Atrial fibrillation is a condition in which the heart's upper chambers, the atria, contract at an abnormally rapid rate. It is a common type of arrhythmia, and occurs in 1-2% of the general population. The prevalence of atrial fibrillation increases with age. Between 50 and 70% of patients with atrial fibrillation lasting <48 hours spontaneously convert to normal sinus rhythm, and drug therapy increases the likelihood of conversion to sinus rhythm. Another treatment option for conversion of atrial fibrillation and atrial flutter is electrical conversion. This is an effective treatment but requires anesthesia.

Current treatment strategy for medical conversion of atrial fibrillation and atrial flutter is to employ drugs that affect ion channel activity in atrial cardiomyocytes. However, such converting drugs all have potentially serious side effects and are expensive. Potassium, sodium, calcium, and magnesium molecules are the most important ions causing electric current in the heart tissue. Our hypothesis is that hypokalemia promotes atrial fibrillation/atrial flutter by a direct effect on cardiomyocytes. Accordingly, we also hypothesize that potassium infusion may convert atrial fibrillation/atrial flutter to normal sinus rhythm. If so, this would be an inexpensive treatment with potentially very few side effects.

Inclusion Criteria:

• Patients with paroxysmal atrial fibrillation/atrial flutter with duration <48 hours
• Plasma potassium ≤4,0 mmol/L.
• Age ≥ 18 år

Exclusion Criteria:

• Plasma potassium > 4,0 mmol/L
• eGFR <30 mL/min
• Patients on antiarrhythmic therapy (flecainid, amiodarone, dronedarone or sotalol)
• Pregnancy
• Breast feeding
• Patients participating in a clinical trial during the last six months
• Addison disease, adynamia episodic hereditary, or Sickle cell anemia
• Metabolic acidosis, pH < 7,2