Left Bundle Branch Block (LBBB)

The Importance and the Treatment of LBBB

left bundle branch block
Left Bundle Branch Block. R Fogoros

Left bundle branch block (LBBB) is an abnormal pattern seen on the electrocardiogram (ECG), which indicates that the cardiac electrical impulse is not being distributed across the ventricles of the heart in the normal way.

What Is Bundle Branch Block?

The bundle branches can be thought of as part of the heart’s “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 heart is coordinated. There are two main bundle branches. The right bundle branch distributes the electrical impulse across the right ventricle. The left bundle branch distributes the impulse across the left ventricle.

With bundle branch block, one or both of the main bundle branches are partially or completely blocked. This block causes the right ventricle to become stimulated before the left ventricle, which can affect the efficiency of the heartbeat. It also produces characteristic changes on the ECG, as illustrated in the figure.

The figure shows what LBBB looks like on the ECG. The top panel shows a normal QRS complex, and the bottom panel shows LBBB. Note that with left bundle branch block the QRS complex is much wider than normal. In the ECG lead that is depicted (which happens to be Lead I), the QRS complex also displays the upright configuration that is characteristic of LBBB.


Right bundle branch block (RBBB) is discussed in a separate article.

Left Bundle Branch Block

In distinction from RBBB (which often turns out to have little medical significance), LBBB can be a problem for two reasons. First, the presence of LBBB most often indicates that a significant underlying heart problem is present.

And second, the LBBB itself can cause the heart to work less efficiently — especially in people who have certain types of heart disease.

LBBB And Underlying Heart Disease

LBBB mainly affects older adults. LBBB is seen in fewer than 1% of people under the age of 50, while nearly 6% of 80-year-olds have LBBB. Most people with LBBB have some form of underlying heart disease.

In the Framingham study, subjects who developed LBBB had an average age of 62 and had a significantly increased incidence of hypertensiondilated cardiomyopathy, or coronary artery disease (CAD). In fact, during the course of the the Framingham study, 89% of the people who developed LBBB were subsequently diagnosed with some form of significant cardiovascular disease.

Anyone, of any age, who is found to have LBBB should have a cardiac evaluation to look for underlying heart disease. That evaluation should include at least an echocardiogram, and if risk factors are present for CAD a stress/thallium study should also be strongly considered.

The most common cardiovascular disorders found in the setting of LBBB include hypertension, CAD, heart failurehypertrophic cardiomyopathy, or valvular heart disease.

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

LBBB and the Efficiency of the Heartbeat

In LBBB, 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, a loss of normal coordination between the two ventricles occurs, which decreases the efficiency of the heart beat. The heart has to work harder to achieve its normal pumping capacity.

In young, healthy people with LBBB, and even in older people with LBBB who may have mild heart disease, the drop-off in cardiac efficiency appears to be pretty trivial, and current evidence indicates that LBBB 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%, LBBB 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.

”LBBB” Caused By Chronic Pacemaker Therapy

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 LBBB.

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

When Does LBBB Require A Pacemaker?

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 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 may eventually result to the extent that a permanent pacemaker might eventually be required. For this reason alone, people with LBBB should make sure they have regular medical checkups.


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.

Curtis AB, Worley SJ, Adamson PB, et al. Biventricular Pacing for Atrioventricular Block and Systolic Dysfunction. N Engl J Med 2013; 368:1585.

Imanishi R, Seto S, Ichimaru S, et al. Prognostic Significance of Incident Complete Left Bundle Branch Block Observed Over a 40-year Period. Am J Cardiol 2006; 98:644.

Schneider JF, Thomas HE Jr, Kreger BE, et al. Newly Acquired Left Bundle-Branch Block: the Framingham study. Ann Intern Med 1979; 90:303.

Continue Reading