Heart arrhythmias occur when the electrical impulses that coordinate the heartbeats do not work properly, causing the heart to beat too fast, too slow or irregularly. They may feel like a fluttering or racing heart and may be harmless. Arrhythmias are classified, not only by where they originate (atria or ventricles) but also by the speed of heart rate they cause. Treatment can often control or eliminate fast, slow or irregular heartbeats.