Bundle branch block (left and right)

Definition

A bundle branch block is a partial or complete interruption of conduction in a bundle branch, a fascicular block (hemiblock) a corresponding disturbance in a fascicle of the left bundle. Normally both ventricles are activated simultaneously and the QRS duration is at most 0.11 s; in bundle branch block one ventricle is activated late and the QRS complex widens.

Classification

• Right bundle branch block (RBBB) and left bundle branch block (LBBB), each complete or incomplete (same criteria, but QRS duration below 0.12 s)
• Left anterior and left posterior hemiblock
• Bifascicular block: RBBB plus left anterior or left posterior hemiblock
• The combination of bifascicular block and first-degree AV block is often imprecisely called "trifascicular"; true trifascicular block is RBBB with alternating left anterior and left posterior hemiblock, or alternating LBBB and RBBB.

Aetiopathogenesis

Conduction blocks can arise in many heart disorders, but also from isolated degeneration of the conduction system without other heart disease.

• RBBB: also in people without heart disease; furthermore in anterior infarction (indicating extensive injury) and transiently after pulmonary embolism as a sign of significant right ventricular dysfunction
• LBBB: more often associated with structural heart disease than RBBB; also in anterior infarction
• Hemiblocks: same associations with heart disease as LBBB; isolated left posterior hemiblock is rare, left anterior hemiblock more common

Clinical features

Bundle branch blocks themselves usually cause no symptoms; symptoms arise from the underlying disease. LBBB can cause mechanical dyssynchrony of the ventricles, which impairs systolic function and may contribute to heart failure, especially in a previously damaged heart. Bifascicular or trifascicular block after infarction indicates extensive damage.

Diagnosis

ECG criteria for right bundle branch block

• QRS duration of at least 0.12 s
• rsr′, rsR′ or rSR′ pattern in V1 or V2 ("M shape")
• delayed right ventricular activation: time from QRS onset to peak of the last R wave in V1 over 0.05 s
• S wave in I and V6 wider than the R wave or longer than 0.04 s; secondary ST depression and T-wave inversion particularly in V1

ECG criteria for left bundle branch block

• QRS duration of at least 0.12 s
• broad notched or slurred R wave and absent Q waves in I, aVL, V5 and V6
• delayed left ventricular activation: time to peak of the last R wave in V5 and V6 over 0.06 s
• secondary ST-T changes opposite in direction to the main QRS deflection (discordant)

Hemiblocks and further evaluation

• Left anterior hemiblock: QRS slightly prolonged (below 0.12 s), frontal axis more negative than −45° (left axis deviation), qR pattern in aVL
• Left posterior hemiblock: frontal axis more positive than +120°, rS pattern in aVL, qR pattern in II and aVF
• RBBB hardly interferes with the ECG diagnosis of a previous infarction. When a bundle branch block is found, an underlying heart disease is sought, especially in LBBB, e.g. by echocardiography.

Keep learning in the app

Further reading (open access)

1. MSD Manual Professional: Bundle Branch Block and Fascicular Block
2. StatPearls: Left Bundle Branch Block
3. StatPearls: Right Bundle Branch Block

Cross-references