What is Bifascicular Block on ECG?
Bifascicular block refers to ECG evidence of impaired conduction below the AV node in two of the three fascicles of the heart's electrical conduction system. 1
ECG Patterns That Define Bifascicular Block
The specific ECG patterns that constitute bifascicular block include:
Right bundle branch block (RBBB) with left anterior fascicular block (LAFB) - the most common combination, showing RBBB pattern with left axis deviation 1
Right bundle branch block with left posterior fascicular block (LPFB) - RBBB pattern with right axis deviation 1
Complete left bundle branch block (LBBB) alone - this is considered bifascicular block because the left bundle divides into anterior and posterior fascicles, so complete LBBB represents block in both 1
Related Terminology
Trifascicular block refers to impaired conduction in all three fascicles (right bundle, left anterior fascicle, and left posterior fascicle), typically manifested as bifascicular block plus first-degree AV block, though this term has been used somewhat inconsistently 1, 2
Alternating bundle branch block (also called bilateral bundle branch block) represents clear ECG evidence of block in all three fascicles on successive ECGs - for example, RBBB with LAFB on one ECG and RBBB with LPFB on another, or RBBB alternating with LBBB 1, 3
Clinical Significance
The rate of progression from bifascicular block to complete heart block is generally slow (1-2% per year) in asymptomatic patients 1. However, certain high-risk features warrant closer attention:
Alternating bundle branch block patterns require urgent pacemaker evaluation even without symptoms, as this represents a Class I indication for pacing 3
Intermittent type II second-degree AV block or intermittent complete heart block with bifascicular block warrants pacing even without symptoms 1, 2
Syncope in the presence of bifascicular block may indicate intermittent complete heart block and requires thorough evaluation, though syncope alone without documented high-grade block is not always associated with increased sudden death risk 1
Common Pitfall
Do not confuse asymptomatic bifascicular block with an indication for prophylactic pacing. Fascicular block without AV block or symptoms is specifically classified as Class III (not indicated) for pacing 1, 2. The presence of bifascicular block alone, without progression to higher-grade AV block or symptoms, does not require pacemaker implantation 1.