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.