Causes

Normally, electrical impulses within the heart muscle cause it to beat (contract). These impulses travel along a pathway, including two branches called the right and the left bundles. If one or both of these branch bundles are damaged — due to a heart attack, for example —the electrical impulses can become blocked, and your heart will beat abnormally.

The cause for bundle branch blocks can differ depending on whether the left or the right bundle branch is affected. Sometimes, there is no known cause.

Causes can include:

Left bundle branch block

• Heart attack (myocardial infarction)
• High blood pressure (hypertension)
• Infection of the heart muscle by bacteria or virus (myocarditis)
• Thickened, stiffened or weakened heart muscle (cardiomyopathy)

Right bundle branch block

• Blood clot in the lungs (pulmonary embolism)
• Heart attack (myocardial infarction)
• Heart defects that are present at birth (congenital) — such as a hole in the wall separating the upper chambers of the heart (atrial septal defect)
• High blood pressure in the pulmonary arteries (pulmonary hypertension)
• Infection of the heart muscle by bacteria or virus (myocarditis)