Heart block

Heart block refers to an abnormality in the way electricity passes through the normal electrical pathways of the heart. The abnormality "blocks" the electrical impulse from continuing through the normal pathways and usually results in a slower heart rate.

What causes heart block?

Heart block can be caused by:

Heart block is more common in older people and may be the result of age and a combination of factors listed above. Heart block can occur in people with heart attacks. When heart attacks cause heart block, it often goes away on its own. But if the heart attack is extensive, the heart block may be permanent and require a pacemaker.

Where does the block occur?

The electrical activity of the heart starts in the sinoatrial (SA) node in the upper chamber (atrium) and travels through the atrioventricular (AV) node to reach the lower chamber (ventricles). Heart block may occur at any point along this electrical pathway. Heart block of the AV node can be of several types, and a doctor generally can diagnose these by looking at the person's electrocardiogram (EKG, ECG).

What are the different types of heart block?

First-degree AV block, which may be caused by medicines, refers to a slowing of the electrical impulse at the AV node but with eventual conduction of the electrical impulse to the ventricles. This does not require a pacemaker. But first-degree AV block may raise your risk of heart rhythm problems, such as atrial fibrillation.

Second-degree AV block refers to intermittent blocking of the electrical impulse so that it does not always reach the ventricles, resulting in "dropped beats."

Second-degree heart block can be categorized into two types:

  • Mobitz type I block (also called Wenckebach) usually occurs in the AV node. It is common in young, healthy people (especially during sleep). It usually does not cause symptoms and rarely requires treatment.
  • Mobitz type II block usually occurs below the AV node in other conduction tissue. It may be part of aging. It is also seen in people with significant heart disease or during a large heart attack. It may cause lightheadedness or fainting (syncope), and it may progress to complete heart block. This type frequently requires a pacemaker.

Complete or third-degree heart block occurs when the electrical impulse is completely blocked at or below the AV node so that electrical signals from the atrium do not reach the ventricles. When this occurs, the atria and ventricles beat at completely different rates.

Complete heart block is caused by the aging process, medicines, heart attacks, infiltrative heart diseases (amyloidosis, sarcoidosis), and infectious diseases (endocarditis, Chagas' disease). It may also occur after heart surgery and can be present from birth (congenital).

Complete heart block frequently causes symptoms of lightheadedness or fainting and usually requires the placement of a permanent pacemaker. People who are born with complete heart block (an uncommon congenital condition) often have no symptoms and may not need treatment initially. But eventually they almost always require pacemaker placement.

Last Updated: June 18, 2009

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