A total of 240 patients will be included. Eligible patients will be recruited from the out-patient clinics at the participation hospitals and through advertisements in local newspapers. After inclusion and a wash-out period of 14 days free from drugs affecting the heart rate, patients will be examined by echocardiography, 12-lead ECG (electrocardiography), 24h Holter monitoring, maximal cardiopulmonary exercise test and venous blood sampling at rest, at maximal exercise and after recovery. Perceived arrhythmia related symptoms, quality of life and level of physical activity will be assessed using self-administered questionnaires.