Bundle Branch Blocks

Definition and Physiological Explanation

Complete bundle branch blocks are characterized by QRS duration >120 ms and represent interruption of conduction through specific pathways of the cardiac conduction system 1
RBBB represents a block in the right bundle branch, causing delayed activation of the right ventricle, while LBBB represents a block in the left bundle branch, causing delayed activation of the left ventricle 2, 3

Alternating bundle branch block refers to RBBB and LBBB appearing on successive ECGs at different times, indicating severe conduction system disease affecting both bundle branches, and is associated with rapid progression to complete heart block 4
Bifascicular block, a combination of RBBB with either left anterior hemiblock or left posterior hemiblock, represents conduction disease in two of the three main fascicles and carries increased risk of progression to complete AV block 1, 4

Bundle branch blocks may develop from primary degenerative lesions of specialized conducting tissue, ischemic or hypertensive heart disease, cardiomyopathies, myocarditis, channelopathies, cardiac tumors, sarcoidosis, Chagas' disease, or congenital heart disease 1, 2, 3, 5
Patients with complex conduction abnormalities require comprehensive cardiac evaluation, consideration for pacemaker therapy in specific scenarios, and assessment for underlying structural heart disease 1, 2, 4
In athletes, complete bundle branch blocks are uncommon (<2%) and warrant thorough investigation 1
In patients with suspected acute myocardial infarction, both RBBB and LBBB can obscure ST-segment analysis 5, 6

Combinations of bundle branch block and hemiblock reflect extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 1
ECG interpretation in patients with complex conduction abnormalities requires careful analysis and correlation with clinical findings 1, 4