Atrial Flutter. Overview. Atrial flutter (AFL) is a type of abnormal heart rate, or arrhythmia. It occurs when the upper chambers of your heart beat too fast. When the chambers in the top of your heart (atria) beat faster than the bottom ones (ventricles), it causes your heart rhythm to be out of sync. Atrial flutter is a similar condition to the more common atrial fibrillation (AFib).