What Is a Bundle Branch Block?

What You Should Know About Bundle Branch Block

bundle branch block
Bundle Branch Block. R Fogoros

If you have been told you have bundle branch block, it means that your electrocardiogram (ECG) is displaying a distinctive, abnormal pattern. The importance of having a bundle branch block varies quite a bit from person to person, so it is important for you to discuss this ECG finding with your own doctor. In the meantime, this article will help you understand bundle branch block so that when you talk to your doctor about it, you should be better able to understand the issues and know which questions to ask.

The Normal Cardiac Electrical System

The bundle branches are an important part of the cardiac electrical system, the system that regulates the heart rhythm and coordinates the pumping action of the heart. Here is a quick primer on the normal cardiac electrical system. You will be able to better follow this discussion on bundle branch block if you have a look at this primer.

To review, the heart beats in response to the heart’s own electrical signal. As the electrical signal is distributed through the heart, the heart muscle contracts. So the organized, well-timed distribution of the electrical impulse is very important to the efficient functioning of the heart.

The heart’s electrical signal originates in the sinus node in the upper right atrium, then spreads across both atria (causing the atria to beat), and then passes through the AV node. Leaving the AV node, the electrical impulse penetrates into the ventricles via the His bundle.

From the His bundle, the electrical impulse enters the two bundle branches, the right bundle branch and the left bundle branch. The right and left bundle branches distribute the electrical impulse across the right and left ventricles, respectively, causing them to beat. When the bundle branches are functioning normally, the right and left ventricles contract nearly simultaneously.

(Read more about the heart’s chambers.)

Panel a) in the figure shows the normal QRS complex on a normal ECG. (For those who are interested, this figure shows lead I from a 12-lead ECG.) The ECG is merely a visual representation of the heart's electrical impulse as it moves through the heart. On the ECG, the QRS complex represents the electrical impulse as it is being distributed, by the bundle branch system, throughout the ventricles. Since normally both ventricles receive the electrical impulse at the same time, the normal QRS complex, as depicted in this figure, is relatively narrow (generally less than 0.1 second in duration.)

A normal QRS complex, like the one in panel a), is formed when the electrical impulse reaches both ventricles at the same time. The simultaneous stimulation of both ventricles depends on the electrical impulse traveling down both the right and left bundle branches at nearly the same rate of speed. 

What Is Bundle Branch Block?

To summarize, the job of the bundle branches is to distribute the spread of the cardiac electrical impulse across the ventricles evenly, so that when the ventricles contract (to eject blood out of the heart), they do so in a coordinated and efficient fashion.

The right bundle branch delivers the electrical impulse to the right ventricle, and the left bundle branch delivers the impulse to the left ventricle.

In bundle branch block, one or both of the two bundle branches are no longer transmitting the electrical impulses normally. This often occurs as a result of disease or damage to one of the bundle branches, as may happen with a myocardial infarction (heart attack), or with cardiomyopathy. On the other hand, an imbalance in the transmission of the electrical signal can also happen for no apparent reason, in completely healthy people. 

When the electrical impulse is delayed in reaching its respective ventricle, the delay shows up as a distinctive pattern on the ECG called a bundle branch block.

The chief effect of a bundle branch block is to disrupt the normal, coordinated and simultaneous contraction of the two ventricles. The contraction of one ventricle (the one whose bundle branch is “blocked”), occurs slightly after the contraction of the other. (Note that while the term bundle branch “block” is used, the affected bundle branch may or may not actually be “blocked.” In many cases, the bundle branch is not really completely blocked, but instead is simply conducting the electrical impulse more slowly than the opposite bundle branch.)

People with bundle branch block usually will have either right bundle branch block (RBBB), or left bundle branch block (LBBB), depending on which of the two bundle branches is "blocked." Panels b) and c) in the figure illustrate the characteristic changes that occur in the QRS complex with left and right bundle branch block. In both cases, the QRS complex becomes wider than normal since it takes longer for the electrical signal to be completely distributed across both ventricles. Also, the specific “shape” of the QRS complex reveals which bundle branch is affected.

Sometimes both bundle branches are affected, and the bundle branch block pattern on the ECG is not clearly identifiable as either right or left bundle branch block. In this case, the bundle branch block is referred to as an intraventricular conduction delay.

How Is Bundle Branch Block Treated?

Whether or not bundle branch block needs to be treated depends on the type of bundle branch block that is present (right or left), and whether or not the bundle branch block is associated with underlying heart disease.

For details on how bundle branch block should be evaluated and treated, read here about right bundle branch block and left bundle branch block.


Surawicz B, Childers R, Deal BJ, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part III: Intraventricular Conduction Disturbances: a Scientific Statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. Endorsed by the International Society for Computerized Electrocardiology. J Am Coll Cardiol 2009; 53:976.

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