Ventricular Depolarization
Ventricular depolarization is the electrical activation of the ventricular myocardium that triggers ventricular contraction, represented on the ECG as the QRS complex. 1
Electrophysiological Basis
Ventricular depolarization occurs when ventricular myocardial cells undergo rapid electrical activation, changing their transmembrane potential from the resting state (approximately -85 mV) to a depolarized state. This electrical wave of activation spreads through the ventricles in a coordinated sequence, typically proceeding from endocardium to epicardium 1.
Key Characteristics:
- Direction of spread: The depolarization wave generally travels from the endocardial surface toward the epicardial surface of the ventricles
- Voltage gradients: During depolarization, significant voltage gradients develop between activated and non-activated regions of the ventricular myocardium
- ECG manifestation: These voltage gradients generate the QRS complex on the surface electrocardiogram 2
Clinical Significance
The QRS complex duration and morphology provide critical information about the integrity and efficiency of ventricular electrical activation. Normal ventricular depolarization produces a narrow QRS complex, while abnormalities indicate disrupted conduction pathways or underlying cardiac pathology 2.
Relationship to Repolarization:
Ventricular depolarization is distinct from but intimately related to ventricular repolarization. Notably, repolarization proceeds in the opposite direction—from epicardium to endocardium—which is opposite to the direction of depolarization 1. This inverse relationship between activation time and action potential duration helps explain normal T-wave morphology.
Abnormalities and Their Implications:
Depolarization abnormalities manifest as changes in QRS morphology, duration, or both, and may indicate:
- Bundle branch blocks
- Ventricular hypertrophy
- Myocardial scarring
- Conduction system disease
- Preexcitation syndromes 2
A VPD (ventricular premature depolarization) QRS duration ≥153 ms predicts increased risk for developing cardiomyopathy (82% sensitivity, 75% specificity), making QRS duration assessment clinically actionable for risk stratification 3.