When the heart beats, the chambers squeeze or contract to pump blood and then relax so they can receive blood again. The process of squeezing and relaxing usually happens regularly. But sometimes, a person develops manifestations of atrial fibrillation, where the atria (upper chambers) start firing abnormal electrical impulses. This overrides the heart’s natural pacemaker, causing the atria to randomly contract, which means they don’t have enough time to relax. Learn more about atrial fibrillation symptoms here, and how you can determine your risk for its potential consequences.