To search for the best marker for the patients in atrial fibrillation with rate control

In an aging society, the prevalence of atrial fibrillation is increasing. The main treatment strategy is to reduce the complications of embolic stroke and heart failure. Among these patients, a group of them will choose to have "rate control" based on the heart conditions and the preference of the patients. The strategy is to control the ventricular rate by drugs to prevent the heart from failure. However, when the physicians attempt to do so, they encounter two questions: how much is the optimal drug dose? And what is the best marker we are looking to? To answer these questions, the investigators design a prospective, cross-over study to explore this issue. The participants will do cardiopulmonary exercise test (CPET) before and after treatment. They will receive another CPET if drug dose is titrated up. With the study, the investigators will understand whether a resting heart rate can be a good marker for optimal drug dosage, and what is the optimal range of heart rate the physicians should target.

Inclusion Criteria:

1. 18~90 years old
2. Rate control is preferred after shared decision making

Exclusion Criteria:

1. cannot cooperate with or complete CPET
2. has contraindication for CPET, such as uncontrolled heart failure, hypertension, angina, valvular heart disease, lung function impariment that SaO2<90% at room air.
3. Ventricular rate below 80/min before any medication
4. Permanent pacemaker implanted