Normally, electrical impulses within your heart's muscle signal it to beat (contract). These impulses travel along a pathway, including a slender cluster of cardiac fibers of the heart's electrical system. One area of these fibers is called the bundle of His. This bundle divides into two branches — the right and the left bundles — one for each of your heart's two lower chambers (ventricles).

If one or both of these branch bundles become damaged — due to a heart attack, for example — this change can prevent your heart from beating normally. The heart's electrical impulses that make your heart beat may be slowed down or blocked. When this occurs, your heart's ventricles no longer contract in perfect coordination with one another.

The underlying cause for bundle branch blocks may differ depending on whether the left or right bundle branch is affected. Most cases of left bundle branch block are due to some type of heart disease. Some people with right bundle branch block do not have any other obvious heart problems. Specific causes may include:

Left bundle branch block

  • Heart disease
  • Congestive heart failure
  • Thickened, stiffened or weakened heart muscle (cardiomyopathy)
  • 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 (hypertension)
  • Scar tissue that develops after heart surgery
  • A blood clot in the lungs (pulmonary embolism)
Apr. 26, 2012

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