Bifascicular Block

When two of three fascicles are blocked, it is termed bifascicular block. While a left bundle branch block (LBBB) technically involves disruption of two fascicles, this term is typically reserved for the combination of a right bundle branch block (RBBB) with a left anterior fascicular block (LAFB) or a left posterior fascicular block (LPFB). Causes include ischemic heart disease (40-60% cases), hypertension (20-25%), aortic stenosis, anterior myocardial infarction (occurs acutely in 5-7%), primary degenerative disease of the conducting system (Lenegre’s/Lev’s disease), congenital heart disease, and hyperkalemia. Recall that RBBB is not normally associated with axis deviation (unlike LBBB); therefore, right axis deviation in the setting of RBBB is most likely explained by the presence of a LPFB. In the same way that a LAFB causes left axis deviation, a LPFB causes a right axis deviation. Bifascicular blocks are clinically relevant because they can progress to complete heart block. In patients with syncope, one early study suggests a 17% cumulative incidence of complete atrioventricular block within five years.



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  2. McAnulty JH, Rahimtoola SH, Murphy E, et al. Natural history of “high-risk” bundle-branch block: final report of a prospective study. The New England journal of medicine. 1982;307(3):137-143.