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 , 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 implies block in both fascicles [1,2].
The term trifascicular block is also confusing since involvement of the right bundle branch and both fascicles of the left bundle branch would be manifested as complete heart block. Thus, trifascicular block is most often inaccurately applied to patients with bifascicular block and prolongation of the PR interval.
A 2009 AHA/ACCF/HRS scientific statement on the standardization and interpretation of the electrocardiogram recommends against using the terms bifascicular and trifascicular block since these patterns do not have unique anatomic and pathologic substrates . Nevertheless, these terms are still widely used.
Left bundle branch block and block involving only one fascicle, as with right bundle branch block (RBBB), left anterior fascicular block (LAFB), or left posterior fascicular block (LPFB), are discussed separately. (See "Left bundle branch block" and "Right bundle branch block" and "Left anterior fascicular block" and "Left posterior fascicular block".)
PROGRESSION TO COMPLETE HEART BLOCK
Progression of chronic bifascicular block and bifascicular block with a prolonged PR interval to complete heart block is probably infrequent. One large study followed 554 such patients and noted that 1 percent per year progressed to complete heart block . As best as could be determined, most deaths were due to tachyarrhythmias rather than complete heart block. The Framingham study also suggested a low incidence of progression to advanced AV block .