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Bundle branch block is a type of conduction block involving partial or complete interruption of the flow of electrical impulses through the right or left bundle branches.

The bundle of His is a group of fibers that conducts electrical impulses from the atrioventricular node. The bundle of His divides into two bundle branches (see A Look Into the Heart). The left bundle branch conducts impulses to the left ventricle, and the right bundle branch conducts impulses to the right ventricle. Conduction may be blocked in the left or right bundle branch.

Bundle branch block usually causes no symptoms. Right bundle branch block is not serious in itself and may occur in apparently healthy people. However, it may also indicate significant heart damage due to, for example, a previous heart attack. Left bundle branch block tends to be more serious. In older people, it often indicates coronary artery disease due to high blood pressure or atherosclerosis.

Bundle branch block can be detected by electrocardiography (ECG—see Symptoms and Diagnosis of Heart and Blood Vessel Disorders: Electrocardiography). Each type of block produces a characteristic pattern. Usually, no treatment is needed for either type. However, an artificial pacemaker (see Keeping the Beat: Artificial Pacemakers) may be implanted in people who are at high risk of complete heart block (such as people with certain types of second-degree heart block) to maintain the heart rate if complete heart block occurs.

Last full review/revision January 2008 by L. Brent Mitchell, MD