What Is Atrial Fibrillation?

Atrial fibrillation (A-tre-al fi-bri-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm.

AF occurs when rapid, disorganized electrical signals cause the atria (AY-tree-uh), the two upper chambers of the heart, to fibrillate. The term "fibrillate" means to contract very fast and irregularly.

In AF, blood pools in the atria and isn't pumped completely into the ventricles (VEN-trih-kuls), the heart's two lower chambers. As a result, the heart's upper and lower chambers don't work together as they should.

Often, people who have AF may not feel symptoms. However, even when not noticed, AF can increase the risk of stroke. In some people, AF can cause chest pain or heart failure, particularly when the heart rhythm is very rapid.

AF may occur rarely or every now and then, or it may become a persistent or permanent heart rhythm lasting for years.

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.

With each heartbeat, an electrical signal spreads from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood. The process repeats with each new 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 fires off 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. This 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 just a little, allowing the ventricles time to finish filling with blood.

The electrical signal then leaves the AV node and travels to the ventricles. This causes the ventricles to contract and pump blood out 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 on 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, and they may spread throughout the atria in a rapid, disorganized way. This can cause the atria to fibrillate.

The abnormal electrical 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 conduct the signals to the ventricles as fast as they arrive. So, even though the ventricles may be beating faster than normal, they aren't beating as fast as the atria.

Thus, the atria and ventricles no longer beat in a coordinated way. This creates a fast and irregular heart rhythm. In AF, the ventricles may beat 100 to 175 times a minute, in contrast to the normal rate of 60 to 100 beats a minute.

When 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 randomness of the atrial beats.

The body may get rapid, small amounts of blood and occasional larger amounts of blood. The amount will depend on how much blood has flowed from the atria to the ventricles with each beat.

Most of the symptoms of AF are related to how fast the heart is beating. If medicines or age slow the heart rate, the symptoms are minimized.

AF may be brief, with symptoms that come and go and end on their own. Or, the condition may be persistent and require treatment. Sometimes AF is permanent, and medicines or other treatments can't restore a normal heart rhythm.

The animation below shows atrial fibrillation. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how the heart's electrical signal begins in a place other than the sinoatrial node, causing the atria to beat very fast and irregularly.

Outlook

People who have AF can live normal, active lives. For some people, treatment can cure AF and restore normal heart rhythms.

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

Revised October 2009