The heart normally beats in a steady rhythm and rate, allowing it to pump a sufficient amount of blood throughout the body. The upper chambers of the heart, called the atria, open to allow blood in and contract to pump blood out into the lower chambers, called the ventricles. The electrical system of the heart regulates the rate and rhythm of the heartbeat.

Atrial fibrillation is a heart condition in which the atria beat too fast and with an irregular rhythm (they fibrillate). AFib is caused by a disruption in the heart’s electrical signaling system, resulting in the atria being out of synch with the ventricles. Insufficient amounts of blood are circulated through the body, and that’s when symptoms may appear.

Because the irregular beats prevent the atria from contracting properly, blood pools inside the heart’s chambers, and over time, this pooled blood can clot. If a clot is pumped into the bloodstream, it can lodge in a major organ and block the blood flow to that organ. For example, if the clot travels to the brain, the person may have a stroke.

Atrial fibrillation is the most common type of irregular heartbeat (arrhythmia), and affects more than 5 million people in the United States.