What Is a Bundle Branch Block?

Learn More About This Abnormal ECG Pattern

bundle branch block
Bundle Branch Block. R Fogoros

If you have been told by a doctor that you have bundle branch block, it means that your electrocardiogram (ECG) is displaying a distinctive, abnormal pattern. Below, find out more about what this means and what questions to ask your physician. 

The Normal Cardiac Electrical System

The bundle branches are an important part of the cardiac electrical system, the system that regulates heart rhythm and coordinates the pumping action of the heart.

The heart beats in response to the heart’s own electrical signal. As the electrical signal is distributed throughout 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, spreads across both atria (causing the atria to beat), and then passes through the AV node. Leaving the AV node, the electrical impulse penetrates the ventricles via a band of cardiac muscle fibers called 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.

What a Normal ECG Looks Like

An ECG is a visual representation of the electrical impulse as it moves through the heart. A QRS complex is what cardiologists call a certain part of an ECG, the part that shows the electrical impulse as it is being distributed by the bundle branch system throughout the ventricles.

In the figure above, Panel A shows what a normal QRS complex looks like on a normal ECG. (For those who are interested, this figure shows lead I from a 12-lead ECG.) Since, normally, both ventricles receive the electrical impulse at the same time, the normal QRS complex is relatively narrow (generally less than 0.1 second in duration.) 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. 

Bundle Branch Block: Definition

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.

However, this 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 that it disrupts the simultaneous contraction of the two ventricles. The contraction of one ventricle (the one that has a "blocked" bundle branch) occurs slightly after the contraction of the other, rather than at the same time. 

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 completely blocked, but instead is simply conducting the electrical impulse more slowly than the opposite bundle branch.

What an Abnormal ECG Looks Like

People who have bundle branch block usually 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 when a person has left or 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. The shape of the QRS complex reveals which bundle branch (right or left) 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.

Symptoms and Complications

One complication of bundle branch block is a slow heart rate (fewer than 60 beats per minute), and that can cause fainting. People who develop a bundle branch block due to a heart attack have higher odds of complications—some of which are serious, such as sudden cardiac death. If you suspect that you have a slow heart rate (which you can learn through taking your pulse or through data from some wrist-based activity trackers) or if you are prone to fainting, make an appointment with a cardiologist to see whether you might have a heart issue. 

How Bundle Branch Block Is 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.


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.

Continue Reading