Press your hand against your chest. Those little thumps you feel are your heart pumping, moving blood into and out of its chambers and through the rest of your body.
Normally, the top part of your heart (the atria) squeezes first, then the bottom part (the ventricles). The timing of these contractions is what moves the blood. But for more than 2 million Americans, the electrical signals that control this system are off-kilter. Instead of working together, the atria do their own thing. This fast, fluttering heartbeat, what doctors call arrhythmia, is atrial fibrillation, or AFib.