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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and Keywords: BBB

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A bundle branch block refers to a defect of the heart's electrical conduction system below the level of the AV node.


Main article: Electrical conduction system of the heart

The heart's electrical activity normally starts in the sinoatrial node (the heart's natural pacemaker), which is situated on the upper right atrium. From there the impulse travels to the left atrium and the atrioventricular node. From the AV node the electrical impulse travels down the Bundle of His and divides into the right and left bundle branches. The right bundle branch contains one fascicle. The left bundle branch subdivides into two fascicles: the left anterior fascicle and the left posterior fascicle. Ultimately, the fascicles divide into millions of Purkinje fibers which in turn interdigitize with individual cardiac myocytes, allowing for rapid, coordinated, and synchronous depolarization of the ventricles.

When a bundle branch or fascicle becomes injured (due to underlying heart disease, myocardial infarction, or cardiac surgery), it may cease to conduct electrical impulses appropriately. This results in altered pathways for ventricular depolarization. Since the electrical impulse can no longer use the preferred pathway across the bundle branch, it may move instead through muscle fibers in a way that both slows the electrical movement and changes the direction of the impulses. As a result, there is a loss of ventricular synchrony, ventricular depolarization is prolonged, and there may be a corresponding drop in cardiac output. When heart failure is present, a pacemaker may be used to resynchronize the ventricles.



A bundle branch block can be diagnosed when the duration of the QRS complex on the ECG exceeds 120 ms. A right bundle branch block typically causes prolongation of the last part of the QRS complex, and may shift the heart's electrical axis slightly to the right. The ECG will show a terminal R wave in lead V1 and a slurred S wave in lead I. Left bundle branch block widens the entire QRS, and in most cases shifts the heart's electrical axis to the left. The ECG will show a QS or rS complex in lead V1 and a monophasic R wave in lead I. Another normal finding with bundle branch block is appropriate T wave discordance. In other words, the T wave will be deflected opposite the terminal deflection of the QRS complex.


Many people with bundle branch blocks may still be quite active, and may have nothing more remarkable than an abnormal appearance to their ECG. However, when bundle blocks are complex and diffuse in the bundle systems, or associated with additional and significant ventricular muscle damage, they may be a sign of serious underlying heart disease. In more severe cases, a pacemaker may be required to re-establish better heart muscle function.

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