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Bundle Branch and Fascicular Block

by L. Brent Mitchell, MD

Bundle branch block is partial or complete interruption of impulse conduction in a bundle branch; fascicular block is similar interruption in a hemifascicle of the bundle. The 2 disorders often coexist. There are usually no symptoms, but presence of either suggests a heart disorder. Diagnosis is by ECG. No specific treatment is indicated.

Conduction blocks can be caused by many heart disorders, including intrinsic degeneration without another associated heart disorder.

Electrical Pathway Through the Heart

Right bundle branch block (RBBB—see Figure: Right bundle branch block.) can occur in apparently normal people. It may also occur with anterior MI, indicating substantial myocardial injury. New appearance of RBBB should prompt a search for underlying cardiac pathology, but often, none is found. Transient RBBB may occur after pulmonary embolism. Although RBBB distorts the QRS complex, it does not significantly interfere with ECG diagnosis of MI.

Right bundle branch block.

Left bundle branch block (LBBB—see Figure: Left bundle branch block.) is associated with a structural heart disorder more often than is RBBB. LBBB usually precludes use of ECG for diagnosis of MI.

Left bundle branch block.

Fascicular block involves the anterior or posterior fascicle of the left bundle branch. Interruption of the left anterior fascicle causes left anterior hemiblock characterized by modest QRS prolongation (<120 msec) and a frontal plane QRS axis more negative than 30° (left axis deviation). Left posterior hemiblock is associated with a frontal plane QRS axis more positive than +120°. The associations between hemiblocks and a structural heart disorder are the same as for LBBB.

Hemiblocks may coexist with other conduction disturbances: RBBB and left anterior or posterior hemiblock (bifascicular block); and left anterior or posterior hemiblock, RBBB, and 1st-degree atrioventricular (AV) block (incorrectly called trifascicular block; 1st-degree block is usually AV nodal in origin).

Trifascicular block refers to RBBB with alternating left anterior and left posterior hemiblock or alternating LBBB and RBBB. Presence of bifascicular or trifascicular block after MI implies extensive cardiac damage. Bifascicular blocks require no direct treatment unless intermittent 2nd- or 3rd-degree AV block is present. True trifascicular blocks require immediate, then permanent pacing.

Nonspecific intraventricular conduction defects are diagnosed when the QRS complex is prolonged (> 120 msec), but the QRS pattern is not typical of LBBB or RBBB. The conduction delay may occur beyond the Purkinje fibers and result from slow cell-to-cell myocyte conduction.

No specific treatment is indicated.

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