Left Bundle Branch Block Overview

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. Therefore, anyone diagnosed with left bundle branch block on their ECG should have a cardiac evaluation. On the other hand, right bundle branch block does not imply an underlying heart condition.

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.


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 identified, presumably due to degeneration of the electrical conducting system. However, it is often associated with a variety of heart problems.

One of the primary causes 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.dilated cardiomyopathy to acute heart attacks and is more common in those with hypertension.

It can be both a cause of dilated cardiomyopathy or a result of it.

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:

  • 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. 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 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, 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.


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. Patients with a dilated cardiomyopathy and low ejection fraction who have significant heart failure with a LBBB and a QRS duration >130 ms.

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.

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.

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.

Arrhythmias Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Old Man
16 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.
  1. Cedars Sinai. Left bundle branch block.

  2. Surkova E, Badano LP, Bellu R, et al. Left bundle branch block: From cardiac mechanics to clinical and diagnostic challenges. Europace. 2017;19(8):1251-1271. doi:10.1093/europace/eux061

  3. Tracy C, Epstein A, Darbar D, et al. 2012 ACCF/AHA/HRS focused update of the 2008 guidelines for device-based therapy of cardiac rhythm abnormalities. J Am Coll Cardiol. 2012;60(14):1297-1313. doi:10.1016/j.jacc.2012.07.009

  4. Baldasseroni S, Opasich C, Gorini M, et al. Left bundle-branch block is associated with increased 1-year sudden and total mortality rate in 5517 outpatients with congestive heart failure: A report from the Italian network on congestive heart failureAmerican Heart Journal. 2002;143(3):398-405. doi:10.1067/mhj.2002.121264

  5. Johns Hopkins Medicine. Dilated cardiomyopathy.

  6. Centers for Disease Control and Prevention. Cardiomyopathy.

  7. Mitchell LB. Bundle branch block. Merck Manual Consumer Version.

  8. Macfarlane PW. New ECG criteria for acute myocardial infarction in patients with left bundle branch blockJAHA. 2020;9(14). doi:10.1161/JAHA.120.017119

  9. Cedars Sinai. Bundle branch block.

  10. Bhardwaj R. Etiology and left ventricular functions in left bundle branch block - a prospective observational study. J Assoc Physicians India. 2016;64(9):36-38.

  11. Schneider JF, Thomas HE Jr, Kreger BE, et al. Newly acquired left bundle-branch block: The Framingham study. Ann Intern Med. 1979;90(3):303-310. doi:10.7326/0003-4819-90-3-303

  12. Linde C, Braunschweig F. Cardiomyopathy and Left Bundle Branch Block: A Farewell to Drugs? J Am Coll Cardiol. 2018 Jan 23;71(3):318-320. doi:10.1016/j.jacc.2017.11.039

  13. Glikson M, Nielsen JC, Kronborg MB, et al. ESC Scientific Document Group, 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: Developed by the Task Force on cardiac pacing and cardiac resynchronization therapy of the European Society of Cardiology (ESC) With the special contribution of the European Heart Rhythm Association (EHRA)European Heart Journal, Volume 42, Issue 35, 14 September 2021, Pages 3427–3520, doi:10.1093/eurheartj/ehab364

  14. Johns Hopkins Medicine. Pacemaker insertion.

  15. American Heart Association. Conduction disorders.

  16. Supariwala AA, Po JR, Mohareb S, et al. Prevalence and long-term prognosis of patients with complete bundle branch block (right or left bundle branch) with normal left ventricular ejection fraction referred for stress echocardiography. Echocardiography. 2015;32(3):483–9. doi:10.1111/echo.12680

Additional Reading

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.