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What Is Atrial Fibrillation?






Understanding the Heart's Electrical System

To understand AF, it helps to understand the heart's internal electrical system. The heart's electrical system controls the rate and rhythm of the heartbeat.


Each electrical signal begins in a group of cells called the sinus node or sinoatrial (SA) node. The SA node is located in the right atrium. In a healthy adult heart at rest, the SA node sends an electrical signal to begin a new heartbeat 60 to 100 times a minute. (This rate may be slower in very fit athletes.)

From the SA node, the electrical signal travels through the right and left atria. It causes the atria to contract and pump blood into the ventricles.

The electrical signal then moves down to a group of cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here, the signal slows down slightly, allowing the ventricles time to finish filling with blood.

The electrical signal then leaves the AV node and travels to the ventricles. It causes the ventricles to contract and pump blood to the lungs and the rest of the body. The ventricles then relax, and the heartbeat process starts all over again in the SA node.

For more information about the heart's electrical system and detailed animations, go to the Diseases and Conditions Index How the Heart Works article.

Understanding the Electrical Problem in Atrial Fibrillation

In AF, the heart's electrical signals don't begin in the SA node. Instead, they begin in another part of the atria or in the nearby pulmonary veins. The signals don't travel normally. They may spread throughout the atria in a rapid, disorganized way. This can cause the atria to fibrillate.

The faulty signals flood the AV node with electrical impulses. As a result, the ventricles also begin to beat very fast. However, the AV node can't send the signals to the ventricles as fast as they arrive. So, even though the ventricles are beating faster than normal, they aren't beating as fast as the atria.


If this happens, blood isn't pumped into the ventricles as well as it should be. Also, the amount of blood pumped out of the ventricles to the body is based on the random atrial beats.





Install flash.
The animation shows how the heart's electrical signal can begin somewhere other than the sinoatrial node.

The animation shows how the heart's electrical signal can begin somewhere other than the sinoatrial node. This causes the atria to beat very fast and irregularly.



For people who have permanent AF, treatment can help control symptoms and prevent complications. Treatment may include medicines, medical procedures, and lifestyle changes.

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Last Updated: September 18, 2014