Atrial fibrillation—aka AFib or AF—is an irregular heartbeat (arrhythmia) in the upper chambers (atria) of the heart. In AFib, random irregular electrical signals cause the atria to quiver. So some of the blood stays behind in the atria, which makes the blood more likely to clot. AFib can cause rapid, fluttering, or pounding heartbeats, lightheadedness, extreme fatigue, shortness of breath, chest pain...or no symptoms at all.