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 many things that affect the electrical system of the heart. Examples include:
- 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.
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 (ventricle). 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 heartbeats." 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.
- 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.
How second-degree AV block is treated can depend on the type and what is causing it. Treatment can also depend on your symptoms. If heart block doesn't cause symptoms, it may not be treated. Treatment may be a pacemaker. You and your doctor can decide what treatment is right for you.
Complete or third-degree block
In third-degree heart block, all of the electrical impulses are completely blocked between the upper and lower chambers of the heart. 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.
What is bundle branch block?
Bundle branch block can affect the heart's rhythm. The heart has structures, like wires, that are called bundle branches. They are part of the heart's electrical pathway. When a branch is diseased, it is called "blocked," because the electrical signals can't travel down the branch.
Some people with bundle branch block don't have any symptoms and don't need treatment. But when a block causes the heart to beat too slowly, it can cause symptoms such as tiredness and fainting spells. A pacemaker may be used to get the heartbeat back to normal.
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Other Works Consulted
- Cheng S, et al. (2009). Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA, 301(24): 2571–2577.
- Olgin JE, Zipes DP (2015). Specific arrhythmias: Diagnosis and treatment. In DL Mann et al., eds., Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 10th ed., vol. 1, pp. 748–797. Philadelphia: Saunders.