Atrial fibrillation is the most common heart arrhythmia with a prevalence of approximately 2% in the western world. Atrial fibrillation is associated with an increased risk of death and morbidity. The comparable effects of a lenient rate control strategy and a strict rate control strategy in patients with atrial fibrillation are uncertain and only one trial has assessed this previously in patients with permanent atrial fibrillation.

The investigators will therefore undertake a randomised, superiority trial at four hospitals in Denmark.

Inclusion Criteria:

1. Participants with atrial fibrillation (ECG confirmed and diagnosed by the treatment provider) who at inclusion have either persistent (defined as atrial fibrillation for more than 7days) or permanent atrial fibrillation (only rate control is considered going forward).
2. Rate control must be accepted as being the primary management strategy going forward. Consideration towards whether rhythm control is more appropriate must be considered, especially given the results of the Early treatment of Atrial fibrillation for Stroke prevention Trial (EAST).
3. Informed consent.
4. Adult (18 years or older).

Exclusion Criteria:

1. No informed consent.
2. Initial heart rate under 80 bpm at rest (assessed via ECG before randomisation).
3. Less than 3 weeks of anticoagulation with new oral anticoagulants or 4 weeks with efficient warfarin if indicated.
4. If the treating physician deems that the participant is not fit to be randomised into both groups based on an individual assessment. Such a decision will be made before randomisation by the treating physician. This can e.g. be participants dependent on a high ventricular rate to maintain a sufficient cardiac output. Such participants could be participants with heart failure, participants with a hemodynamically significant valve dysfunction, or severely dehydrated participants.
5. Participants who are haemodynamically unstable and therefore require immediate electrical cardioversion

We will share anonymised data in a data repository.

Not possible to describe yet but will be made available within a timely manner after publication of results.

Sharing will abide by the General Data Protection Regulation and the Danish data protections laws.

Outcome assessors will be blinded. Participants will not be informed of the heart rate target or actual heart rate. Treatment providers managing the heart rate target will not be blinded as the intervention requires information of the heart rate. Other treatment providers will not be informed of the heart rate target.