Left bundle branch block and right bundle branch block represent distinct patterns of delayed ventricular depolarization visible on the surface electrocardiogram, and differentiating lbbb ecg vs rbbb is essential for accurate clinical interpretation. While both conditions disrupt the normal sequence of electrical activation, they arise from different anatomical sites of conduction delay and carry unique implications for underlying heart disease. Recognizing the specific ECG morphology, associated findings, and clinical context allows clinicians to refine diagnosis, risk stratify patients, and guide appropriate management strategies.
Understanding Bundle Branch Block Physiology
Before comparing lbbb ecg vs rbbb, it is helpful to review the basics of ventricular conduction. The His-Purkinje system rapidly distributes electrical impulses from the atrioventricular node to the ventricles, ensuring near-synchronous contraction. A bundle branch block occurs when there is a significant delay or complete block along either the left or right bundle branch, slowing depolarization of the affected ventricle. This delayed activation alters the direction and magnitude of electrical vectors recorded on the ECG, producing the characteristic wide QRS complexes and specific lead patterns that define each block type.
Key ECG Features of Left Bundle Branch Block
In lbbb ecg, the impulse reaches the right ventricle normally but must travel slowly through the interventricular septum to reach the left ventricle, producing a sequence-specific pattern. The QRS duration is typically widened to 120 milliseconds or greater, with a broad monophasic R wave in lateral leads I, aVL, V5, and V6. Leads V1 and V2 often show a deep, wide S wave, while the frontal plane QRS axis may be normal, left deviated, or extremely variable. Additional supportive criteria include the presence of notched or slurred R waves in lateral leads and the absence of q waves in the lateral territory, reflecting altered activation.
Key ECG Features of Right Bundle Branch Block
By contrast, rbbb presents with a characteristic pattern where the initial septal depolarization proceeds normally, but the left ventricle is activated late via the slow transseptal pathway. The classic rbbb ECG shows a wide QRS complex with a distinct rSR' or M-shaped morphology in leads V1 and V2, where the initial small r wave reflects right ventricular activation followed by the delayed leftward septal pull. In lateral leads, a wide, slurred S wave is evident, and the QRS duration again exceeds 120 milliseconds. The frontal plane axis is usually normal or rightward deviated, further supporting the diagnosis when correlated with the precordial findings.
Differentiating lbbb ecg vs rbbb on the ECG
When comparing lbbb ecg vs rbbb side by side, the lead-specific morphology provides the most reliable discriminators. In lbbb, the dominant deflection in left-sided leads is upward, with deep S waves in V1, whereas in rbbb, the initial deflection in V1 is upward, producing the rS pattern. The width of the R wave in V6 often exceeds the depth of the S wave in V1 in lbbb, while rbbb typically demonstrates a clear second upward deflection (R') in the right precordial leads. Axis deviation is more common in rbbb, whereas lbbb tends to show left axis deviation or extreme axis deviation when present.
Clinical Associations and Underlying Causes
Both lbbb and rbbb can be idiopathic or related to underlying structural heart disease. In lbbb ecg, associations include ischemic heart disease, hypertension, aortic valve disease, and cardiomyopathies, with new-onset complete left bundle branch block potentially indicating acute myocardial ischemia or infarction. In rbbb, causes are often related to conditions that stretch the right ventricle or disrupt conduction along the right bundle branch, such as pulmonary embolism, chronic lung disease, congenital heart disease, or right ventricular infarction. Evaluating the clinical context, including symptoms, risk factors, and imaging findings, is crucial to determine the significance of each block.
