Atrial fibrillation is the most common heart rhythm disorder, and the incidence is rapidly increasing. Cardioversion using an electrical shock (DC-cardioversion) is an important treatment to reduce symptoms and improve patient's quality-of-life. The treatment is performed by applying gel electrodes to the chest. Cardioversion is not always successful, and it is unknown which electrode-position provides the optimal efficacy.

This study aims to compare two electrode positions, which are in clinical use: Anterior-posterior (left front and left back) versus anterior-lateral (right front and left side of the chest).

Inclusion Criteria:

1. Atrial fibrillation (documented on an ECG-12)
2. Able and willing to sign informed consent
3. Age ≥ 18 years
4. Anticoagulation according to guidelines (Patients with atrial fibrillation for >48 hours will be required to have a documented weekly international normalized ratio (INR) ≥2.0 (including within 48 hours of cardioversion) or treatment with non-vitamin K oral anticoagulant for three weeks or longer. Alternatively, a transoesophageal echocardiogram documenting the absence of intracardiac thrombi is accepted and cardioversion can be performed on treatment with low molecular weight heparin).

Exclusion Criteria:

1. Implanted pacemaker and/or cardioverter defibrillator (ICD)
2. Prior enrollment in the trial
3. Hemodynamically unstable atrial fibrillation
4. Untreated hyperthyroidism
5. Known or suspected pregnancy