Fascicular VT

The ECG: What Should We Look For?

The major diagnostic challenge is that fascicular VT often produces a QRS complex that is considerably narrower than we would expect in conventional ventricular tachycardia.

1. Regular tachycardia

2. Right bundle branch block morphology

Fascicular VT usually originates in the left ventricular conduction system, resulting in ventricular activation that produces an RBBB-like morphology. This is characteristic of left posterior fascicular VT. However, RBBB morphology is not diagnostic by itself. SVT with aberrant conduction may produce an identical broad pattern.

3. Left axis deviation

The combination of RBBB morphology and left axis deviation is an important clue. It suggests a left posterior fascicular VT pattern, although SVT with RBBB and left anterior fascicular block remains a significant mimic.

4. Relatively narrow QRS complexes

This is one of the most important points. In conventional ventricular tachycardia, we often expect markedly prolonged QRS complexes, sometimes exceeding 160 ms. In fascicular VT, the QRS duration is commonly less than 140 ms. The reason is that ventricular activation involves the specialised conduction system, allowing relatively rapid propagation through the ventricles.

A QRS duration of less than 140 ms does not exclude ventricular tachycardia.

5. Additional morphological clues

Several additional findings can help distinguish posterior fascicular VT from SVT with aberrancy:

• A positive QRS complex in aVR.
• A deep S wave in V6, producing an R/S ratio of 1 or less.
• A relatively short RS interval, approximately 60–80 ms.
• No clearly identifiable associated P waves.

These findings should be interpreted collectively rather than as isolated diagnostic criteria.

Why Does Verapamil Work?

Most conventional ventricular tachycardias involve circuits or mechanisms that are not dependent on slow calcium-mediated conduction in the same way. In fascicular VT, the re-entry circuit includes a slow-conducting component associated with Purkinje tissue. This component is sensitive to calcium channel blockade. This is why fascicular VT is often described as verapamil-sensitive VT.

But Isn’t Verapamil Contraindicated in VT?

Yes. Verapamil can cause serious haemodynamic deterioration when administered to patients with undifferentiated wide-complex tachycardia, particularly those with structural heart disease or ventricular dysfunction.

The exception is confidently identified, haemodynamically stable fascicular VT.

References

1. Khan R, Shahid M. Man in his 40s with a sudden onset of palpitations. Emergency Medicine Journal. 2026;43:657. doi:10.1136/emermed-2025-215665.
2. Michowitz Y, Tovia-Brodie O, Heusler I, et al. Differentiating the QRS Morphology of Posterior Fascicular Ventricular Tachycardia From Right Bundle Branch Block and Left Anterior Hemiblock Aberrancy. Circulation: Arrhythmia and Electrophysiology. 2017;10.
3. Brugada P, Brugada J, Mont L, et al. A new approach to the differential diagnosis of a regular tachycardia with a wide QRS complex. Circulation. 1991;83:1649–1659.
4. Kapa S, Gaba P, DeSimone CV, et al. Fascicular Ventricular Arrhythmias. Circulation: Arrhythmia and Electrophysiology. 2017;10.