Can Short-Long-Short Sequence Cause VF?
Yes, short-long-short (S-L-S) ventricular sequences can definitively precipitate ventricular fibrillation (VF) and represent a well-established proarrhythmic mechanism that requires recognition and intervention.
Mechanism and Clinical Evidence
The S-L-S sequence creates critical electrophysiologic conditions that promote VF through two primary mechanisms:
Dispersion of repolarization: The pause (long cycle) following a premature beat (short cycle) causes differential prolongation of repolarization across myocardial layers—particularly affecting midmyocardial and endocardial sites more than epicardial sites. This increased dispersion creates zones of functional conduction block and slowed conduction 1.
Reentrant excitation: When a second premature beat (the terminal "short") encounters this altered repolarization pattern, it can trigger reentrant circuits leading to polymorphic VT/VF 1.
Clinical Contexts Where S-L-S Triggers VF
Idiopathic VF
In patients with idiopathic polymorphic VT/VF, catheter ablation of triggering PVCs with consistent QRS morphology is useful 2. These patients typically have short-coupled PVCs arising from the Purkinje system that initiate VF through S-L-S sequences 2. The recurrence risk is very high—87% of patients undergoing ablation had experienced VF episodes in the preceding year 2.
Pacemaker-Facilitated VF
S-L-S sequences can be actively facilitated by bradycardia pacing systems, constituting an important yet overlooked proarrhythmic mechanism 3:
- Pacing-facilitated S-L-S accounted for 14.8% of VT/VF episodes in DDD/R mode, 9.4% in VVI/R mode, and 8.2% in MVP mode 3
- Pause durations differed by mode: DDD/R 793±172 ms, MVP 865±278 ms, VVI/R 1180±414 ms 3
- Ventricular safety pacing algorithms can inadvertently create these sequences during rapid atrial pacing 4
Long QT Syndrome
In LQT syndrome, S-L-S sequences (often from ventricular bigeminy) are a classic trigger for torsades de pointes. The mechanism involves enhanced dispersion of repolarization during the pause, with the subsequent premature beat encountering heterogeneous recovery 1.
Clinical Recognition and Management
Key Diagnostic Features
Look for these patterns on telemetry or device interrogation:
- Premature ventricular beat → compensatory pause → second premature beat → VF initiation
- Consistent PVC morphology preceding VF episodes
- Clustering of VF episodes with stereotypical initiation patterns 5
Management Algorithm
For patients with recurrent S-L-S-initiated VF:
ICD implantation is mandatory if meaningful survival >1 year is expected 2—the recurrence risk is too high for medical therapy alone
Catheter ablation should be performed for the triggering PVC focus when a consistent morphology is identified 2. Success rates are high, though 18% experience recurrence requiring repeat ablation 2
Device programming optimization in pacemaker patients 3, 4:
- Decrease pacing rate to minimize pauses
- Adjust AV intervals to reduce ventricular pacing
- Consider disabling ventricular safety pacing if causing S-L-S sequences
- Use atrial-based timing when possible
Antiarrhythmic therapy has limited efficacy except quinidine/hydroquinidine in specific contexts 2
Address underlying triggers: In atrial fibrillation patients, rhythm control with amiodarone can eliminate both AF and the S-L-S sequences 5
Critical Pitfalls
- Subcutaneous ICDs may not be optimal for idiopathic VF patients due to higher risk of T-wave oversensing 2
- Don't rely on ablation alone—even after apparently successful procedures, patients need ICD protection given the 18% recurrence rate 2
- Review device electrograms carefully in ICD patients with recurrent shocks—pacing-facilitated S-L-S is easily missed without thorough retrospective analysis 6
The evidence consistently demonstrates that S-L-S sequences are not merely associated with VF but actively precipitate it through predictable electrophysiologic changes that create the substrate for reentrant excitation 1.