Bundle branch block refers to a disorder of the heart's electrical conducting system.

Normal Bundle Branch Function

The heart's electrical activity normally starts in the sinoatrial node of the upper right atrium (the heart's built-in "pacemaker"), and travels to the atrioventricular node. From the AV node the electrical impulse travels down the Bundle of His, and divides into two branch bundles, one for each ventricle.

The function of the bundles is to speed the electrical impulse and distribute it in a pattern which makes later heart muscle contraction forceful and coordinated. As electrical impulses travel down these bundles, they spread over the associated ventricle to the muscle fibers, stimulating the fibers to contract in a rhythmic manner, squeezing blood out of the ventricles and into the arterial circulation.

Because the left ventricle is larger, the left bundle divides into an anterior left bundle and a posterior left bundle, the former controlling the front wall of the left ventricle, and the latter controlling the back wall of the left ventricle.

Bundle Branch Blocks

When bundles are injured, as in a myocardial infarction, or because of underlying heart disease, a bundle or a branch of a bundle may cease normal function. The result is an altered pathway for electrical heart activity. Since an electrical impulse can no longer use the bundle to travel, 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, the ability of the ventricles to effectively pump blood is impaired, and cardiac output — the amount of blood the ventricles can pump into the arterial circulation — is reduced.

Diagnosis and Treatment

Bundle branch blocks can commonly be diagnosed by the shape and width of the QRS complex on the EKG (ECG). Right bundle branch blocks typically cause some prolongation of the last part of the QRS wave complex, and may cause the heart's electrical axis to be more rightward. Left bundle branch blocks widen the entire QRS, making the EKG look quite abnormal, and may move the heart's electrical axis dramatically leftward.

Many people with bundle branch blocks may still be quite active, and may have nothing more remarkable than an abnormal appearance to their EKG. 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.

Related Topics

• Electrical conduction system of the heart
• Cardiac pacemaker
• Heart blocks
  • First degree heart block
  • Second degree heart block
  • Trifascicular heart block

Reference

• Cecil Textbook of Medicine. W.B. Sanders. 2004. Chapters 50; 58.
• Rakel: Textbook of Family Practice, 6th ed., 2002 W. B. Saunders Company. pp. 699-732.