Atrial fibrillation is when the heart's atria quiver instead of beating effectively. This is due to multiple reentrant circuits meandering through the atria as a result of cardiac arrhythmias, and causes disorganized continuous atrial activity on the electrocardiogram (EKG). As a result, blood is not pumped effectively, and so may pool and clot. On EKG, the R-R (QRS - QRS) intervals are irregular.
Atrial fibrillation is the most common sustained arrhythmia, and accounts for a third of all admissions to hospital for arrhythmia. It also accounts for more total hospital days than other arrhythmia, and is a significant risk factor for stroke, since a loose piece of blood clot (thrombus or embolus) may result in a stroke if it leaves the heart and becomes lodged in an artery of the brain.
A more concerted form of atrial fibrillation is atrial flutter. Unlike atrial flutter, fibrillation has different electrical paths that are being pursued with each beat, and there is truly chaotic activation. Because the atria are activated in such a chaotic fashion, the ventricles are also irregularly activated, resulting in irregular RR intervals.
Patients presenting with atrial fibrillation are managed by first being given stroke prophylaxis, then having their heart rate and heart rhythm controlled.