10. Idiopathic and Structural Ventricular Tachycardias
Ventricular tachycardias range from benign, outflow-tract and fascicular arrhythmias occurring in structurally normal hearts to scar-related reentrant circuits in ischemic and non-ischemic cardiomyopathy.
1. Idiopathic Ventricular Tachycardias (Structurally Normal Hearts)
| VT Subtype | Anatomical Origin & Circuit | 12-Lead Electrocardiographic Signature | Pharmacological Responsiveness & Catheter Ablation |
|---|---|---|---|
| Idiopathic Left Fascicular VT (Belhassen VT) | Reentry in the Purkinje network around the Left Posterior Fascicle ($90\%$) or Anterior Fascicle ($10\%$). | RBBB morphology with Left Axis Deviation (Posterior fascicle origin) or Right Axis Deviation (Anterior fascicle). Narrower QRS ($110-140\text{ ms}$) with sharp upstroke. | Verapamil-Sensitive (IV Verapamil $5-10\text{ mg}$ terminates tachycardia); Highly curable via Catheter Ablation of Purkinje potentials (P-potentials). |
| Right Ventricular Outflow Tract (RVOT) VT | Triggered activity (cAMP-mediated delayed afterdepolarizations) in the RVOT. | LBBB morphology with Inferior Axis (tall R in II, III, aVF); Late precordial transition ($\ge V_4$). | Sensitive to Adenosine, Beta-blockers, and CCBs; Catheter ablation of earliest activation site ($>95\%$ success). |
| Left Ventricular Outflow Tract (LVOT) / Aortic Cusp VT | Focus in coronary cusps (LCC, RCC, non-coronary) or aortomitral continuity. | LBBB or RBBB with inferior axis; Early precordial transition ($\le V_2 - V_3$); broad R wave in $V_1$. | Catheter ablation in aortic sinus of Valsalva. |
2. Structural & Scar-Related Ventricular Tachycardias
- Post-Infarction Scar VT: Macroreentrant circuits around dense fibrotic myocardium (LGE on CMR); fragmented diastolic potentials, entrainment criteria.
- Bundle Branch Reentrant VT (BBR-VT): Macroreentry down the right bundle and up the left bundle in dilated cardiomyopathy; typical LBBB morphology with rapid rate $\to$ treated with Right Bundle Branch RF Ablation.
- Bidirectional VT: Frontal QRS axis alternates by $180^\circ$ on beat-to-beat basis; pathognomonic of Digitalis Toxicity and CPVT.
3. Multimodality Investigations & 3D Mapping
- Cardiac MRI: Identifies ischemic transmural scar vs mid-myocardial (DCM/myocarditis) vs subepicardial (sarcoid/ARVC) arrhythmogenic substrates.
- 3D Electroanatomical Voltage Mapping (CARTO / EnSite): Defines dense scar ($<0.5\text{ mV}$) and border zone channels ($0.5-1.5\text{ mV}$) for substrate-based catheter ablation.