Fascicular ventricular tachycardia (VT) is a specific type of idiopathic ventricular tachycardia characterized by a relatively narrow QRS complex with a right bundle branch block (RBBB) morphology. It originates from one of the fascicles (specifically the left posterior or left anterior fascicle) of the left bundle branch in the heart's ventricular conduction system.
Understanding Fascicular VT
Fascicular VT is considered "idiopathic" because it typically occurs in individuals with structurally normal hearts, unlike ventricular tachycardias associated with heart disease. It is a re-entrant tachycardia, meaning that an electrical impulse circulates in a loop within the fascicle, causing rapid and repetitive ventricular contractions.
Key Characteristics:
- Origin: Arises from the fascicles of the left bundle branch (most commonly the left posterior fascicle).
- QRS Morphology: Typically exhibits a relatively narrow QRS complex (compared to other VTs) with a right bundle branch block (RBBB) pattern and a left axis deviation.
- Underlying Cause: Usually idiopathic, meaning it occurs in the absence of structural heart disease.
- Mechanism: Re-entrant circuit within the fascicle.
- Common Location: More frequently arises from the left posterior fascicle, resulting in a superior axis deviation. Less commonly originates from the left anterior fascicle, leading to an inferior axis deviation.
- Response to Verapamil: Often sensitive to verapamil, a calcium channel blocker, which helps distinguish it from other types of ventricular tachycardia.
Electrocardiographic Features:
The electrocardiogram (ECG) plays a crucial role in diagnosing fascicular VT. Key features include:
- Ventricular Tachycardia: A wide complex tachycardia with a rate typically between 150 and 250 beats per minute.
- QRS Duration: The QRS duration is usually less than 120 milliseconds, making it "relatively narrow" compared to other VTs.
- RBBB Morphology: The QRS complex resembles a right bundle branch block pattern in the precordial leads (V1-V6).
- Axis Deviation: The axis deviation depends on the location of the tachycardia:
- Left Posterior Fascicular VT: Superior axis deviation.
- Left Anterior Fascicular VT: Inferior axis deviation.
- AV Dissociation: Atrioventricular dissociation may be present, where the atria and ventricles beat independently.
Treatment Options:
Fascicular VT can often be effectively treated with:
- Verapamil: A calcium channel blocker that is effective in terminating the tachycardia.
- Radiofrequency Ablation: A catheter-based procedure to eliminate the re-entrant circuit within the fascicle. This is often a curative option.
Example:
Imagine a patient presenting with palpitations and dizziness. An ECG reveals a ventricular tachycardia with a rate of 180 bpm, a relatively narrow QRS complex exhibiting RBBB morphology, and a superior axis deviation. This presentation is highly suggestive of left posterior fascicular VT.
In conclusion, fascicular VT is a specific type of ventricular tachycardia with unique ECG characteristics and treatment options, usually occurring in individuals without structural heart disease.
