What Is a Left Bundle Branch Block?

A Disruption to Your Heartbeat That’s Usually Not Serious

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Left bundle branch block (LBBB) is a problem where the electrical impulse that controls your heartbeat is not working in the usual way. It is considered an abnormal finding on an electrocardiogram (ECG).

LBBB doesn't always indicate a problem with your heart, especially if you have no underlying health conditions. However, it can sometimes be an indication of a cardiac problem, so anyone diagnosed with left bundle branch block on their ECG should have a cardiac evaluation.

This article explains the symptoms, causes, diagnosis, and treatment of LBBB.

what is left bundle branch block

Verywell / Laura Porter

Bundle Branch Function

The bundle branches are part of the heart's electrical "wiring." They are the pathways that spread the heart's electrical impulse evenly through the ventricles of the heart. This ensures that the ventricles contract (beat) in a coordinated way.

With left bundle branch block, the bundle branch that distributes the electrical impulse to the left ventricle is wholly or partially blocked. This blockage delays the contraction of the left ventricle. As a consequence, the right ventricle contracts before the left one.

For the heart to beat efficiently, both ventricles should contract simultaneously. As such, left bundle branch block can reduce the efficiency of the heartbeat.

In someone whose heart is otherwise healthy, this reduced function may be trivial. However, in someone with heart failure, left bundle branch block can produce a significant decrease in the heart's efficiency.

This reduced efficiency can accelerate the deterioration of heart failure and make symptoms significantly worse.


Most people with LBBB do not experience symptoms directly related to LBBB. However, if you have other heart conditions in addition to LBBB, you are more likely to experience symptoms. Symptoms may include:

  • Shortness of breath
  • Fatigue
  • Syncope (fainting)


LBBB may develop without any specific cause. However, it is often associated with a variety of heart problems.

One of the primary causes of LBBB is dilated cardiomyopathy. This is a heart disease where a weakened heart can't contract normally. As the heart muscle tries to compensate for this weakness, it stretches and becomes enlarged. LBBB can be both a cause of dilated cardiomyopathy and a result of it.

Most of the time, healthcare providers can't identify a cause for dilated cardiomyopathy. However, certain things can increase the risk of developing the disease, including:

  • Genetics
  • Heart valve abnormalities
  • Infections that can lead to heart damage (Lyme disease and Chagas disease)
  • Ischemia (lack of oxygen to living tissue)
  • Hypertension (high blood pressure)
  • Some medications (anthracyclines)
  • Alcohol, cocaine
  • HIV, diabetes, thyroid problems
  • Peripartum cardiomyopathy (pregnancy-related heart failure)
  • Infiltrative cardiomyopathies (a group of heart diseases where abnormal substances in the heart cause it to malfunction)

LBBB mainly affects older adults. It occurs in less than 1% of people under 50; in contrast, nearly 6% of 80 year olds have left bundle branch block.

Significance of LBBB

Left bundle branch block most often occurs as a result of some underlying heart problem.

The LBBB itself causes the heart to work a bit less efficiently, which makes a big difference in people with certain heart disease types.


Left bundle branch block produces characteristic changes on an ECG. Healthcare providers are typically able to diagnose this condition by examining these test results.

The standard ECG recording shows 12 different views of the heart's electrical activity. Ten electrodes (or "leads") that are attached to the body transmit these images.

QRS Complex

The portion of the ECG called the QRS complex represents the electrical impulse distributed across the ventricles.

Typically, because both ventricles are stimulated simultaneously, the QRS complex is between 0.08 and 0.1 seconds in duration. However, the QRS complex is much wider with left bundle branch block, often greater than 0.12 seconds.

Differential Diagnoses

Most people with left bundle branch block have some form of underlying heart disease. This means anyone of any age who has LBBB should have a cardiac evaluation to look for underlying heart disease.

Frequent underlying conditions include:

In one study, 47.7% of people with left bundle branch block also had high blood pressure.

During an early landmark study in cardiology, 89% of the people who developed left bundle branch block were subsequently diagnosed with some form of significant cardiovascular disease.

If you have CAD risk factors, your healthcare provider may order additional tests, including:

If, after a thorough cardiac evaluation, no evidence of heart disease is found, especially if you are under 50, your prognosis is quite good. In these cases, the left bundle branch block is considered a benign (harmless), incidental ECG finding.

About one-third of people with heart failure also have LBBB.


For people without underlying heart disease, treatment may not be necessary. However, cardiac resynchronization therapy (CRT) or a pacemaker may be options for those with an underlying condition.


CRT is a type of pacemaker that re-coordinates the contraction of the ventricles. It can improve the heart's efficiency in people with LBBB and heart failure.

Unless there is a reason to insert a CRT pacemaker to re-coordinate the function of the ventricles, most people with LBBB never require a pacemaker.

Chronic Pacemaker Therapy

The typical permanent pacemaker paces the heart from a pacing lead located in the right ventricle. Since the electrical impulse from the pacemaker stimulates the right ventricle before the left ventricle, people with permanent pacemakers, in effect, have a pacemaker-induced left bundle branch block.

In recent years, some evidence has suggested that people with permanent pacemakers who have reduced left ventricular ejection fraction—when the heart is less efficient at pumping blood—may have an increased risk of developing heart failure due to the pacemaker-induced left bundle branch block.

For this reason, some experts now routinely use CRT pacemakers (which avoid pacemaker-induced left bundle branch block) in people with reduced ejection fractions who are entirely dependent on permanent pacemakers.

If you have an underlying heart condition, the heart's electrical signal can become disrupted in several ways. When that happens, significant bradycardia (slow heart rate) may develop. If it does, you might require a permanent pacemaker. For this reason alone, people with LBBB should make sure they have regular medical checkups.

Managing LBBB

If you have LBBB, make sure you see your healthcare provider regularly, even if you don't have any symptoms. Your healthcare provider will want to do regular evaluations of your heart to make sure it remains healthy.

Is LBBB Reversible?

LBBB doesn't resolve on its own. In some cases, treatment can help correct the problem, but many people continue to live with LBBB long term.

Preventing LBBB

Left bundle branch block can't always be prevented. However, you may be able to reduce your chances of developing left bundle branch block by building healthier habits. For example:

  • Eat heart-healthy foods like fruits and vegetables, whole grains, and fish.
  • Exercise at least 30 minutes a day five times each week.
  • Monitor your blood pressure and reduce high blood pressure.
  • Lose weight if you need to.
  • If you smoke, quit.
  • Keep your cholesterol levels under control.


LBBB is a condition in which cardiac electrical impulses do not distribute in a balanced way. Often, but not always, the presence of LBBB indicates an underlying heart condition.

Healthcare providers diagnose LBBB during an ECG. If no underlying heart condition is present, you may not need treatment. However, a CRT pacemaker or permanent pacemaker may be necessary when a heart condition is present, especially heart failure.

Frequently Asked Questions

  • What is a left bundle branch block on ECG?

    A left bundle branch block (LBBB) is an abnormal pattern seen on an electrocardiogram (ECG). If LBBB is identified, cardiac electrical impulses are not following a normal distribution pattern across the ventricles. This can be a sign of underlying heart disease. 

  • What is the life expectancy with left bundle branch block?

    A left bundle branch block can indicate a severe heart condition, or it can be benign. On its own, LBBB does not seem to shorten your lifespan. When combined with other risk factors, LBBB is associated with an increased risk of death.

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14 Sources
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Additional Reading
Richard N. Fogoros, MD

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.