Idiopathic Fascicular Left Ventricular Tachycardia

ECG Features of Idiopathic Fascicular Left Ventricular Tachycardia
  • Monomorphic ventricular tachycardia eg. fusion complexes, AV dissociation, capture beats
  • QRS duration 100 – 140 ms — this is narrower than other forms of VT
  • Short RS interval (onset of R to nadir of S wave) of 60-80 ms — the RS interval is usually > 100 ms in other types of VT
  • RBBB Pattern
  • Axis deviation depending on anatomical site of re-entry circuit (see classification)

  • Fascicular tachycardia is the most common idiopathic VT of the left ventricle
  • It is a re-entrant tachycardia, typically seen in young healthy adults without structural heart disease Attacks can be precipitated by exercise, excitement, and infection
  • Characterized by a right bundle branch block pattern and left axis deviation
  • Verapamil is the first line treatment

AKA: Fascicular tachycardia, Belhassen-type VT, verapamil-sensitive VT or infrafascicular tachycardia.

Idiopathic VT
  • 90% of patients presenting with monomorphic VT are secondary to structural heart disease, including ischaemic heart disease, congenital heart disease, valvular dysfunction and myocardial dysfunction
  • 10% of cases of monomorphic VT occur in the absence of structural heart disease and are termed idiopathic VT
  • 75-90% of idiopathic VT arise from the right ventricle — e.g right ventricular outflow tract tachycardia
  • 10-25% of idiopathic VT arise from an ectopic focus within the left ventricle, especially the left posterior fascicle (Fascicular VT). First described as a unique electrophysiologic entity in 1981 by Belhassen et al [Belhassen VT; fascicular or intrafascicular tachycardia; or verapamil sensitive VT; narrow complex VT]


Usually occurs in young healthy patients (15-40 years of age; 60-80% male). Most episodes occur at rest but may be triggered by exercise, stress and beta agonists. The mechanism is re-entrant tachycardia due to an ectopic focus within the left ventricle.

NB. A similar ECG pattern of fascicular VT may occur with digoxin toxicity, but here the mechanism is enhanced automaticity in the region of the fascicles.


Fascicular tachycardia can be classified based on ECG morphology corresponding to the anatomical location of the re-entry circuit:

  1. Posterior fascicular VT (90-95% of cases): RBBB morphology + left axis deviation; arises close to the left posterior fascicle
  2. Anterior fascicular VT (5-10% of cases): RBBB morphology + right axis deviation; arises close to the left anterior fascicle
  3. Upper septal fascicular VT (rare): atypical morphology – usually RBBB but may resemble LBBB instead; cases with narrow QRS and normal axis have also been reported. Arises from the region of the upper septum

Diagnosis and Management
  • Diagnosis can be difficult and this rhythm is often misdiagnosed as SVT with RBBB; the diagnosis is made by observing specific features of VT, e.g. fusion/capture beats, AV dissociation.
  • Idiopathic fascicular tachycardia may prove difficult to treat as it is often unresponsive to adensoine, vagal maneouvers, and lignocaine. However, it characteristically responds to verapamil
  • Digoxin-induced fascicular VT is responsive to Digoxin Immune Fab

ECG Examples
Example 1
Belhassen-VT-with-capture-beat1 2

Idiopathic Fascicular VT:

  • Broad-complex complex tachycardia with modest increase in QRS width (~120 ms)
  • RBBB morphology (RSR’ in V1)
  • Left axis deviation (-90 degrees)
  • Narrow-complex capture beat (complex #6)
  • Several dissociated P waves are seen in the lead II rhythm strip (associated with the 3rd, 10th, 14th, 18th and 22nd QRS complexes)

This rhythm could easily be mistaken for SVT with bifascicular block (RBBB + LAFB) — however, the presence of dissociated P waves and a narrow-complex capture beat indicates that this is fascicular VT arising from the left posterior fascicle.

This ECG is reproduced from Heart Pearls.com

Example 2
Idiopathic Fascicular Left Ventricular Tachycardia 2
Idiopathic Fascicular VT: Presentation

Idiopathic Fascicular Left Ventricular Tachycardia 2a
Idiopathic Fascicular VT: Post Verapamil

Example 3
ECG Primary idiopathic fascicular VT 3a
Idiopathic Fascicular VT – Presentation

ECG Primary idiopathic fascicular VT 3b reversion
Idiopathic Fascicular VT – Post verapamil


Advanced Reading



LITFL Further Reading


Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

One comment

  1. Should example 3 be SVT with bifacicular block (RBBB+Left posterior fascicular block)? Because there is only wide complex QRS in all leads, no narrow complex, also no fusion beats, capture beat are found in all leads. Thank you for your concern

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