An estimated 2.2 million Americans are living with AF. That makes it the most common "serious" heart rhythm abnormality.

AF isn't life-threatening, but it can lead to other rhythm problems, chronic fatigue, congestive heart failure and, worst of all, stroke. When your left atrium pumps too fast and unevenly, blood doesn't empty completely into your left ventricle. Instead, leftover blood "pools" in the atrium. This blood can form clots. If a clot breaks loose, enters the bloodstream and travels to the brain, it can plug an artery and cause a stroke. This doesn't happen to everyone with AF, but your chances of having a stroke are five times higher if you have AF. About 15–20 percent of all strokes occur in people with AF.

Atrial flutter
This condition occurs when rapidly fired signals cause the muscles in the atria to contract quickly, leading to a very fast, steady heartbeat.

Premature atrial contraction (PAC)
Premature beats or extra beats most often cause irregular heart rhythms. Those that start in the upper chambers are called premature atrial contractions (PACs). If you've ever felt your heart "skip a beat," it was probably from this type of arrhythmia. In reality, the heart doesn't skip a beat. Instead an extra beat comes sooner than normal.

Premature beats are very common in normal children and teenagers — most people have them at some time. Usually no cause can be found and no special treatment is needed. The premature beats may disappear later. Occasionally premature beats may be caused by disease or injury to the heart. Your doctor may recommend more tests to make sure your heart is OK.

Sick sinus syndrome
When the sinus node doesn't fire its signals properly, the heart rate slows down. This is called sick sinus syndrome. In this condition, the rate sometimes changes back and forth between a slow rate (bradycardia) and a fast rate (tachycardia).

Sinus arrhythmia
This refers to cyclic changes in the heart rate during breathing. It's common in children and often found in adults.

Sinus tachycardia
Sinus tachycardia is a normal increase in the heart rate caused when the sinus node (the heart's natural pacemaker) sends out electrical signals faster than usual. It occurs with fever, excitement and exercise. No treatment is needed. Rarely, diseases such as anemia (low blood count) or increased thyroid activity cause this fast heart rate. In these cases, when the disease is treated, the tachycardia goes away.

Supraventricular tachycardia (SVT)
Supraventricular tachycardia (SVT) is also called paroxysmal atrial tachycardia (PAT) or paroxysmal supraventricular tachycardia (PSVT). In this condition a series of early beats in the atria speeds up the heart rate. Many people with it don't need medical therapy. Treatment is considered if episodes are prolonged or occur often. This is the most common form of arrhythmia in children.

Wolff-Parkinson-White (WPW) syndrome
Abnormal pathways between the atria and ventricles cause the electrical signal to arrive at the ventricles too soon and to be transmitted back into the atria. Very fast heart rates may develop as the electrical signal ricochets between the atria and ventricles. Many people with WPW syndrome don't have symptoms or episodes of tachycardia.

This content is reviewed regularly. Last updated 09/21/07.