Atrial fibrillation, supraventricular arrhythmia in the heart and treatments. Atrial fibrillation is a supraventricular arrhythmia. In the design of a heart seen in section, the stasis of the flow induced by the anarchic contractions of the atria led to the formation of a clot (thrombus) in the left auricle. The electrical conduction of the heart starts at the sinus node of Keith and Flack (orange), spreads to the auricles and the atrioventricular node (yellow), then the bundle of His and the Punkinje fibers (green). At the top of the image a normal ECG (electrocardiogram) shows the different colors corresponding to the electrical conduction on the heart. Venous and arterial blood flow is represented in transparency.