Left bundle branch block (LBBB) and right bundle branch block (RBBB) represent the most common intraventricular conduction delays encountered in clinical cardiology, each altering the electrical activation sequence of the heart in distinct ways. Interpreting these patterns on the surface electrocardiogram (ECG) requires a systematic approach that goes beyond simple recognition, demanding an understanding of the underlying pathophysiology, axis deviation, and associated morphological nuances. This analysis provides a detailed comparison of LBBB versus RBBB on ECG, focusing on diagnostic criteria, key differentiating features, and clinical implications.
Core Pathophysiology and Activation Sequence
The fundamental difference between LBBB and RBBB stems from where the conduction delay occurs within the His-Purkinje system. In LBBB, the block is within the left bundle branch, delaying activation of the left ventricle which now receives impulses via cell-to-cell spread from the right ventricle, resulting in a secondary leftward axis deviation. Conversely, RBBB involves a delay in the right bundle branch, forcing the right ventricle to wait for activation signals traveling slowly through the myocardium from the left side, a sequence that classically produces a rightward axis deviation. Recognizing this underlying mechanism is essential for predicting the ECG morphology in any given patient.
Diagnostic Criteria for Left Bundle Branch Block
The diagnosis of LBBB relies on specific criteria that reflect the delayed and abnormal sequence of left ventricular activation. The presence of a wide QRS complex greater than 120 milliseconds is a prerequisite, indicating the prolonged time required for the impulse to spread through the myocardium. The QRS complex in leads V1 and V2 should be characteristically wide, monophasic, and predominantly negative, often described as QS or rS morphology, signifying initial septal depolarization moving away from the lead. Furthermore, the lateral leads (I, aVL, V5, and V6) must demonstrate a wide monophasic positive R wave, indicating that the terminal activation of the left ventricle is occurring via transmyocardial spread rather than the normal rapid conduction system.
Diagnostic Criteria for Right Bundle Branch Block
Identifying RBBB is equally methodical, resting on a triad of specific ECG findings. The first requirement is a QRS duration exceeding 120 milliseconds, consistent with any intraventricular conduction delay. The second hallmark is the presence of a wide, slurred S wave in leads I and V6, a feature that reflects the delayed activation of the left ventricle. The third critical component is the characteristic "rsR'" pattern, often termed the "rabbit ear," observed in leads V1 and V2, where the initial r wave is followed by a deep S wave and then a tall, wide secondary R' wave representing the late activation of the right ventricle.
Key Morphological Comparison in Lead V1
Lead V1 provides the most direct visualization of the interventricular septum and serves as a primary location for differentiating these two blocks. In LBBB, the initial deflection is usually negative, as the wave of depolarization moves away from the electrode, creating an initial Q wave or QS complex. The terminal component is also negative but wider, representing the delayed activation of the lateral and posterior left ventricle. In contrast, RBBB in V1 presents with an initial positive r wave, corresponding to the rightward depolarization of the interventricular septum, followed by the characteristic deep S wave and the prominent R' peak that defines the block.
Axis Deviation and Associated Findings
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