Electrophysiology of Fascicular Ventricular Tachycardia
Fascicular VT is a reentrant arrhythmia involving the left ventricular Purkinje system, with a verapamil-sensitive slow conduction zone as the anterograde limb and the left posterior fascicle (most commonly) as the retrograde limb of the circuit. 1
Reentrant Circuit Components
The reentrant mechanism involves two distinct pathways with specific electrophysiological properties:
- Anterograde limb: A zone of slow conduction with verapamil-sensitive tissue located along the left ventricular septum, extending from the basal to apical regions, which exhibits calcium-dependent conduction properties 1
- Retrograde limb: The normal left posterior fascicle (in >90% of cases) or left anterior fascicle (in <10% of cases) of the His-Purkinje system, providing rapid conduction back to complete the circuit 1
Electrophysiological Characteristics During Tachycardia
Critical diagnostic potentials recorded during mapping include:
- Purkinje potentials (PP): High-frequency presystolic potentials representing activation of the fascicular tissue in the retrograde limb, preceding the QRS by approximately 25 ms 2, 3
- Diastolic potentials (DP): Mid-diastolic potentials representing electrical activity in or near the slow conduction zone, which are the optimal ablation targets 2, 4, 5
- His-ventricular interval: Shorter during VT compared to sinus rhythm, confirming participation of the Purkinje system 2
Mechanism Classification
While reentry is the predominant mechanism, two distinct electrophysiological subtypes exist:
- Reentrant fascicular VT (most common): Macroreentrant circuit involving extensive left interventricular septum with reproducible induction and entrainment 4, 5, 6
- Non-reentrant focal fascicular VT (2.8% of cases): Focal mechanism arising from Purkinje tissue, characterized by non-entrainable VT, spontaneous occurrence, and lack of verapamil response 3
Induction Characteristics
The electrophysiological properties during EP testing reveal:
- Reproducible induction with programmed atrial or ventricular stimulation in reentrant forms, though contact inhibition during endocardial mapping frequently limits reproducibility 2, 4
- Burst pacing can be effective for induction when single extrastimuli fail 1
- Isoproterenol infusion is generally not required for fascicular VT (unlike outflow tract VT), as the mechanism is reentrant rather than triggered activity 1
Verapamil Sensitivity Mechanism
The hallmark verapamil sensitivity reflects the calcium-dependent slow conduction zone:
- Intravenous verapamil (5 mg) terminates VT in the majority of cases by blocking the slow conduction zone in the anterograde limb 1, 7
- Partial verapamil response (slowing without termination) occurs in approximately 50% of papillary muscle variants, indicating these may have different electrophysiological properties 2
- The verapamil-sensitive zone is distinct from the Purkinje tissue itself, representing partially depolarized fibers with altered calcium handling 5
ECG-Electrophysiology Correlation
Surface ECG morphology directly reflects the circuit location:
- Left posterior fascicular VT (>90%): RBBB with superior axis (left axis deviation) due to retrograde activation through the left posterior fascicle and anterograde exit near the inferior septum 1, 7
- Left anterior fascicular VT (<10%): RBBB with right axis deviation reflecting retrograde conduction through the left anterior fascicle 1, 7
- QRS duration: Relatively narrow (typically <140 ms) compared to other VTs because the circuit involves the specialized conduction system rather than myocardium 1, 3
Common Pitfall
Contact inhibition during endocardial mapping frequently terminates the tachycardia, making conventional activation mapping challenging and limiting ablation success when relying solely on exit site targeting 4. Targeting sites with simultaneous recording of both diastolic and Purkinje potentials yields superior outcomes compared to exit site ablation alone 4, 6.