Fibrillation and atrial flutter are very rapid electrical discharges that cause rapid contraction of the atria; Some of these electrical impulses reach the ventricles and also cause them to contract irregularly, faster and less efficiently than normal.

Fibrillation and atrial flutter are more common in older people and in those with heart disease. Atrial fibrillation is much more common than atrial flutter. In many cases, when there is atrial flutter there are also episodes of atrial fibrillation. Both can be intermittent or constant.