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The heart has four areas, or chambers. During each heartbeat, the two upper chambers (atria) contract, followed by the two lower chambers (ventricles). This is directed by the heart's electrical system.

The electrical impulse begins in an area called the sinus node, located in the right atrium. When the sinus node fires, an impulse of electrical activity spreads through the right and left atria, causing them to contract, forcing blood into the ventricles.

Then the electrical impulses travel in an orderly fashion to areas called the atrioventricular (AV) node and HIS-Purkinje network. The AV node is the electrical bridge that allows the impulse to go from the atria to the ventricles. The HIS-Purkinje network carries the impulses throughout the ventricles. The impulse then travels through the walls of the ventricles, causing them to contract. This forces blood out of the heart to the lungs and the body. The pulmonary veins empty oxygenated blood from the lungs to the left atrium. A normal heart beats in a constant rhythm -- about 60 to 100 times per minute at rest.

Atrial fibrillation (also referred to as AF or AFib) is the most common type of irregular heartbeat. It is found in about 2.2 million Americans. Its frequency increases with age. If you have AF, the electrical impulse does not travel in an orderly fashion through the atria. Instead, many impulses begin simultaneously and spread through the atria and compete for a chance to travel through the AV node.

The firing of these impulses results in a very rapid and disorganized heartbeat. The rate of impulses through the atria can range from 300 to 600 beats per minute. Luckily, the AV node limits the number of impulses it allows to travel to the ventricles. As a result, the pulse rate is often less than 150 beats per minute, but this is often fast enough to cause symptoms.

You may have atrial fibrillation without having any symptoms at all. If you have symptoms, they may include:

 Via Webmd