Fasciculoventricular bypass tracts: Electrocardiographic and electrophysiologic features

Abstract Fasciculoventricular accessory pathways are rare variants of preexcitation. The differential diagnosis of fasciculoventricular accessory pathways from other preexcitation variants can be challenging. Based on two cases, we discuss the specific electrocardiographic and electrophysiologic features of fasciculoventricular bypass tracts.


| INTRODUC TI ON
Fasciculoventricular (FV) accessory pathways (AP) are uncommon preexcitation variants. 1,2 The differential diagnosis of FV APs from anteroseptal atrioventricular APs and nodoventricular (NV)/nodofascilular (NF) APs can be challenging. 1,2 Based on two cases, we discuss the specific electrocardiographic and electrophysiologic features of FV bypass tracts.

| C A S E REP ORT 1
A 31-year-old male was referred for an electrophysiologic study (EPS) because of overt preexcitation on 12 lead electrocardiogram (ECG). He never complained for palpitations or tachycardia. He had no history of structural heart disease and the transthoracic echocardiogram was normal. The QRS duration was 105 milliseconds with normal frontal plane axis, normal PR interval with minimal preexcitation (absence of septal q waves), and precordial transition (R/S wave ratio > 1) in lead V 3 ( Figure 1A). The differential diagnosis included the presence of anteroseptal AP or NV/NF AP or FV AP. During EPS, the AH interval was normal and the HV interval was 28 milliseconds Sudden prolongation of the HV interval and loss of preexcitation occurred either spontaneously or following atrial pacing at slow rates, confirming a long effective refractory period of the AP ( Figure 1B). Interestingly, during EPS, junctional beats with identical preexcitation pattern were recorded, while the HV interval remained short and unchanged, suggesting the presence of an infranodal AP ( Figure 1A and C). Para-Hisian pacing produced a near perfect match with the baseline ECG ( Figure 1D). Programmed atrial pacing with or without isoproterenol failed to induce any supraventricular tachycardias. Based on these findings, the presence of a FV AP was established. No ablation was performed and the patient was discharged and remains asymptomatic.

| C A S E REP ORT 2
A 17-year-old female was referred for an EPS owing to overt preexcitation on 12-lead ECG. She never complained for palpitations or tachycardia. The ECG showed a wide QRS (>120 milliseconds) with normal frontal plane axis, a short PR interval with minimal preexcitation (positive delta waves in leads II, aVF, V4-6), and precordial transition in lead V 3 (Figure 2A). The echocardiographic study was normal. The differential diagnosis included the presence of anteroseptal AP or NV/NF AP or FV AP. During EPS, the baseline HV interval was short, at 15 milliseconds, and stayed fixed during incremental atrial pacing until AP refractoriness was reached ( Figure 2B). Of note, the degree of preexcitation remained the same during incremental atrial pacing ( Figure 2B). The administration of 18mg adenosine induced different atrioventricular (AV) conduction patterns (prolongation of PR interval), AV block as well as junctional rhythm without producing any change in the configuration of the QRS complex ( Figure 2C and D), an event that favored the diagnosis of FV AP. Programmed atrial pacing with or without isoproterenol failed to induce any supraventricular tachycardias.
No ablation was attempted and the patient remains asymptomatic until today. The ECG is characterized by normal frontal plane axis, similar to an anteroseptal APs, a subtle preexcitation, and a normal PR interval.

| D ISCUSS I ON
R/S transition in the precordial leads is mostly recorded in lead V 2 . 3 Anteroseptal APs display significantly higher delta wave amplitudes (4.8 ± 2.0 mm vs 1.9 ± 1.3 mm), shorter PR intervals (94.6 ± 12.5 milliseconds vs 106.8 ± 13.2 milliseconds), and longer QRS intervals (133.6 ± 19.0 milliseconds vs 118.7 ± 24.7 milliseconds) compared to FVFs. 4 In the later study, the delta wave amplitude was the only independent predictor of WPW syndrome. 4 Figure 3 shows these ECG differences between anteroseptal and FVAPs.  Table 1. Although FV APs are not implicated in clinical tachycardias, they have been associated with structural abnormalities and sudden cardiac death. In the setting of PRKAG2, gene mutations have a high incidence of syncope, ventricular hypertrophy, atrial arrhythmias, sinus bradycardia, and complete AV block. 6 In conclusion, the diagnosis of FV APs is essential, mainly for two reasons. First, in the setting of a supraventricular tachycardia, the recognition of the bystander nature of these APs is extremely important in order to avoid unnecessary catheter ablation and inadvertent damage of the AV node. Second, subjects with FV APs require further investigation in order to exclude a PRKAG2 gene mutation phenotype which has been associated with sudden cardiac death.
Although challenging, prompt differentiation of FVs from anteroseptal APs is therefore extremely important. Even in the absence of  and their association with genetic syndromes related to structural heart disease and sudden cardiac death should be excluded.

CO N FLI C T O F I NTE R E S T S
The authors declare no conflict of interests for this article.