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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Atrial fibrillation is a heart rhythm disorder that causes a rapid and irregular heartbeat.

Atrial fibrillation affects the upper two chambers of the heart, the atria. All blood circulates through both atria.

The heart is a muscle. The walls of the chambers of the heart are made of muscle cells.

Normally, the muscular walls of the atria contract at the same time, pumping blood into the lower two chambers (the ventricles). Then the walls of the ventricles contract at the same time, pumping blood to the rest of the body.

For the heart to work efficiently, the atria need to pump first, and then the ventricles need to pump. How is this coordinated? Normally, each heartbeat starts with an electrical impulse that comes from a small part of the atrium called the sinus node. That signal first causes the atria to beat, pumping blood into the ventricles. Then the signal travels to another part of the heart called the atrioventricular node. From there, the signal travels down to the ventricles, and causes them to beat, sending blood throughout the body.

In contrast, during atrial fibrillation, instead of one coordinated signal that causes all parts of the atria to pump at the same time, there are multiple uncoordinated signals. Instead of pumping efficiently, the atria just quiver.

As a result, the atria do not pump all of their blood into the ventricles. Also, the ventricles sometimes pump when they don't have a lot of blood in them. So the heart is not pumping efficiently.

In atrial fibrillation, the heartbeat is rapid and irregular. A normal heartbeat is 60 to 100 beats per minute, and very regular: beat…beat…beat…beat. During atrial fibrillation, the heart beats at 80 to 160 beats per minute, and is very irregular: beat..beat…..beat….beat.beat.beat….beat.

Atrial fibrillation can lead to the formation of blood clots inside the atria. That's because blood tends to form clots when it's not moving. The quivering atria don't move all of the blood along to the ventricles. Some blood just pools inside the atria, and the pool of still blood tends to form clots.

Such blood clots can cause serious problems. They can travel out of the heart and get stuck in an artery to the lungs (causing a pulmonary embolism), an artery to the brain (causing a stroke) or an artery elsewhere in the body.

The major factors that increase the risk of atrial fibrillation are:

  • Age

  • Coronary artery disease

  • Rheumatic heart disease

  • High blood pressure

  • Diabetes

  • An excess of thyroid hormones

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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