An arrhythmia is an abnormal heart rhythm resulting from any change, deviation or malfunction in the heart's conduction system – the system through which normal electrical impulses are generated and travel through the heart. An arrhythmia may result in a heartbeat that is unusually fast (tachycardia), unusually slow (bradycardia), regular or irregular. Some arrhythmias are signs of more serious heart problems while others are not. An arrhythmia may be brief and unnoticeable, or it may be startling, obvious or even fatal.

Skips, pauses and palpitations (strong, fast, “galloping” heartbeats) commonly occur in the general population. In the majority of cases, a skipped beat is not medically significant. The most serious arrhythmias, however, contribute to approximately 500,000 deaths in the United States each year according to the American Heart Association. One type of arrhythmia (ventricular fibrillation) causes most of the 330,000 sudden cardiac deaths (SCD) that occur each year. SCD occurs when a patient dies following an episode of cardiac arrest, in which the heart suddenly stops beating. It is fatal unless the patient receives immediate medical attention. 

In general, the probable outcome of an arrhythmia is largely dependent on any structural heart abnormalities. Outcomes tend to be worse among patients with poor heart function such as patients who have experienced large heart attacks or cardiomyopathy.