Electrophysiology of PVCs in Non-Ischemic Hearts
In non-ischemic hearts, PVCs arise primarily through three electrophysiological mechanisms: enhanced or abnormal automaticity in the His-Purkinje system or ventricular myocardium, triggered activity from delayed afterdepolarizations (most common in structurally normal hearts), and reentry around areas of patchy fibrosis in cardiomyopathic substrates 1, 2.
Core Mechanisms
Automaticity
- Normal automaticity occurs when subordinate pacemaker cells in the His-Purkinje system exhibit enhanced phase 4 spontaneous depolarization, reaching threshold prematurely and firing before the next sinus beat 1, 2
- The rate depends on three factors: maximum diastolic potential, slope of phase 4 depolarization, and threshold potential
- Abnormal automaticity arises from partially depolarized membrane potentials (closer to calcium channel activation potential), though this is less common in non-ischemic hearts since it typically requires injury currents from ischemia 1
- Enhanced automaticity can result from increased intracellular calcium content or heightened sensitivity of pacemaker cells 2
Triggered Activity
- Delayed afterdepolarizations are the predominant triggered activity mechanism in non-ischemic hearts 3
- These result from spontaneous calcium release from the sarcoplasmic reticulum, activating a depolarizing sodium-calcium exchange current 2
- Abnormal calcium handling appears crucial—animal models demonstrate this as a key mechanism underlying PVC-induced cardiomyopathy 4
- Early afterdepolarizations (occurring during phase 2-3 of the action potential) are less relevant in structurally normal hearts, being more associated with QT prolongation syndromes 2, 5
Reentry in Non-Ischemic Substrates
- While reentry is the dominant mechanism in ischemic heart disease, it also occurs in non-ischemic cardiomyopathy around patchy fibrosis 1, 2
- The substrate includes areas of myocardial fibrosis and left ventricular hypertrophy—increasingly recognized as important SCD substrates 2
- Both anatomical reentry (around fixed obstacles) and functional reentry (around areas of functional block) can occur 2
Critical Role of Tissue Heterogeneity
A novel concept is the "gerrymandering" effect: structural and functional tissue heterogeneities can electrically isolate small clusters of pathological cells 6. This overcomes the normal "source-sink mismatch" where healthy surrounding myocardium suppresses abnormal signals. Regions of weak electrical coupling or specific cellular arrangements allow abnormal cell clusters to reach the critical mass needed to initiate PVCs 6.
Dynamic Modulation
The electrophysiological substrate is dynamically influenced by 1, 2:
- Cardiac metabolism
- Electrolyte disturbances
- Signaling pathways
- Autonomic nervous system effects
- Ion channel or transporter function changes
- Cell-to-cell coupling alterations
Clinical Pitfalls
Common misconception: Assuming all PVCs in non-ischemic hearts are benign. While the substrate differs from ischemic disease, patchy fibrosis in cardiomyopathy creates reentrant circuits that can sustain ventricular tachycardia and increase SCD risk 2.
Key distinction: The non-ischemic substrate lacks the acute injury currents and ischemia-related abnormal automaticity seen in coronary disease, making triggered activity and enhanced normal automaticity relatively more important mechanisms 1, 3.
Risk factors for progression: QRS duration of the PVC, coupling interval, and male sex predict development of PVC-induced cardiomyopathy, with abnormal calcium handling as the underlying mechanism 4. Shorter coupling intervals prolong QRS duration and worsen hemodynamics, particularly when PVCs originate from the left ventricle 7.