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Heart Blocks


The Manual's Editorial Staff

Reviewed/Revised Jul 2022 | Modified Sep 2022
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Your heart is a muscle that pumps blood through your body. Your heart rate is how fast your heart beats. Your heart should always have a regular, rhythmic beat, like the ticking of a clock.

Your heart has four chambers. The atria are the two upper chambers in your heart. The ventricles are the two lower chambers in your heart. The atria pump blood into the ventricles. The ventricles pump blood to your lungs and your body (see also Biology of the Heart Biology of the Heart The heart is a hollow organ made of muscle. The heart and blood vessels are part of your cardiovascular system. Your heart pumps blood through your blood vessels Blood carries oxygen and nutrients... read more ).

Special pacemaker cells in a part of the atria called the SA node (sinoatrial node) send out regular electrical signals to your heart muscle to make it contract.

The Conduction System

Your heart's conduction system has tiny strips of tissue sort of like electrical wires. The conduction system carries the pacemaker signals from the SA node to the rest of your heart. The signals must get to all your heart muscle cells at just the right time so your heart gives a good, strong beat that pumps blood properly.

What is a heart block?

What causes a heart block?

Your heart's conduction system carries electrical signals from the atrium through a gateway called the AV node (atrioventricular node). The AV node controls how signals pass from the upper chambers of your heart (atria) to the lower chambers (ventricles). From the AV node, the signals pass through several fiber bundles (bundle branches) before they spread out through your ventricles. A heart block can occur in any part of the conduction system.

Some people are born with a heart block. Other people develop a heart block later in life. Heart blocks can be caused by damage from:

What are the types of heart block?

Doctors group heart blocks based on what part of the conduction system is involved and how severe they are.

The 2 main locations for heart block are:

  • Atrioventricular (AV) block

  • Bundle branch block

What's an atrioventricular (AV) block?

An AV block occurs at or near the AV node. AV blocks have 3 levels, depending on how severe they are:

  • 1st degree: Each electric pulse only slows for a fraction of a second as it moves through your heart (your heart rate is normal)

  • 2nd degree: Some of the signals don't get through the AV node and your heart skips some beats

  • 3rd degree: No signals get through the AV node and your heart can stop or beat dangerously slowly

What's a bundle branch block?

A bundle branch block occurs in the fiber bundles of the conduction system in your ventricles. There's a left bundle branch and a right bundle branch. A block of the left bundle branch tends to be more serious because it's usually caused by underlying heart disease.

What are the symptoms of a heart block?

Most heart blocks don't cause symptoms. If you have a severe heart block, you may feel tired and dizzy and you may faint.

How can doctors tell if I have a heart block?

Doctors feel your pulse and do:

An ECG is a quick, painless test that measures your heart’s electrical activity using stickers and cables on your chest, arms, and legs.

How do doctors treat a heart block?

Most types of heart block don't need treatment. But you may need treatment for the heart disease that caused your heart block.

However, if you have a 2nd or 3rd degree AV heart block, you may need a pacemaker. A pacemaker is a small electrical device that doctors put in your chest or belly to signal your heart when to beat.

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