Heart Block

In heart block, the electrical currents that pass through the tissues between the atria and ventricles are delayed or blocked.

Depending on its severity, heart block may cause no problems or serious problems, such as loss of consciousness or fainting. Heart block is classified as first, second, or third degree based on its severity. Heart block is more common among older people. Electrocardiography (ECG) is needed to make the diagnosis.

In first-degree heart block, the electrical currents flowing between the atria and ventricles are slightly delayed. First-degree heart block may occur in healthy people. But it may be caused by rheumatic fever, disorders that damage the heart's electrical system, and other heart disorders. It may also be caused by drugs, particularly those that slow the heart rate (such as beta-blockers, diltiazem, verapamil, and amiodarone). This disorder produces no symptoms. No treatment is needed.

In second-degree heart block, the electrical currents are sometimes (intermittently) blocked. The heart may beat slowly, irregularly, or both. Common causes include heart attack, scarring of the heart's electrical system, heart infections (myocarditis), and use of digoxin or other drugs that slow the heart rate in doses that are too high. Second-degree heart block may cause dizziness and fainting. Sometimes second-degree heart block progresses to third-degree heart block. If second-degree heart block causes symptoms and its cause cannot be eliminated, an artificial pacemaker is usually implanted.

In third-degree (complete) heart block, the electrical currents flowing between the atria and ventricles are completely blocked. Another part of the heart then acts as a pacemaker. This substitute pacemaker is slower than the heart's normal pacemaker and is often irregular and unreliable. Common causes include heart attack, scarring of the heart's electrical system, and toxicity due to digoxin or other drugs that slow the heart rate. Third-degree heart block commonly causes fatigue, dizziness, and fainting. It is a serious disorder that can affect the heart's ability to pump. Heart failure can result. Third-degree heart block can cause death if the substitute pacemaker is too slow or if the heart stops beating.

In an emergency, third-degree heart block may be treated with a temporary external pacemaker until a permanent one can be implanted. A temporary pacemaker may also be used when doctors think that heart rhythm is likely to return to normal after the cause of the heart block resolves—for example, after a person recovers from a heart attack or after digoxin is discontinued. For nearly all other people with third-degree heart block, a permanent artificial pacemaker is needed.