In most people, bundle branch block doesn't cause symptoms. Some people with the condition don't know they have a bundle branch block.

Signs and symptoms in people who have them might include:

• Fainting (syncope)
• Feeling as if you're going to faint (presyncope)

Normally, electrical impulses within your heart's muscle signal it to beat (contract). These impulses travel along a pathway, including the right and the left bundles. If one or both of these branch bundles become damaged — due to a heart attack, for example — this can block the electrical impulses and cause your heart to beat abnormally.

The cause for bundle branch blocks can differ depending on whether the left or right bundle branch is affected. It's also possible that this condition can occur without a known cause.

Causes can include:

Left bundle branch block

• Heart attacks (myocardial infarction)
• Thickened, stiffened or weakened heart muscle (cardiomyopathy)
• A viral or bacterial infection of the heart muscle (myocarditis)
• High blood pressure (hypertension)

Right bundle branch block

• A heart abnormality that's present at birth (congenital) — such as atrial septal defect, a hole in the wall separating the upper chambers of the heart
• A heart attack (myocardial infarction)
• A viral or bacterial infection of the heart muscle (myocarditis)
• High blood pressure in the pulmonary arteries (pulmonary hypertension)
• A blood clot in the lungs (pulmonary embolism)

Risk factors for bundle branch block include:

• Increasing age. Bundle branch block is more common in older adults than in younger people.
• Underlying health problems. Having high blood pressure or heart disease increases your risk of having bundle branch block.

The main complication of bundle branch block, right or left, is to progress to a complete block of the electric conduction from the upper chambers of the heart to the lower. This can slow your heart rate, which can cause fainting and lead to serious complications and abnormal heart rhythms.

People who have a heart attack and develop a left bundle branch block have a higher chance of complications, including sudden cardiac death, than do people who don't develop the condition after a heart attack.

Because bundle branch block affects the electrical activity of your heart, it can sometimes complicate the accurate diagnosis of other heart conditions, especially heart attacks, and lead to delays in proper management of those problems.