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Bundle Branch Block

By L. Brent Mitchell, MD, Professor of Medicine, Department of Cardiac Services, Libin Cardiovascular Institute of Alberta, University of Calgary

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. 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.

The left bundle branch further divides into two branches, called the anterior and posterior fascicles. If conduction is blocked through one of these fascicles, the blockage is called a hemiblock or a fascicular block.

Tracing the Heart’s Electrical Pathway

The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract.

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 atherosclerosis.

Bundle branch block can be detected by electrocardiography (ECG). Each type of block produces a characteristic pattern.

Usually, no treatment is needed.

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