Arrhythmias are caused by a disruption of the normal functioning of the electrical conduction system of the heart. Normally, the atria and
ventricles contract in a coordinated manner. In atrial fibrillation and
flutter, the atria are stimulated to contract very quickly. This results in ineffective and uncoordinated contraction of the atria.
The impulses may be transmitted to the ventricles in an irregular fashion, or only some of the impulses may be transmitted.