What Is a Bundle Branch Block?

Learn More About This Abnormal ECG Pattern

bundle branch block
Bundle Branch Block. R Fogoros
In This Article

Bundle branch block is a heart conduction disorder that occurs due to the disruption of nerves that electrically stimulate the heart muscles, sometimes due to underlying cardiac disease. Left bundle branch block and right bundle branch block don't usually cause noticeable effects, but may lead to fainting. Abnormal heart rhythm caused by bundle branch block can be diagnosed with an electrocardiogram (ECG) or echocardiogram. Treatment depends on the severity and may be unnecessary, or can include implantation of a pacemaker and/or management of underlying heart disease.

The Heart's Electrical System

The heart has four chambers that pump rhythmically by sequentially contracting and relaxing to circulate blood throughout the body and the lungs. Heart muscles are controlled by the cardiac electrical system, which is a branched distribution of nerves embedded in the heart muscle.

The sinus node is a bundle of nerves located in the right atrium. It controls the heart's electrical system by sending electrical signals across the heart's left and right atria, stimulating their contraction. The message also passes through the atrioventricular (AV) node to the ventricles via a band of cardiac nerve fibers called the bundle of His.

The right and left bundle branches distribute the electrical impulse from the bundle of His across the right and left ventricles, causing them to beat. When the bundle branches are functioning normally, the right and left ventricles contract regularly and nearly simultaneously. This is described as normal sinus rhythm.

Types of Bundle Branch Block

If you have a bundle branch block, either the left bundle branch or the right bundle branch (or both) isn't functioning as it should. This leads to a disruption in your heart rhythm as it pumps.

Note that while the term bundle branch “block” is used, the affected bundle branch may or may not actually be fully “blocked.” In many cases, the bundle branch is not completely blocked, but instead is simply conducting the electrical impulse more slowly than normal.

The chief effect of a bundle branch block is that it interferes with the coordinated contraction of the left and right 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.

Left Bundle Branch Block

The left side of the heart receives oxygenated blood into the left atrium from the lungs. This blood flows from the left atrium into the left ventricle, which contracts powerfully, sending oxygenated blood to the body.

A left bundle branch block interferes with the contraction of the left ventricle.

Right Bundle Branch Block

The right side of the heart receives deoxygenated blood from the body's circulation into the right atrium and sends this blood to the right ventricle, and then to the lungs to be replenished with oxygen.

The contraction of the right ventricle is normally slightly less powerful than the contraction of the left ventricle. A right bundle branch block disrupts the contraction of the right ventricle.


In most cases, bundle branch block does not produce symptoms. However, it can cause intermittent effects, including dizziness or syncope (fainting).

You're more likely to experience symptoms of bundle branch block if you also have a cardiovascular disease, such as hypertension (high blood pressure), heart failure, or cardiomyopathy (disease of the heart muscle).

And symptoms are also more common if the bundle branch block is severe or involves both the right and left bundle branches.

Risks of bundle branch block:

  • Overall, bundle branch block slightly increases the risk of health problems, including a decline in heart function.
  • Having a bundle branch block may increase the risk of a bad outcome if you are hospitalized for a heart condition.
  • Sometimes bundle branch block can pose a risk if you are having heart surgery for another cardiac problem, so it's important for your doctors to know if you have this condition.

With a very severe bundle branch block, the electrical impulse may suddenly stop reaching the ventricles altogether, leading to a complete heart block—which can produce syncope or sudden cardiac death. 


There are risk factors that increase the chances of bundle branch block, but the condition can occur without an identifiable cause, especially on the right side. Typically, when there is not an identifiable cause, the health effects of the bundle branch block are less significant and might even be completely negligible.

However, bundle branch block may be caused by underlying heart disease. In fact, left bundle branch block is almost always associated with a cardiac condition of some type.

The most common cardiac conditions associated with bundle branch block include: 

Right bundle branch block can develop due to all of these causes, and it can also occur as a result of lung disease, including:


Identifying a bundle branch block relies on diagnostic testing that includes a 12-lead ECG and an echocardiogram. An ECG is a non-invasive test that uses electrodes placed on the surface of your chest to detect your heart rhythm. And an echocardiogram is a non-invasive ultrasound test that visualizes your heart as it beats, potentially detecting disruptions in rhythm.

In many cases, one test alone will show the pattern consistent with bundle branch block, so you won't necessarily need both tests.

Bundle Branch Block and ECG

An ECG is a visual representation of the electrical impulse as it moves through the heart. A computer-generated wave pattern reflecting your heart rhythm is produced based on electrical impulses detected with metal plates that are placed on the surface of your chest.

The QRS complex is a portion of the ECG that represents 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. Since both ventricles normally 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.

With a bundle branch block, it takes longer for the electrical signal to be completely distributed across both ventricles and the QRS complex widens.

  • Panel B in the figure illustrates the characteristic changes that occur in the QRS complex when a person has a left bundle branch block. The complex widens and is deflected up. This change can make interpretation of an ECG less reliable when it's used to evaluate other conditions.
  • Panel C in the figure illustrates the ECG changes that occur in the QRS complex with a right bundle branch block—a widened QRS wave with a downward deflection.

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.”

Cardiac Evaluation

Depending on your symptoms and risk factors, you might also need more testing to evaluate your heart and/or lungs, although this isn't necessarily part of the evaluation of an uncomplicated bundle branch block without symptoms or risk factors. For example, you may need to have a diagnostic cardiac catheterization to evaluate the blood vessels in your heart or a stress test if you are having angina (chest pain).


Most of the time, bundle branch block does not require treatment. However, you and your doctor will have to discuss your condition and determine whether your symptoms (if you have any) are caused by bundle branch block or another health condition, such as heart disease.

Occasionally, a severe bundle branch block may require the surgical placement of a pacemaker to prevent episodes of complete heart block.

In addition, significant heart failure accompanied by left bundle branch block may be treated with implantation of a cardiac resynchronization therapy (CRT) device. CRT is a specialized pacemaker that re-coordinates the beating of the two ventricles and can improve cardiac efficiency in some situations.

Associated Conditions

Also, if you have another type of heart disease along with your conduction block, the associated heart condition typically needs to be treated.

This might involve approaches like prescription blood thinners for CAD, antihypertensive medication to alleviate high blood pressure, bronchodilators to treat COPD, or surgical repair for a malfunctioning heart valve.

A Word From Verywell

Bundle branch block is not an uncommon incidental finding on the ECG. People who have a bundle branch block usually will have a noninvasive cardiac evaluation to look for underlying heart disease. While sometimes bundle branch block itself causes cardiac issues, in most cases it does not pose a significant problem.

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