Atrial fibrillation (say "A-tree-uhl fih-bruh-LAY-shun") is an irregular heart rhythm (arrhythmia) that starts in the upper parts (atria) of the heart. Atrial fibrillation is the most common type of persistent irregular heartbeat (arrhythmia).
Normally, the heart beats in a strong, steady rhythm. In atrial fibrillation, a problem with the heart's electrical system causes the atria to quiver, or fibrillate. The quivering upsets the normal rhythm between the atria and the lower parts (ventricles) of the heart. The lower parts may beat fast and without a regular rhythm.
Atrial fibrillation is dangerous because it greatly increases the risk of stroke. If the heart doesn't beat strongly, blood can collect, or pool, in the atria. Pooled blood is more likely to form clots. If the heart pumps a clot into the bloodstream, the clot can travel to the brain and block blood flow, causing a stroke. Atrial fibrillation can also lead to heart failure.
Conditions that damage or strain the heart commonly cause atrial fibrillation.