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Chronic bifascicular blocks

William H Sauer, MD
Section Editor
N A Mark Estes, III, MD
Deputy Editor
Brian C Downey, MD, FACC


Bifascicular block, a pattern seen on the surface electrocardiogram (ECG), results when normal physiologic activation in the His-Purkinje system is interrupted. The normal sequence of activation is altered dramatically in patients with bifascicular block, with a resultant characteristic appearance on the ECG that varies depending upon the exact fascicles which are blocked. Interruptions in conduction may result in right bundle branch block (RBBB), left anterior fascicular block (LAFB), or left posterior fascicular block (LPFB), with bifascicular block resulting when two of these three are identified from the ECG.

A 2009 American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society (AHA/ACCF/HRS) scientific statement on the standardization and interpretation of the electrocardiogram recommends against using the term bifascicular block (and also trifascicular block) since these patterns do not have unique anatomic and pathologic substrates [1]. However, these terms are still widely entrenched in clinical practice and scientific literature, meriting their discussion here.

The anatomy, clinical manifestations, differential diagnosis, prognostic implications, and treatment of bifascicular block (RBBB with either LAFB or LPFB) will be reviewed here. Though technically a type of bifascicular block, complete LBBB is discussed separately, as are conduction system abnormalities involving only a single fascicle. (See "Left bundle branch block" and "Right bundle branch block" and "Left anterior fascicular block" and "Left posterior fascicular block" and "Left median (middle or septal) fascicular block".)


Bifascicular block – The term bifascicular block most commonly refers to conduction disturbances below the atrioventricular (AV) node in which the right bundle branch and one of the two fascicles (anterior or posterior) of the left bundle branch are involved. Although this definition is used in the 2008 American College of Cardiology/American Heart Association/Heart Rhythm Society (ACC/AHA/HRS) guidelines for device-based therapy of cardiac rhythm abnormalities [2], some authors, including those of the guidelines on the management of syncope published by the European Society of Cardiology (ESC), include left bundle branch block (LBBB) in the definition of bifascicular block since LBBB, as noted, implies block in both fascicles [2,3].

Trifascicular block – The term trifascicular block is most commonly used to describe bifascicular block associated with prolongation of the PR interval (ie, first degree AV block). However, this description, though commonly used in clinical practice, is inaccurate as the conduction delay resulting in the PR interval prolongation does not usually occur in a fascicle, but in the AV node. True trifascicular block would involve block of the right bundle branch and both fascicles of the left bundle branch; this manifests as third degree (complete) heart block and is referred to as such. Sinus rhythm with alternating left/right bundle branch block or right bundle branch block (RBBB) with alternating fascicular blocks on a beat-to-beat basis is a very rare manifestation of trifascicular block, usually heralding complete AV block. (See "Third degree (complete) atrioventricular block".)

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Literature review current through: Dec 2017. | This topic last updated: Feb 03, 2017.
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