What Is a Bundle Branch Block?

What You Should Know About Bundle Branch Block

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If you have been told you have bundle branch block (BBB), it means that your electrocardiogram (ECG) is displaying a distinctive, abnormal pattern. The importance of having a BBB varies quite a bit from person to person, so it is important for you to discuss this ECG finding with your own doctor. In the meantime, this article will help you understand BBB, so that when you talk to your doctor about it, you should be better able to understand the issues, and know which questions to ask

The Normal Cardiac Electrical System

The bundle branches are an important part of the cardiac electrical system, the system that regulates the heart rhythm and coordinates the pumping action of the heart. Here is a quick primer on the normal cardiac electrical system. You will be able to better follow this discussion on BBB if you have a look at this primer.

The normal cardiac electrical system is also depicted in Figure 1 on this page.

Figure 1 - The Normal Electrical System: AVN = AV node, His = His bundle, RBB = right bundle branch, LBB = left bundle branch, RA = right atrium, RV = right ventricle, LA = left atrium, LV = left ventricle

To review, the heart's electrical impulse originates in the in the sinus node in the upper right atrium, then spreads across both atria, then travels through the AV node. Leaving the AV node, the electrical impulse penetrates into the ventricles via the His bundle.

From the His bundle, the electrical impulse enters the two bundle branches (the right and the left). The right and left bundle branches send the electrical impulse to the right and left ventricles, respectively. When the bundle branches are functioning normally, the right and left ventricles contract nearly simultaneously.

Figure 2 shows the normal QRS complex on a normal ECG. The ECG is merely a visual representation of the heart's electrical impulse as it moves through the heart. On the ECG, the QRS complex represents the electrical impulse as it is being distributed, by the bundle branch system, throughout the ventricles. Since normally both ventricles receive the electrical impulse at the same time, the normal QRS complex, as depicted in this figure, is relatively narrow (generally less than 0.1 second in duration.)

Figure 3 shows, in more detail, how the bundle branches normally distribute the heart's electrical impulse to the two ventricles. A normal QRS complex is formed when the electrical impulse reaches both ventricles at the same time. In this figure, purple arrows indicate the electrical impulse as it travels down the bundle branches and causes the ventricles to beat simultaneously.

What Is Bundle Branch Block?

As we have just seen, the bundle branches work to evenly distribute the spread of the cardiac electrical impulse across the ventricles, so that when the ventricles contract (to eject blood out of the heart), they do so in a coordinated and efficient fashion.

The right bundle branch delivers the electrical impulse to the right ventricle, and the left bundle branch delivers the impulse to the left ventricle.

In BBB, one or the other (or both) of these bundle branches no longer conduct electrical impulses normally. This can occur from disease or damage to one of the bundle branches, or it may occur for no apparent reason in completely healthy people. When the electrical impulse is delayed in reaching its respective ventricle, the delay shows up as a distinctive pattern on the ECG called a BBB. The chief effect of a BBB is to disrupt the normal, coordinated and simultaneous contraction of the two ventricles. The contraction of one ventricle (the one whose bundle branch is blocked) occurs slightly after the contraction of the other.

People with BBB usually will have either right bundle branch block (RBBB) or left bundle branch block (LBBB), depending on which of the two bundle branches is "blocked." Sometimes both bundle branches are affected, and the BBB pattern on the ECG is not clearly identifiable as either right or left BBB - in this case, the BBB is referred to as an intraventricular conduction delay (IVCD).

Right bundle branch block (RBBB)

In RBBB, the right bundle branch no longer conducts electricity normally, and the heart's electrical impulse enters the ventricles using only (or mainly) the left bundle branch - which means the left ventricle receives the electrical impulse first. Then, from the left ventricle, the electrical impulse finally makes its way to the right ventricle. As a result, the two ventricles no longer receive the electrical impulse simultaneously.

First the left ventricle receives the electrical impulse, then the right.

Left Bundle Branch Block (LBBB)

Figure 4 depicts left bundle branch block, LBBB. In LBBB, the opposite occurs. Here, the left bundle branch no longer conducts electricity normally. So the electrical impulse enters the ventricles through the right bundle branch, and is carried first to the right ventricle (first panel). From there, it finally spreads to the left ventricle (second panel). Once again, the two ventricles no longer receive the electrical impulse simultaneously. First the right ventricle receives the electrical impulse, then the left.

With either type of BBB, therefore, the electrical signal spreads across the ventricles sequentially, as opposed to simultaneously. This sequential spread of the impulse (i.e., first the right ventricle, then the left, or vice versa) means that it takes longer for the impulse to spread throughout both ventricles.

And this means that, with either type of BBB, the QRS complex on the ECG is wider than normal.

What the ECG Looks Like in BBB

Figure 5 depicts what the ECG looks like when BBB is present.

The top image in this figure shows a normal QRS complex. The middle image shows RBBB, and the bottom image shows LBBB. Notice that with both kinds of BBB, the QRS is wider than normal, and is misshapen. This is because the spread of the electrical impulse across the ventricles in BBB takes longer than normal (therefore widening the QRS complex), and the pattern of that spreading is also abnormal (therefore changing the shape of the QRS complex).

The characteristic shapes of the QRS complex that occur with RBBB and LBBB allow your doctor to determine whether the right or the left bundle branch is blocked.

What Is the Significance of Right Bundle Branch Block?

RBBB is a relatively common ECG finding, and while it occurs in a variety of medical conditions, it also is found in some people whose hearts are completely normal - in which case RBBB usually has no medical significance.

RBBB often occurs in medical conditions that affect the right ventricle or the lungs, so a finding of RBBB on the ECG ought to trigger a screening exam for such conditions. These include pulmonary embolus (blood clots to the lung), chronic obstructive pulmonary disease (COPD), coronary artery disease (CAD), atrial septal defect, ventricular septal defect, and valvular heart disease.

