Left Bundle Branch Block Overview

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Left bundle branch block (LBBB) is an abnormal pattern seen on an electrocardiogram (ECG). More specifically, it indicates that the cardiac electrical impulse is not distributed across the heart's ventricles in the usual way.

LBBB is important because it often indicates that some form of underlying cardiac disease is present. On the other hand, right bundle branch block does not imply an underlying heart condition. Therefore, 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, ensuring the coordinated contraction of the two ventricles.

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 response of the left ventricle. As a consequence, the right ventricle activates and contracts before the left ventricle is activated.

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 people who have heart failure and a left ventricular ejection fraction reduced to less than 50%, left bundle branch block can produce a significant drop-off in cardiac efficiency.

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


LBBB is often asymptomatic. 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)


The primary cause of LBBB is dilated cardiomyopathy, 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.

Most of the time, doctors can not identify a cause for dilated cardiomyopathy. However, certain things can increase the risk of developing the disease, including:

LBBB mainly affects older adults. For example, 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. So when a doctor finds it, it is pretty likely that some significant underlying cardiac condition is also present.
  • 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. So doctors 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 relatively narrow—usually 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.

Doctors can examine the heart's electrical activity to get an idea of the location of various heart problems. For example, with left bundle branch block, the wide QRS complex appears upright in certain leads and downward in others.

Differential Diagnoses

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

The most frequent comorbidities 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 doctor may order additional tests, including:

If, after a thorough cardiac evaluation, your doctor finds no evidence of heart disease, especially if you are under 50, the prognosis is quite good. In these cases, the left bundle branch block is considered a benign, 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.


People who have significant heart failure may be good candidates for cardiac resynchronization therapy (CRT). CRT is a type of pacemaker that re-coordinates the contraction of the ventricles. It can substantially improve cardiac 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. However, in some cases, the presence of LBBB may indicate an underlying heart condition.

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 reduced left ventricular ejection fractions with permanent pacemakers 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 eventually develop. In that case, you might require a permanent pacemaker. For this reason alone, people with LBBB should make sure they have regular medical checkups.


LBBB is a condition where cardiac electrical impulses do not distribute in a balanced way. Often, but not always, the presence of LBBB indicates an underlying heart condition. The most common heart condition that leads to LBBB is dilated cardiomyopathy.

Doctors 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.

A Word From Verywell

If you were diagnosed with LBBB, be sure to have a cardiac evaluation to look for underlying heart disease. Rest assured that not everyone with LBBB has heart disease, but it is important to rule it out.

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. 

  • How serious is a left bundle branch block?

    It depends. A left bundle branch block can signify a serious underlying heart condition, but not always. In people under 50 without other comorbidities, LBBB is typically benign.

    However, further testing is warranted if you have LBBB and other risk factors for cardiovascular disease. Other tests your cardiologist may order include an echocardiogram and a stress test with imaging. 

  • 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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11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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