What is left bundle branch block?

Left bundle branch block is a problem with the heart’s electrical wiring (conduction) system.

Your heart has 4 chambers. The 2 upper chambers are called atria, and the 2 lower chambers are called ventricles. In a healthy heart, the signal to start your heartbeat begins in the upper right chamber of the heart (right atrium). From there, the signal activates the left atrium and travels to the lower chambers (right and left ventricles) of the heart. As the signal travels along the heart's conduction system, it triggers nearby parts of the heart to contract in a coordinated manner.

Two bundle branches carry the electrical signal through the ventricles to the bottom of the heart and cause the ventricles to beat. These are termed the right bundle and left bundle. In left bundle branch block, there is a problem with the left branch of the electrical conduction system. The electrical signal can’t travel down this path the way it normally would. The signal still gets to the left ventricle, but it is slowed down. That's because the signal has to spread from the right bundle branch through the heart muscle and slowly activate the left ventricle. So the left ventricle contracts a little later than it normally would. This can cause an uncoordinated contraction of the heart. As a result, the heart may eject blood less efficiently. For most people, this is not a big problem. But if you have underlying heart failure, left bundle branch block can make it worse.

In some people, a left bundle branch block is present all the time. In others, it happens off and on, depending on the heart rate. Exercise, for example, might bring it on for some people. Some people can even have an incomplete left bundle branch block. This would be a sign that a person may be developing a left bundle branch block.

Left bundle branch block happens more often in older people. It is rare in healthy young people. It usually happens in people who have some type of underlying heart problem.

What causes left bundle branch block?

Left bundle branch block can result from a number of heart conditions. These include:

• Coronary artery disease

• High blood pressure

• Heart valve disease

• Enlarged or weakened heart muscle (cardiomyopathy)

• Heart infection (myocarditis)

• Heart attack

• Congenital heart defects

• Certain heart rhythm medicines

All of these conditions increase the risk for left bundle branch block. Sometimes, though, left bundle branch block happens on its own even when the heart may be structurally normal. Researchers aren’t sure what causes these cases of left bundle branch block, but they are more often seen in older people.

What are the symptoms of left bundle branch block?

Usually, left bundle branch block by itself does not cause symptoms. You are more likely to have symptoms if you have other problems in addition to your left bundle branch block. In people with heart failure, left bundle branch block can sometimes make those symptoms such as shortness of breath and fatigue worse.

In very rare cases, both the right and left bundles become blocked. This is a form of complete heart block. When this happens, the impulses from the top chambers of the heart do not activate the lower chambers of the heart. This can lead to long episodes in which the heart stops. The treatment for this would be a pacemaker.

How is left bundle branch block diagnosed?

Your healthcare provider can diagnose left bundle branch block with the help of an electrocardiogram (ECG). This test provides information about the heart rhythm. People often first find out they have left bundle branch block when having an ECG done for some other reason.

Your healthcare provider will probably want to check you for other heart conditions associated with left bundle branch block, like high blood pressure and coronary artery disease. This evaluation will include a thorough history and physical exam along with an ECG. It might also include the following tests:

• Echocardiogram to check blood flow in the heart and heart motion

• Cardiac stress testing to look for coronary artery disease or other abnormalities

• Blood work to assess cholesterol levels and other factors

How is left bundle branch block treated?

In young and healthy people, left bundle branch block is rare. This condition seems to have little effect on how long you live if you have no other underlying heart problems. You may not need any treatment at all, especially when you have no other disease affecting your heart. But people without any symptoms should still undergo careful evaluation.

In older people with coronary artery disease, left bundle branch block is associated with greater risk of death. This is especially true for people with heart failure. Left bundle branch block is also linked to a greater risk of death after a heart attack.

Some people may have left bundle branch block for many years without any problems. But a newly diagnosed left bundle branch block may mean there is some underlying heart condition that requires prompt treatment. An aggressive evaluation may be necessary if you have new onset of a left bundle branch block.

Some people with left bundle branch block may need a permanent pacemaker. A pacemaker helps keep the heart beating at the correct rate. This is usually only needed if you are having symptoms or have another conduction problem along with left bundle branch block.

People with heart failure and left bundle branch block may need cardiac resynchronization therapy or CRT. This is a type of pacemaker therapy that helps the ventricles contract at the same time. CRT can increase the amount of blood that the heart ejects and can improve symptoms of shortness of breath and fatigue. It can improve survival.

How do I manage left bundle branch block?

• Monitor your symptoms carefully. Make sure you see your healthcare provider regularly, even if you don’t have any symptoms.

• Make sure all your healthcare providers know about your left bundle branch block.

• Be aware that left bundle branch block may make it harder for a healthcare provider to diagnose a heart attack.

When should I call my healthcare provider?

See a healthcare provider right away if you have chest pain, fatigue, transient loss of consciousness, or shortness of breath. If you have any new symptoms, plan to see your healthcare provider as soon as possible.

Key points about left bundle branch block

• Left bundle branch block affects the heart’s electrical conduction system. When you have left bundle branch block, the left branch of this conducting system is partially or completely blocked. This causes the left ventricle to contract a little later than it should.

• Follow your healthcare provider's instructions carefully. Take all your medicines as prescribed.

• If you have left bundle branch block, see your healthcare provider for monitoring as advised. This is important even if you don’t have any symptoms.

• Keep in mind that you may need treatment for other heart conditions.

• Be aware that some people with left bundle branch block may not require treatment, but others need pacemakers or cardiac resynchronization therapy.

• Tell your healthcare provider if you have severe symptoms or if your symptoms increase.

Next steps

Tips to help you get the most from a visit to your healthcare provider:

• Know the reason for your visit and what you want to happen.

• Before your visit, write down questions you want answered.

• Bring someone with you to help you ask questions and remember what your provider tells you.

• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.

• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are.

• Ask if your condition can be treated in other ways.

• Know why a test or procedure is recommended and what the results could mean.

• Know what to expect if you do not take the medicine or have the test or procedure.

• If you have a follow-up appointment, write down the date, time, and purpose for that visit.

• Know how you can contact your provider if you have questions.