Idiopathic Fascicular Left Ventricular Tachycardia


  • Fascicular tachycardia is the most common idiopathic VT of the left ventricle.
  • It is a re-entrant tachycardia, typically seen in young patients without structural heart disease.
  • Verapamil is the first line treatment.

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

Idiopathic VT

  • Only 10% of cases of VT occur in the absence of structural heart disease, termed idiopathic VT.
  • The majority of idiopathic VTs (75-90%) arise from the right ventricle — e.g right ventricular outflow tract tachycardia.
  • Fascicular VT is the most common type of idiopathic VT arising from the left ventricle (10-15% of all idiopathic VTs).


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.

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 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

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

Related Topics


  • Edhouse J, Morris F. ABC of clinical electrocardiography: Broad complex tachycardia-Part II. BMJ. 2002 Mar 30;324(7340):776-9. Full Text.
  • Eynon CA, Howe L, Firoozan S. Fascicular tachycardia: uncommon or just unrecognised? Emerg Med J 2002;19:477–478. Full Text
  • Francis J, Venugopal K, Khadar SA, Sudhayakumar N, Gupta AK. Idiopathic Fascicular Ventricular Tachycardia. Indian Pacing Electrophysiol J. 2004 Jul-Sep; 4(3): 98–103 Full text
  • Moreno Reviriego S. Idiopathic fascicular left ventricular tachycardia. An article from the E-Journal of the ESC Council for Cardiology Practice, Dec 2010. Full text.
  • Quimby TW, Clark AA, Fix ML. Idiopathic Ventricular Tachycardia: Belhassen Type. West J Emerg Med. 2010; 11(4):389-390 Full Text
  • Wieland JM, Marchlinski FE. Electrocardiographic Response of Digoxin-Toxic Fascicular Tachycardia to Fab Fragments: Implications for Tachycardia Mechanism Pacing Clin Electrophysiol. 1986 Sep;9(5):727-38. PMID: 2429280

LITFL Further Reading

Advanced Reading




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

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