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Left Bundle Branch Block (LBBB)

The Importance and the Treatment of LBBB


Updated May 16, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

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 heart in the normal way.

A separate article gives an overview of bundle branch block (BBB), shows what it is and how it occurs, and illustrates how it affects the ECG. In this present article, we will concentrate on the clinical significance of LBBB; that is, on what it means if you have LBBB.

LBBB can be significant for two reasons. First, the presence of LBBB most often indicates the presence of underlying heart disease. 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. Less than one half of one percent of people under the age of 50, have it, while nearly 6% of 80-year-olds have LBBB.

LBBB is often linked to underlying heart disease. In one important study known as the Framingham study, subjects who developed LBBB as the study progressed were older (average age 62), and had a significantly increased incidence of hypertension, an enlarged heart, 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 CAD, heart failure, or some other form of cardiovascular disease.

In contrast, when younger people (those under the age of 50) develop LBBB, their odds of having heart disease, while still elevated, are substantially lower than in older people such as those in the Framingham study.

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 which are found in the setting of LBBB include hypertension, CAD, heart failure, hypertrophic cardiomyopathy, or valvular heart disease. But almost any cardiac problem can be associated with LBBB.

If no heart disease is found after such a cardiac evalation in a young person with LBBB, the prognosis is excellent. Generally in these cases the LBBB can be written off as an incidental finding of no medical significance.

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. Specifically, the left ventricle in a person with LBBB is stimulated to contract only after the right ventricle is stimulated. This loss of normal coordination between the two ventricles decreases the efficiency of the heart beat, so that the heart has to do more work 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 patients with 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 these patients. CRT is a type of pacemaker that re-coordinates the contraction of the ventricles and improves cardiac efficiency.

LBBB and Chronic Pacemaker Therapy

The typical permanent pacemaker paces the heart from a pacing lead that is placed 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, accumulating evidence suggests 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. These reports have led several experts to recommend CRT pacemakers (which avoid pacemaker-induced LBBB) in people with reduced ejection fractions who are completely dependent on permanent pacemakers. But the potential benefits of CRT in these patients needs to be weighed against the added complexity of using CRT pacemakers, and as yet the experts have not reached a consensus on this issue.

Other Times LBBB May Require A Pacemaker

Simply having a bundle branch block, of any type, does not require a person to receive a permanent pacemaker. However, there are a few clinicial situations which, if they occur in a person who also has a bundle branch block, may require a pacemaker to be implanted. You can read about these uncommon clinical situations here.


Schneider JF, Thomas HE Jr, Kreger BE, et al. Newly acquired left bundle-branch block: the Framingham study. Ann Intern Med 1979; 90:303.

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.

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