A person with atrial flutter has a very fast pulse because the atria of the heart are beating too rapidly.

Atrial flutter is an abnormal heart rhythm that causes the upper chamber of the heart to beat at a very high rate. During a normal heartbeat, the upper chambers of the heart fill the lower chambers of the heart, just before the lower chambers pump blood to the body. This sequence of events occurs too quickly in atrial flutter, weakening the pumping action of the heart. Left untreated, atrial flutter can lead to heart attack or heart failure.