Cardiac arrhythmias are a common term for all deviations from the normal heart rhythm (sinus rhythm). Many heart rhythm disorders are harmless, while others require medical treatment. Cardiac arrhythmias can cause rapid, slow or irregular heartbeats. Cardiac arrhythmias may feel like irregular heartbeats, palpitations, dizziness or sometimes as fainting. A common heart rhythm disorder is atrial fibrillation. The heart beats irregularly and often too fast during atrial fibrillation. Atrial fibrillation increases the risk of blood clots in the heart. The blood clots can follow the large blood vessels to the brain and cause a stroke. Anticoagulation therapy can prevent blood clots.