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Core EP Topics

Fascicular VT (Belhassen VT)

Verapamil-sensitive idiopathic left ventricular tachycardia — the most common idiopathic VT in young patients

Idiopathic VT Verapamil-Sensitive Left Posterior Fascicle
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Mechanism

Fascicular ventricular tachycardia (also known as Belhassen VT or idiopathic left ventricular tachycardia) is a macroreentrant VT involving the Purkinje fiber network of the left ventricle. It is the most common form of idiopathic VT, typically affecting young, otherwise healthy patients with structurally normal hearts. First described by Zipes in 1979 and subsequently characterized by Belhassen in 1984 for its hallmark sensitivity to intravenous verapamil, this arrhythmia has been the subject of decades of electrophysiologic investigation.

The reentrant circuit involves an abnormal region of the Purkinje fiber network in the left ventricular septum. The circuit has three critical components: (1) a zone of slow conduction dependent on L-type calcium channels (the verapamil-sensitive zone, generating the P1 or diastolic Purkinje potential), (2) a zone of rapid Purkinje fiber conduction (generating the P2 or pre-systolic Purkinje potential), and (3) the ventricular myocardium as the exit site. The slow conduction zone is the critical element — its dependence on calcium channel–mediated conduction is what makes the tachycardia uniquely responsive to verapamil.

Circuit Components
  • P1 (diastolic Purkinje potential): generated by the verapamil-sensitive slow conduction zone in the mid to apical left ventricular septum
  • P2 (pre-systolic Purkinje potential): generated by the distal Purkinje fiber just before ventricular activation — the exit point of the circuit
  • Ventricular myocardium: the “bystander” tissue activated by the Purkinje exit, producing the QRS
  • Retrograde limb: retrograde fast conduction via the normal fascicular system back to the slow conduction zone

Three anatomic variants exist based on which fascicle is involved. The left posterior fascicular VT is by far the most common (90–95%), involving the left posterior fascicle and producing an RBBB with left axis deviation (superior axis) morphology. The left anterior fascicular VT involves the left anterior fascicle, producing RBBB with right axis deviation (inferior axis). The rare upper septal variant involves the upper interventricular septum near the His bundle bifurcation, producing a narrow QRS with a normal or left axis.

Unlike scar-mediated VTs, fascicular VT occurs in a structurally normal heart and carries an excellent prognosis. The mechanism is not automaticity or triggered activity (although some fascicular VTs show features of triggered activity) — the dominant mechanism is reentry within the Purkinje network, supported by the ability to entrain the tachycardia and the response to programmed stimulation.

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ECG Clues

The surface ECG is often the first clue to fascicular VT, and the morphology is distinctive enough to make a confident diagnosis in the appropriate clinical context. However, the relatively narrow QRS and rate frequently lead to misdiagnosis as SVT with aberrant conduction.

Key ECG Features — Left Posterior Fascicular VT (Classic Belhassen)
  • RBBB morphology with left axis deviation (superior axis)
  • Relatively narrow QRS: 120–140 ms (narrower than typical scar-mediated VT)
  • Rate: 150–200 bpm, usually regular
  • RS interval in precordial leads typically <100 ms
  • Precordial R/S transition in V4–V6
  • AV dissociation or fusion/capture beats confirm VT
  • Terminates with IV verapamil — highly suggestive and nearly pathognomonic

The relatively narrow QRS (compared to the typical >160 ms of scar-mediated VT) occurs because the Purkinje fiber system provides rapid ventricular activation, similar to the normal conduction system. This is the most common reason fascicular VT is misdiagnosed as SVT with RBBB aberrancy. The key distinguishing features are AV dissociation (present in most cases), fusion beats, and the response to verapamil (SVT with aberrancy does not terminate with verapamil unless it is AVNRT, which would have a different morphology).

