What is the electrophysiological mechanism underlying the short‑long‑short sequence of premature ventricular contractions?

Short-Long-Short Electrophysiological Mechanism

The short-long-short (SLS) sequence is a pause-dependent arrhythmogenic mechanism where a premature ventricular contraction (short interval) is followed by a compensatory pause (long interval), which then triggers another premature beat (short interval) that initiates ventricular tachyarrhythmias, most characteristically torsades de pointes in long QT syndrome. 1

Core Electrophysiological Mechanism

The SLS sequence operates through triggered activity from early afterdepolarizations (EADs) that occur during the prolonged action potential following the pause 2. Here's how it unfolds algorithmically:

The Three-Step Sequence:

1. First "Short": A premature ventricular contraction occurs with a relatively short coupling interval to the preceding beat

2. "Long": This creates a compensatory pause, during which:

   • Action potential duration becomes markedly prolonged
   • The membrane remains depolarized longer than normal
   • Repolarizing potassium currents are reduced
   • Inward calcium currents or late sodium currents increase 2

3. Second "Short": During this prolonged repolarization phase, EADs develop when:

   • L-type calcium channels reactivate before the membrane returns to normal resting potential
   • Spontaneous calcium release from the sarcoplasmic reticulum activates depolarizing sodium-calcium exchange currents 2
   • This generates the triggering PVC that initiates the arrhythmia

Disease-Specific Patterns

The SLS mechanism manifests differently across conditions:

Long QT Syndrome: This is the classic setting where SLS sequences predominate. Recent data shows that 54% of ventricular fibrillation episodes in LQTS patients are initiated by SLS sequences, with trigger coupling intervals exceeding 450 ms in 68% of cases 3. The guideline explicitly states that torsades de pointes "often has a long-short initiating sequence with a long coupling interval to the first VT beat" 1.

Bradycardia-Related: High-grade AV block can create the prolonged pauses necessary for SLS sequences, facilitating torsades de pointes 1. This is particularly relevant when backup pacing rates are programmed too low (e.g., 35 bpm), which may inadvertently create the substrate for SLS-triggered arrhythmias 4.

Clinical Implications and Pitfalls

ICD Programming Considerations:

A critical pitfall occurs with single-chamber ICDs using combined onset and stability criteria. The SLS sequence can prevent appropriate therapy delivery because the varying cycle lengths fail to meet onset detection thresholds, even during sustained VT 4. The initial short-long transition creates rate variability that the device misinterprets as non-ventricular in origin.

To prevent this:

• Avoid requiring both onset AND stability criteria simultaneously
• Lower VT detection rates appropriately (e.g., from 160 to 150 bpm)
• Increase backup pacing rates from very low settings (35 bpm) to at least 55 bpm to prevent pause-dependent triggers 4

Autonomic Nervous System Role:

Variable coupling intervals in PVCs—which characterize SLS sequences—have particularly destabilizing effects on the intrinsic cardiac nervous system. Variable coupling intervals affect convergent neurons (local reflex processors) more than fixed intervals, creating both neuronal and electrical instability 5. This suggests the SLS pattern is not merely a trigger but actively destabilizes cardiac control mechanisms.

Substrate Requirements

For SLS sequences to successfully trigger sustained arrhythmias, there must be:

• Temporal dispersion: The prolonged action potential creates heterogeneous repolarization across the myocardium 2
• Spatial substrate: Regions of structural or functional heterogeneity that allow small clusters of triggered cells to overcome source-sink mismatch 6
• Adequate trigger strength: The second "short" beat must generate sufficient current to depolarize surrounding tissue

The mechanism differs fundamentally from reentrant VT, which requires both a trigger and a structural substrate with an excitable gap 2. In contrast, SLS-triggered arrhythmias depend on abnormal impulse formation rather than abnormal impulse propagation.