Atrial fibrillation is a type of cardiac arrhythmia. The condition is characterized by an abnormal heart rhythm that originates in the atria and is propagated in a circular pattern. From the atria, the impulses are transmitted to the ventricles, and produce a change in the heart rate depending on whether this occurs at a slow (bradycardia) or a fast rate (tachycardia). Atrial fibrillation can be present long-term (permanent), or it may be experienced as recurring episodes (paroxysmal). The condition is always associated with an irregular heartbeat, which is therefore its cardinal symptom. While not dangerous in itself, some patients may experience the symptoms associated with the condition as extremely uncomfortable. Symptoms, however, vary greatly from one patient to another, with some patients experiencing no symptoms at all.

The main complication of the condition is stroke, which may occur as a result of blood clots travelling to the brain from the left atrium.

During atrial fibrillation, the atria can no longer pump blood effectively. The result is reduced flow velocity, which increases the chance of blood clots forming, particularly in the left atrium. If these blood clots leave the left atrium, the risk of embolisms increases. The risk of stroke is particularly high, but there is also an increased risk of embolisms affecting organs in the rest of the body. Following an initial risk assessment, most patients with atrial fibrillation will need to start anticoagulant medication to prevent blood clots from forming. Aside from traditional, oral anticoagulants (blood thinning medication), treatment may include new drugs such as Falithrom /Marcumar [phenprocoumon] and, in rare cases, even drugs such as Dabigatran and Rivaroxaban, both of which are members of a new generation of blood thinning drugs.