An abnormal heartbeat (arrhythmia) is any variation in the normal heartbeat. Abnormal heartbeats occur when the heart has an irregular heart rhythm, beats too fast (tachycardia), or beats too slow (bradycardia).

The electrical system of the heart creates signals that trigger the heart to pump. These electrical signals control the heart rate and rhythm. Normally, the heart beats in a regular rhythm and at a rate that is appropriate for the work the body is doing. An arrhythmia results from a problem in the electrical system of the heart. Things that can cause the heart to beat abnormally include heavy smoking, alcohol use, excess caffeine or other stimulants, stress, thyroid disease, and fever.

Many arrhythmias are minor, causing only occasional abnormal heartbeats and requiring no treatment. Others, such as atrial fibrillation, can be life-threatening because they increase the risk of blood clots and strokes. Arrhythmias are of special concern in people who also have heart disease or heart failure. Some arrhythmias can be treated with medicine. Others may require an electrical shock (cardioversion), surgery, or a pacemaker.