Electrophysiology of PVCs and R-on-T Phenomenon
PVCs arise from ectopic foci within the ventricles through three primary mechanisms: triggered activity (delayed afterdepolarizations), abnormal automaticity, and reentry, with the most common sites being the right ventricular outflow tract (RVOT), Purkinje system, and less frequently the ventricular myocardium 1, 2.
Cellular Mechanisms of PVCs
The fundamental electrophysiology involves:
- Triggered activity from delayed afterdepolarizations - the most likely mechanism for idiopathic PVCs, particularly those originating from the outflow tracts 2
- Abnormal automaticity - spontaneous depolarization of ventricular myocytes or Purkinje fibers
- Reentry circuits - particularly in the setting of structural heart disease
Ionic Current Alterations
When PVCs become frequent enough to cause cardiomyopathy, significant cellular remodeling occurs 3:
- Reduced outward currents: Decreased transient outward current (Ito) and inward rectifier current (IK1), leading to prolonged action potential duration
- Reduced inward currents: Decreased L-type calcium current (ICaL), impairing excitation-contraction coupling
- Heterogeneous reduction across myocytes creates marked dispersion of action potential configurations and durations - a critical substrate for sudden cardiac death 3
- Impaired calcium-induced calcium release from the sarcoplasmic reticulum due to decreased junctophilin-2 and misalignment of Cav1.2 channels with ryanodine receptors 3
Regional Electrophysiological Properties
The RVOT demonstrates unique characteristics that explain its arrhythmogenic susceptibility 4:
- Shorter action potential duration at 80% repolarization compared to the right ventricular apex
- Adrenergic sensitivity: Isoproterenol shortens action potential duration and increases PVC frequency
- Cholinergic modulation: Acetylcholine partially reverses isoproterenol-induced changes and reduces PVC incidence 4
- Susceptibility to alternans: Both concordant and discordant alternans patterns during rapid pacing
R-on-T Phenomenon (Short-Coupled PVCs)
The R-on-T phenomenon occurs when a PVC falls on the vulnerable period of the preceding T wave, creating a critical substrate for ventricular fibrillation (VF), particularly in idiopathic VF where short-coupled PVCs from the Purkinje system trigger malignant arrhythmias 5.
Critical Features:
- Short coupling interval - the hallmark characteristic distinguishing these dangerous PVCs
- Consistent QRS morphology - typically arising from a single focus in the Purkinje system of either ventricle 5
- Very high recurrence risk: Among 38 patients with idiopathic VF triggered by short-coupled PVCs, 87% had experienced VF episodes in the preceding year 5
Clinical Significance:
The R-on-T pattern represents a serious risk for sudden cardiac death, particularly when occurring:
- Late after myocardial infarction
- In the presence of left ventricular dysfunction
- With frequent, multifocal, or successive patterns 6
Important caveat: While historically emphasized, the R-on-T phenomenon is most clinically relevant in the context of idiopathic VF where short-coupled PVCs consistently trigger polymorphic VT/VF. In this setting, ICD placement is mandatory given the very high recurrence risk, and catheter ablation of the triggering focus is highly effective (82% freedom from VF at median 63-month follow-up, though 18% experienced recurrence) 5.
Management Implications:
For idiopathic VF triggered by short-coupled PVCs 5:
- ICD is indicated to prevent sudden cardiac death (Class I recommendation)
- Catheter ablation is highly effective but requires ICD backup due to substantial recurrence risk
- Subcutaneous ICD may not be optimal due to higher risk of T-wave oversensing in this population
The electrophysiological substrate involves electrical instability where the PVC occurs during the vulnerable repolarization period when ventricular tissue has heterogeneous refractoriness, creating conditions favorable for reentry and degeneration into VF.