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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2]

Synonyms and Keywords: BBB

For detailed medical information on Left Bundle Branch Block click here.

For detailed medical information on Right Bundle Branch Block click here.

Overview

A bundle branch block refers to a defect of the heart's electrical conduction system below the level of the AV node. Depending upon the level and location of the block/defect in the conducting pathway bundle branch block can be classified into left bundle branch block and right bundle branch block.

Physiology

Normal Anatomy

  • The conduction system of the heart consists of specialized cells designed to conduct electrical impulse faster than the surrounding myocardial cells.
  • Anatomically, the AV node is divided into three regions as follows:
  • The left bundle branch penetrates the membranous portion of the interventricular septum and divides into several smaller branches. Parts of the left bundle branch include a pre-divisional segment, anterior fascicle/hemibundle and posterior fascicle/hemibundle. Rarely a median fascicle is present in some hearts.
    • The anterior fascicle supplies the anterior papillary muscle and the Purkinje network of the antero-lateral surface of the left ventricle.
    • The posterior fascicle supplies the posterior papillary muscle and the Purkinje network of the postero-inferior surface of the left ventricle.
    • Left bundle branch receives its blood supply from left anterior descending artery.
Structure of the heart's conduction system

Normal Physiology

  • The normal cardiac conduction proceeds in a way so as to allow time for the atrium to relax during atrial diastole.
  • The electrical impulse generated in the SA node travels through the internodal pathways towards the AV node.
  • The conduction through the AV node is slowed down as it travels through it. This decrease in velocity of conduction allows time for the atrium to contract ahead of the ventricle so that the blood from the atria can fill up the ventricles through the atrio-ventricular valves.
  • As the impulse flows through the compact AV node, it rapidly conducts through the ventricular myocardial cells. Once the depolarization is complete, the ventricle relaxes during diastole in preparation for the next impulse.


Classification

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Bundle Branch Block
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Left Bundle Branch Block
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Right Bundle Branch Block
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Based on Duration
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Based on Extent of Block
 
 
 
Proximal RBB
 
 
 
 
Intermediate RBB
 
 
 
 
Distal RBB
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
New LBB
 
 
 
Old LBB
 
 
 
LBB of unknown duration
 
 
 
Complete LBB
 
 
 
 
 
 
 
Incomplete LBB
 
 
 

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References

de:Schenkelblock


Cardiology