Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm characterized by rapid and irregular beating. Often it starts as brief periods of abnormal beating which become longer and possibly constant over time. Most episodes have no symptoms. Occasionally there may be heart palpitations, fainting, shortness of breath, or chest pain. The disease increases the risk of heart failure, dementia, and stroke.

Hypertension and valvular heart disease are the most common alterable risk factors for AF. Other heart related risk factors include heart failure, coronary artery disease, cardiomyopathy, and congenital heart disease. In the developing world valvular heart disease often occurs as a result of rheumatic fever. Lung related risk factors include COPD, obesity, and sleep apnea. Other factors include excess alcohol intake, diabetes mellitus, and thyrotoxicosis. However, half of cases are not associated with one of these risks. Diagnosis is by feeling the pulse which may be confirmed using an electrocardiogram (ECG). The ECG shows no P waves and an irregular ventricular rate.

AF is often treated with medications to slow the heart rate to a near normal range (known as rate control) or to convert the rhythm to normal sinus rhythm (known as rhythm control). Electrical cardioversion can also be used to convert AF to a normal sinus rhythm and is often used emergently if the person is unstable. Ablation may prevent recurrence in some people. Depending on the risk of stroke either aspirin or anti-clotting medications such as warfarin may be recommended. While these medications reduce this risk they increase rates of major bleeding.

Atrial fibrillation is the most common abnormal heart rhythm. In Europe and North America, as of 2014, it affects about 2% to 3% of the population. This is an increase from 0.4 to 1% of the population around 2005. In the developing world about 0.6% of males and 0.4% of females are affected. The percentage of people with AF increases with age with 0.14% under 50 years old, 4% between 60 and 70 years old, and 14% over 80 years old being affected. A-fib and atrial flutter resulted in 112,000 deaths in 2013, up from 29,000 in 1990. The first known report of an irregular pulse was by John Baptist Senac in 1749. This was first documented by ECG in 1909 by Thomas Lewis.

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