What Is It? Atrial fibrillation is a heart rhythm disorder that
causes a rapid and irregular heartbeat. The atria are the upper
two chambers of the heart that receive blood from the rest of
the body. They pump blood into the lower two chambers (the
ventricles). Then the ventricles pump blood to the rest of the
body. During atrial fibrillation, the atria do not beat
normally. Instead, they quiver or "fibrillate." Normally,
electrical impulses from a small part of the atrium called the
sinus node cause the atria to beat. The electrical signal then
goes through another part of the heart called the
atrioventricular node. Then it goes down to the ventricles, and
causes them to beat. First the atria, then the ventricles: it is
coordinated, and so the heart pumps blood efficiently. The atria
pump, filling the ventricles with blood, and then the ventricles
pump. In contrast, during atrial fibrillation, the electrical
impulses come from all over the atria. Instead of pumping
efficiently, the atria just quiver. There i