BAsic information description An abnormal heart rhythm. Fibrillation pertains to a "quivering" of muscles. Atrial pertains to the atria, the upper chambers of the heart. The abnormal rhythm reduces the flow of blood to the brain and other body parts and can cause symptoms. It usually affects older adults and men more than women. frequent signs and symptoms There may be no symptoms. Irregular and often rapid beating of the heart. Weakness, dizziness, shortness of breath, chest pain,...