An Overview of Left Bundle Branch Block

Left bundle branch block (LBBB) is an abnormal pattern seen on an electrocardiogram (ECG), which indicates that the cardiac electrical impulse is not being distributed across the ventricles of the heart in the normal way. Left bundle branch block is important because (in contrast to right bundle branch block), left bundle branch block usually indicates that some form of underlying cardiac disease is present. For this reason, anyone who is discovered to have left bundle branch block on their ECG should have a cardiac evaluation.


The bundle branches can be thought of as part of the heart's electrical "wiring." They are the electrical pathways designed to spread the heart's electrical impulse evenly through the ventricles. This assures that the contraction of the two ventricles is coordinated.

With left bundle branch block, the bundle branch that distributes the electrical impulse to the left ventricle is partially or completely blocked. This blockage delays the electrical activation of the left ventricle. As a consequence, the right ventricle is activated, and begins to contract, before the left ventricle is activated.

In order for the heart to beat as efficiently as possible, both ventricles should contract simultaneously. So left bundle branch block can reduce the efficiency of the heartbeat. This reduction in cardiac efficiency may be trivial in someone whose heart is otherwise normal, but it can have a significant impact in people with certain types of heart disease—especially heart failure.


Left bundle branch block produces characteristic changes on the ECG, so doctors are able to diagnose this condition simply by examining an ECG.

The portion of the ECG called the QRS complex represents the electrical impulse being distributed across the ventricles. Normally, because both ventricles are being stimulated at the same time, the QRS complex is quite narrow—normally, between 0.08 and 0.1 seconds in duration. With left bundle branch block, the QRS complex is much wider, often greater than 0.12 seconds.

Furthermore, the standard ECG recording shows 12 different "views" (called "leads") of the heart's electrical activity, and doctors can examine these different leads to get an idea of the location of various heart problems. With left bundle branch block, the wide QRS complex appears upright in certain leads, and downward in others. By examining both the duration of the QRS complex and its pattern on the various leads of the ECG, it is usually quite easy to diagnose left bundle branch block when it is present.

Why a Left Bundle Branch Block Is Significant:

  • Left bundle branch block most often occurs as a result of some underlying heart problem. So when it is found, it is quite likely that some significant underlying cardiac condition is also present.
  • The left bundle branch block itself can cause the heart to work less efficiently in people who have certain types of heart disease.

Underlying Heart Disease

Left bundle branch block mainly affects older adults. It is found in less than 1% of people under the age of 50; in contrast, nearly 6% of 80-year-olds have left bundle branch block.

Most people with left bundle branch block have some form of underlying heart disease. In the Framingham study, subjects who developed left bundle branch block had an average age of 62 and had a significantly increased incidence of hypertensiondilated cardiomyopathy, or coronary artery disease (CAD).

During the course of the Framingham study, 89% of the people who developed left bundle branch block were subsequently diagnosed with some form of significant cardiovascular disease.

What this means is that anyone, of any age, who is found to have left bundle branch block should have a cardiac evaluation to look for underlying heart disease.

People with LBBB should have a further cardio workup, including an echocardiogram, and if risk factors are present for CAD, a stress test with imaging should also be strongly considered. The most common cardiovascular disorders found in the setting of LBBB include hypertension, CAD, heart failure, hypertrophic cardiomyopathy, or valvular heart disease.

If no heart disease is found after a thorough cardiac evaluation in a person with left bundle branch block, especially in people under 50, the prognosis turns out to be quite good. In these cases, the left bundle branch block is best thought of as a benign, incidental ECG finding.

Efficiency of the Heartbeat

In left bundle branch block, the heart's two ventricles are being stimulated by the cardiac electrical impulse in sequence, instead of simultaneously. That is, the left ventricle is stimulated only after the right ventricle is stimulated. Thus, left bundle branch block causes a loss of normal coordination between the two ventricles, which decreases the efficiency of the heartbeat. The heart has to work harder to achieve its normal pumping capacity.

In young, healthy people with left bundle branch block, and even in older people with left bundle branch block who may have mild heart disease, the drop-off in cardiac efficiency appears to be pretty trivial, and current evidence indicates that left bundle branch block itself does not pose a problem in these people.

However, in people who have heart failure and a left ventricular ejection fraction which is reduced to less than 35%, 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.

The use of cardiac resynchronization therapy (CRT) should be strongly considered in people like this. CRT is a type of pacemaker that re-coordinates the contraction of the ventricles, and can substantially improve cardiac efficiency in people with left bundle branch block and heart failure.

Chronic Pacemaker Therapy and Left Bundle Branch Block

The typical permanent pacemaker paces the heart from a pacing lead located in the right ventricle. Because the electrical impulse (which in this case comes from the pacemaker) stimulates the right ventricle prior to 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 who have permanent right ventricular pacemakers that are actually pacing all or most of the time 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 completely dependent on permanent pacemakers.

Does Left Bundle Branch Block Require a Permanent Pacemaker?

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

However, in some cases, the presence of left bundle branch block indicates a more general disorder of the cardiac electrical conducting system. In such people, the heart's electrical signal can become disrupted in several ways, and significant bradycardia (slow heart rate) may eventually develop, and a permanent pacemaker might be required. For this reason alone, people with left bundle branch block should make sure they have regular medical checkups.

A Word From Verywell

Left bundle branch block is a disorder of the heart's electrical conduction system. Anyone diagnosed with left bundle branch block should have a cardiac evaluation to look for underlying heart disease that requires treatment. And in some cases—especially in people who have heart failure in addition to left bundle branch block—the bundle branch block itself may require treatment with a CRT pacemaker.

However, if a complete cardiac evaluation shows no underlying heart disease, the left bundle branch block can usually be considered a benign condition.

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Additional Reading
  • Badheka AO, Singh V, Patel NJ, et al. QRS Duration on Electrocardiography and Cardiovascular Mortality (from the National Health and Nutrition Examination Survey-III). Am J Cardiol 2013; 112:671.
  • Barsheshet A, Goldenberg I, Garty M, et al. Relation Of Bundle Branch Block To Long-Term (Four-Year) Mortality In Hospitalized Patients With Systolic Heart Failure. Am J Cardiol 2011; 107:540.
  • Curtis AB, Worley SJ, Adamson PB, et al. Biventricular Pacing for Atrioventricular Block and Systolic Dysfunction. N Engl J Med 2013; 368:1585.
  • Schneider JF, Thomas HE Jr, Kreger BE, et al. Newly Acquired Left Bundle-Branch Block: the Framingham study. Ann Intern Med 1979; 90:303.