If you are told you have RBBB, however, keep in mind that this ECG pattern also occasionally occurs in normal, healthy people, and your medical evaluation may turn up no medical problems at all.

Fortunately, while the presence of RBBB means that the right ventricle begins to contract slightly after the left ventricle, in normal hearts this delay in right ventricular contraction causes no measurable decrease in cardiac function. So in general isolated RBBB has no apparent medical significance, and usually can be written off as a "normal variant" and safely ignored.

What Is the Significance of Left Bundle Branch Block?

LBBB is somewhat less common than RBBB, and its presence usually indicates underlying cardiac pathology. LBBB is commonly seen with dilated cardiomyopathy, hypertrophic cardiomyopathy, hypertension, aortic valve disease, CAD, and a variety of other cardiac conditions. While occasionally LBBB occurs in apparently healthy people, its appearance should trigger a thorough search (as opposed to a simple screening) for underlying cardiac problems.

If the cardiac evaluation reveals no underlying heart problems, the medical significance of LBBB (especially in young people) is generally regarded as minimal. However, in some people - especially in patients with heart failure and reduced left ventricular ejection fractions - the fact that the contraction of the left ventricle is delayed in LBBB actually does cause a measurable decrease in cardiac efficiency and cardiac function. So for many patients with heart failure and LBBB, doctors should strongly consider using cardiac resynchronization therapy (CRT), which helps to re-coordinate the contraction of the left ventricle and improve cardiac function.

What If Both Bundle Branches Are Blocked?

If both the right and left bundle branches are completely blocked (a condition called "bilateral" BBB), the electrical impulse will not be able to reach the ventricles at all. This is a form of "complete heart block," and unless spontaneous electrical impulses arise within the ventricles (a so-called "ventricular escape rhythm"), the ventricles stop beating and death occurs. Fortunately, this form of complete heart block is uncommon.

More commonly, both bundle branches can be damaged to the extent that they conduct the electrical impulses more slowly than normal, but neither is completely blocked. This kind of problem leads to a widened QRS complex (since activation of the ventricles is slowed), but the pattern on the QRS complex reflects neither classic RBBB, nor classic LBBB. This kind of QRS abnormality is referred to as an "intraventricular conduction delay," or IVCD. IVCD has the same medical significance as LBBB.

The major significance of BBB is that a damaged bundle branch may indicate the presence of previously unknown underlying cardiovascular disease. When BBB is found, therefore, a search for such underlying disease ought to be carried out. Since RBBB more often occurs in normal hearts than does LBBB, the search for underlying disease generally should be more aggressive with LBBB than with RBBB. LBBB is especially important in people who have heart failure, where the resultant dis-coordination of ventricular contraction often worsens cardiac efficiency. In these people, CRT therapy may produce a marked improvement.

When Does Bundle Branch Block Need to Be Treated?

Normal cardiac function depends on the two bundle branches. If they both were to completely stop working (thus producing complete heart block), the electrical impulse could not be delivered to the ventricles, and the heart would stop beating.

Fortunately, it is quite uncommon for stable RBBB or LBBB to progress to complete heart block. So despite the fact that BBB is not a rare finding on routine ECGs, it is uncommon for either kind of BBB to require a pacemaker.

Still, there are three general circumstances in which people with BBB might require pacemakers:

1) When disease in both the right and left bundle branches appears after an acute heart attack. When a patient develops new evidence of disease in both the right and left bundle branches right after a heart attack, there is a greatly increased chance of developing complete heart block within the next few weeks or months. For this reason, pacemakers are most often recommended in these patients.

2) When bundle branch block is associated with syncope (loss of consciousness). When a person with BBB - especially LBBB - experiences syncope, that person may have an increased risk of developing complete heart block. In general, an electrophysiology study should be considered at that point, to test for impending complete heart block. A permanent pacemaker eliminates the problem.

Notably, however, the most common cause of syncope in a person with LBBB and underlying heart disease is not complete heart block, but ventricular tachycardia.

The elecrophysiology study will also test for this possibility - so anyone with LBBB and new-onset syncope ought to be referred to a cardiac electrophysiologist (a heart rhythm specialist) for an evaluation.

3) When BBB is associated with heart failure and a reduced left ventricular ejection fraction. In patients with heart failure and a reduced left ventricular ejection fraction who also have wide QRS complexes indicating BBB, cardiac resynchronization therapy (CRT) should be strongly considered.

CRT "re-coordinates" the contraction of the two ventricles, and can greatly improve the efficiency of the heart muscle. This improvement in efficiency - allowing the heart to do more work, using less effort - can significantly improve the symptoms of heart failure, and has been shown to prolong the survival of some patients with heart failure.


If you have been told you have RBBB or LBBB, your doctor should do an evaluation looking for the kinds of underlying heart or lung diseases that have been associated with BBB. If your heart and lungs are normal and you have RBBB, you can stop worrying.

While LBBB can also be seen in people whose hearts otherwise appear entirely normal, most often it indicates underlying heart disease. So if you have LBBB, you should make especially sure your doctor has done a thorough cardiac evaluation.

Unless you have had a recent heart attack, or have had syncope, or have heart failure, the presence of BBB generally requires no specific treatment - beyond performing that thorough cardiac evaluation.

If you have significant heart failure, BBB can cause your heart to work less efficiently and can produce a worsening of your symptoms. So anyone with heart failure and BBB should discuss with their doctors the potential need for CRT pacing.


Fahy GJ, Pinski SL, Miller DP, et al. Natural history of isolated bundle branch block. Am J Cardiol 1996; 77:1185.

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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