Atrial fibrillation is usually caused by a barrage of uncoordinated electrical signals that trigger cells in the atria to contract independently of one another. The atria themselves don't actually contract because the individual heart muscles aren't synchronized. The result is a fast and irregular rhythm. Atrial flutter, in contrast, is a fast and regular rhythm caused by a small, tight circle of electrical activity. Every time the wave of excitement goes around the loop, the atria contract. The term "atrial flutter" comes from the fact that when it is occurring, the edges of the atria look like the flapping wings of a bird.
Although some people have one or the other, many people have episodes of atrial flutter and stretches of atrial fibrillation.
In addition to making the atria beat faster than normal, both conditions cause the heart's lower chambers (the ventricles) to beat faster, too, sometimes up to 175 beats per minute. This can cause symptoms such as lightheadedness, breathlessness, fatigue, and low blood pressure. Some of the same medications and procedures are used to treat both.
In the past, doctors thought that atrial flutter was less likely than atrial fibrillation to cause a stroke, but research indicates that atrial flutter also boosts the chances of having a stroke. As with atrial fibrillation, the decision to start warfarin or other stroke-preventing medication is based on an individual's overall stroke risk.