the onset of ventricular activation on the surface ECG never preceded the His bundle depolarisation. No supraventricular tachycardia was inducible. No ablation therapy was performed, and the patient was discharged and remains asymptomatic. These findings point to the question of the diagnosis and conduction mechanism for the patient.

Discussion

In the present case, the PR interval was shortened and accompanied by an initial delta wave, indicating the presence of pre-excitation. Intracardiac electrograms demonstrated a shortened AH interval and a nearly normal HV interval. The earlier presentation of localised V waves appeared near the His bundle, suggesting premature excitation of the septal basal segment of the local ventricular myocardium. The surface ECG characteristic of the patient demonstrated an rS pattern exhibited by the V1 lead, indicating that it may not correspond to a superoparaseptal accessory pathway (AP).¹

During atrial extrastimulation, the AH interval was prolonged, whereas the HV interval, pre-excitation degree and QRS complex remained unchanged. As per previous case reports and studies, the above electrophysiological features are highly suggestive of a fasciculoventricular AP (FVAP).^2,3 Additionally, the presence of pre-excited junctional beats favours this diagnosis. The patient’s cardiac structure appeared normal, with no evidence of other arrhythmias or any familial form of pre-excitation syndrome. Based on these findings, it is unlikely that this patient exhibits protein kinase adenosine monophosphate-activated non-catalytic subunit γ 2 (PRKAG2) cardiomyopathy.⁴ Therefore, we consider this to be a sporadic case of FVAP.

FV fibres arise from the His bundle and bundle branches.⁵ Theoretically, in a FVAP case, the HV interval is always short (<35 ms).⁶ Block at the FVAP during incremental atrial pacing results in abrupt HV prolongation and a narrow QRS complex without signs of pre-excitation.² However, contrary to previously documented electrophysiological properties of FV bypass tracts, the present case displayed a nearly normal HV interval.^7,8 Two hypotheses could account for this atypical phenomenon.^2,9 First, the proximal insertion point of the bypass may be located more inferiorly (possibly the distal His bundle or the proximal segments of the right bundle branch). Second, the conduction velocity of the FV bypass may be relatively slower, resulting in local ventricular myocardial excitation distal to the insertion point of the bypass failing to occur more in advance of the earliest excitable ventricular myocardium conducted through the His–Purkinje system.

The most challenging aspect of the present case was the observation of fusion complexes with elongated HV intervals during an atrial pacing cycle length of 280 ms, coinciding with AV Wenckebach conduction. This finding definitively supports the diagnosis of a functional decremental conduction pathway extending from the His bundle to the ventricles. Moreover, it substantiates the existence of an AP besides the normal AV conduction system. The delayed or obstructed conduction via the bypass tract separates the merged His bundle and local ventricular deflection, leading to the prolonged HV interval. This separation is associated with variably fused QRS complexes, a phenomenon particularly significant in the context of diagnosis, given the near-normal HV interval observed in this patient. As subtle changes in the pre-excitation degree are often easily overlooked, the prolongation of the HV interval must be meticulously noted to avoid a missed diagnosis.

Some scenarios exist that could contribute to the varying HV interval. Besides delays in the His–Purkinje system below the takeoff of the bypass tract and block of conduction within the FV bypass tract, delays in a bundle branch proximal to the takeoff of the bypass tract can also affect the degree of pre-excitation.¹⁰ In the latter situation, the proximal insertion is located in the right bundle branch rather than the His bundle. Histological findings reveal that FV pathways are common in humans, with some individuals exhibiting multiple FV pathways.^9,11 As with the FV pathways themselves, it remains unclear whether these pathways are functionally significant. Multiple conduction pathways could also result in varying HV intervals.

Despite the absence of a family history of genetic disorders or any noticeable structural cardiac abnormalities and other types of arrhythmias, genetic testing was not conducted for this patient. It is important to note that decremental conduction over FVAP was previously reported in the setting of PRKAG2 mutation.^4,12,13

It is crucial to focus on nodoventricular/nodofascicular APs, which partially traverse the AV node, exhibiting decremental conduction properties. Therefore, the degree of pre-excitation may change with rapid atrial pacing, and the HV interval may even decrease to negative values. Furthermore, a significant proportion of familial APs identified in association with PRKAG2 mutations exhibit decremental conduction.⁴ Notable examples include FV pathways and nodoventricular fibres, except for atriofascicular fibres.¹³ The presence of FVAP decremental conduction is particularly unusual, as this infra-Hisian connection suggests a broader involvement of the PRKAG2 protein in the development of the cardiac conduction system.