Feature Posterior Fascicular VT Anterior Fascicular VT Upper Septal VT
Bundle branch pattern RBBB RBBB Narrow QRS or RBBB
Axis Left axis (superior) Right axis (inferior) Normal axis
QRS duration 120–140 ms 120–140 ms 100–130 ms
Prevalence 90–95% 5–8% <2%
Verapamil response Yes (terminates) Variable Variable
Fascicle involved Left posterior Left anterior Near His bifurcation
Clinical Pearl: The most common misdiagnosis is SVT with RBBB aberrancy. A young patient with a regular wide-complex tachycardia with RBBB + left axis that terminates with verapamil is fascicular VT until proven otherwise. Always look for AV dissociation — even subtle dissociation favors VT.
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EP Study Findings

The electrophysiology study in fascicular VT serves to confirm the diagnosis, localize the circuit, and guide ablation. The hallmark findings involve characteristic Purkinje potentials recorded during tachycardia.

Induction

Fascicular VT is typically induced with programmed ventricular or atrial stimulation, often requiring isoproterenol to enhance inducibility. The tachycardia may also be initiated by burst atrial pacing that conducts down the His-Purkinje system. The ability to induce and entrain the tachycardia confirms a reentrant mechanism and distinguishes it from automatic or triggered mechanisms.

Purkinje Potentials During VT

The P1-P2-QRS Sequence

During fascicular VT, careful mapping along the left ventricular septum reveals two distinct Purkinje potentials:

  • P1 (diastolic Purkinje potential): a sharp, high-frequency potential recorded during diastole, preceding the QRS by 30–60 ms or more. It represents activation of the verapamil-sensitive slow conduction zone and is the critical ablation target.
  • P2 (pre-systolic Purkinje potential): recorded just before or at the onset of the QRS (<20 ms). It represents the distal Purkinje fiber activation at the exit point of the circuit.
  • The P1–to–P2 interval represents the conduction time through the slow zone and is the component prolonged by verapamil.

Confirming the Diagnosis

VA dissociation is present during fascicular VT, confirming ventricular tachycardia rather than SVT with aberrancy. The atrium is not a part of the circuit, and atrial pacing or adenosine (which blocks AV nodal conduction) does not terminate the tachycardia — unless the retrograde limb of the fascicular circuit passes through the His-AV node (which can occur in some variants).

Entrainment

Entrainment from the left ventricular septum is feasible and confirms reentry. Concealed entrainment (manifest fusion absent, QRS identical to tachycardia morphology) at a site with a diastolic Purkinje potential confirms that the pacing site is within the circuit and is an excellent ablation target. The post-pacing interval (PPI) minus tachycardia cycle length (TCL) of <30 ms at such sites confirms the critical isthmus.

Verapamil Response

IV verapamil (5–10 mg) terminates fascicular VT in the vast majority of posterior fascicular VTs, and this response is considered nearly diagnostic. Verapamil acts by blocking the L-type calcium channels in the slow conduction zone, interrupting the reentrant circuit. Importantly, verapamil also prevents re-induction, which can be used as a diagnostic criterion. During sinus rhythm, the fascicular system appears normal in most patients, and there may be no identifiable electrophysiologic abnormality.

Clinical Pearl: If VT with RBBB + left axis deviation terminates with verapamil and becomes non-inducible, the diagnosis of fascicular VT is essentially confirmed. If it does not respond to verapamil, consider scar-mediated VT (even in a young patient — obtain cardiac MRI to evaluate for scar or infiltrative disease).
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Ablation Targets & Strategy

Catheter ablation is the definitive treatment for fascicular VT, offering cure rates of 90–95%. The approach depends on whether the VT can be induced and sustained during the procedure.

Activation Mapping (Preferred)

When VT is inducible and hemodynamically tolerated (which is usually the case), activation mapping during VT is the preferred strategy. The ablation catheter is advanced to the left ventricle (retrograde aortic approach) and mapped along the mid to apical left ventricular septum. The target is the site recording the earliest diastolic Purkinje potential (P1) with the longest P1-to-QRS interval. This represents the entrance to the slow conduction zone, which is the critical isthmus of the reentrant circuit.

