Sometimes things go wrong in the heart's electrical system. The heartbeat becomes irregular or changes its rate inappropriately. This is called an arrhythmia. These impulses cause the upper heart to contract very rapidly, often so fast that the chambers quiver instead of really pumping. This fibrillation, or rapid contracting, of the atria interrupts the heart's normal efficient pumping.
Atrial fibrillation is a kind of tachycardia-an abnormally fast heart rhythm.
A normal, steady heart rhythm typically beats 60-80 times a minute. These very fast, irregular signals can cause a number of problems. First, the quivering atria no longer pump efficiently, and some blood may stay in the atria with each heartbeat. The pooled blood could possibly clot, increasing the risk of stroke. Second, the many impulses coming from the atria are trying to follow the electrical pathway to the ventricles and make them contract at the same rate.