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:

• Scarring (fibrosis) of the heart's electrical system caused by aging. This is the most common cause of heart block.
• Heart attacks.
• Infection of the heart valves (endocarditis).
• Complication of Lyme disease.
• Sarcoidosis or hemochromatosis.
• Use of certain medicines, especially beta-blockers, calcium channel blockers, and digoxin.

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 is atrioventricular (AV) block?

First-degree AV block

In first-degree block, the electrical impulses take longer to travel between the upper chamber (atrium) and lower chamber (ventricle) of the heart. This type of heart rhythm may or may not be associated with a slow heart rate.

It does not usually require treatment. But this type of heart block may raise your risk of heart rhythm problems, such as atrial fibrillation.

Second-degree AV block

In second-degree heart block, some of the electrical impulses are blocked between the upper and lower chamber of the heart. These electrical impulses may or may not have a clear pattern. The blocking of the impulse can come and go, resulting in "dropped heart beats." A second-degree type II block may progress to complete or third-degree heart block.

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.