Bundle Branch Blocks Part 1

The electrical conduction system of the heart begins at the sinoatrial node which is located in the upper right atrium. An electrical impulse propagated by the sinoatrial node travels to the left and right atria and inferiorly to the atrioventricular node. From the AV node the impulse continues to bundle of his which divides into the right and left bundle branches. The right bundle branch is composed of single fascicle. The left bundle branch further branches into two fascicles: the left anterior fascicle and the left posterior fascicle.

A block in either the right or left bundle branch produces a delay in the conduction through the affected ventricle.   Normally both ventricles are depolarized simultaneously, however when one of the bundle branches is blocked, then the unaffected ventricle will begin to depolarize first and the affected ventricle will depolarize more slowly.   On the EKG strip, this produces a widened QRS complex, greater than 0.12 sec.  A widened QRS complex of .10-.11 suggests an incomplete bundle branch block or an undetermined intraventricular conduction delay. 

Incomplete right bundle branch block

If there is a bundle branch block present, you will detect it by examining leads I, V1, V5, and V6.   The right chest lead is V1 and is used to detect a right bundle branch block.  In a right bundle branch block, the QRS in V1 will measure > .12 sec and the QRS complex may have an RSR pattern that has an M appearance or a notched appearance.

Right bundle branch block

The left chest leads are I, V5, and V6 and are used to detect a left bundle branch block.  In a left bundle branch block the QRS complex in lead I, V5, or V6 will measure > .12 sec and may also have a notched appearance or produce a monophasic R wave.  

Left bundle branch block