In the heart we have a normal pacemaker. This is a specialized group of cells in the heart's right upper chamber (right atrium). Somewhere between 60 and 100 times a minute, this pacemaker sends out an electrical impulse. This impulse then travels throughout the heart on a specified route. As the impulse passes through, the heart muscle contracts (beats).

The impulse first travels through the upper chambers (the atria). Before it can go to the lower chambers (the ventricles), it must pass through one small group of cells called the A-V node. The A-V node is located between the atria and the ventricles. After the impulse goes through this A-V node, it goes along a track called the bundle of His (hiss). From there, this bundle divides into a right bundle and a left bundle. These two bundles go to the right and left lower chambers of the heart. All of this is much like following the roads on a freeway map.

Normally, the electrical impulse travels down both the right and left branches at the same speed. Thus, both ventricles contract at the same time. But occasionally, there's a block in one of the branches, and electrical impulses are slowed or blocked as they travel through the specialized conducting tissue in the ventricles (His).

Usually a person with bundle branch block shows no symptoms, but this block shows up on the electrocardiogram (ECG or EKG) as an abnormality. If you have bundle branch block, it may have only been noticed when you had an ECG. You may feel fine, although some people may either faint (syncope) or feel as if they're going to faint (presyncope).

If both bundles are diseased, heart block may result, producing syncope or presyncope.  When this happens, the heartbeat may be so slow that an artificial pacemaker is implanted.

Bundle heart block may be caused by damage to the heart muscle resulting from a heart attack.

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