AKA Fascicular tachycardia, Belhassen-type VT, verapamil-sensitive VT or infrafascicular 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.
- 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.
- 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:
- Posterior fascicular VT (90-95% of cases): RBBB morphology + left axis deviation; arises close to the left posterior fascicle.
- Anterior fascicular VT (5-10% of cases): RBBB morphology + right axis deviation; arises close to the left anterior fascicle.
- 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.
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.
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