Ablation Targets
  • Primary target: site of the diastolic Purkinje potential (P1) during VT, typically at the mid to apical posterior left ventricular septum
  • Confirmatory target: concealed entrainment at the P1 site with PPI − TCL <30 ms
  • Alternative: elimination of P1 during sinus rhythm pace-mapping if VT is not inducible
  • For anterior fascicular VT: target is along the anterosuperior left ventricular septum

Substrate Mapping (When VT Not Inducible)

If fascicular VT is not inducible (often after verapamil administration or in the post-ablation setting), an anatomical/substrate approach targets the Purkinje fiber network along the left ventricular septum. Mapping during sinus rhythm for abnormal or split Purkinje potentials can identify the substrate. Pace mapping from the septum to replicate the clinical VT morphology can also guide ablation. Some operators use a strategy of ablating along the distal left posterior fascicle (for posterior fascicular VT) in a linear fashion.

Endpoints

Procedural Endpoints
  • Termination of VT during ablation at a site with diastolic Purkinje potential
  • Elimination of the diastolic Purkinje potential during sinus rhythm at the ablation site
  • Non-inducibility: VT cannot be re-induced with programmed stimulation ± isoproterenol
  • Observation period of 30 minutes to confirm no spontaneous recurrence
Warning — Fascicular Block: Ablation along the left ventricular septum near the posterior fascicle can cause left posterior hemiblock (LPHB). For posterior fascicular VT, this is often an unavoidable consequence (and may occur even with precise ablation). Similarly, ablation of anterior fascicular VT can cause left anterior hemiblock (LAHB). Isolated hemiblock is generally well-tolerated and does not require pacing, but patients should be counseled about this possibility.

Success Rates and Recurrence

Acute success rates are 90–95%, with long-term freedom from VT in approximately 85–90% of patients. Recurrence is more common when ablation is performed using substrate/anatomic mapping alone (without VT activation mapping). The anterior fascicular variant is more challenging to ablate due to its less predictable circuit location. Cryoablation has been used in some centers, particularly when the target is close to the His bundle (upper septal variant), but its role is less well-established compared to AVNRT ablation.

Clinical Pearl: Fascicular VT carries an excellent long-term prognosis even without ablation. Medical therapy with oral verapamil is a reasonable alternative for patients who prefer not to undergo ablation, or for the rare patient with recurrence post-ablation. Unlike scar-mediated VT, fascicular VT does not cause sudden cardiac death and ICD implantation is not indicated.

Key References

  1. Belhassen B, Rotmensch HH, Laniado S. Response of recurrent sustained ventricular tachycardia to verapamil. Br Heart J. 1981;46(6):679–682. DOI: 10.1136/hrt.46.6.679
  2. Zipes DP, Foster PR, Troup PJ, Pedersen DH. Atrial induction of ventricular tachycardia: reentry versus triggered automaticity. Am J Cardiol. 1979;44(1):1–8. DOI: 10.1016/0002-9149(79)90242-X
  3. Nogami A. Idiopathic left ventricular tachycardia: assessment and treatment. Card Electrophysiol Rev. 2002;6(4):448–457. DOI: 10.1023/A:1021100828459
  4. Ma FS, Ma J, Tang K, et al. Left posterior fascicular block: a new endpoint of ablation for verapamil-sensitive idiopathic ventricular tachycardia. Chin Med J. 2006;119(5):367–372. DOI: 10.1097/00029330-200603010-00003
  5. Kapa S, Gaba P, DeSimone CV, Asirvatham SJ. Fascicular ventricular arrhythmias: pathophysiologic mechanisms, anatomical constructs, and advances in approaches to management. Circ Arrhythm Electrophysiol. 2017;10(1):e002476. DOI: 10.1161/CIRCEP.116.002476