Ventricular tachycardia: An abnormally rapid heart rhythm that originates from a ventricle, one of the lower chambers of the heart. Although the beat is regular, ventricular tachycardia is life-threatening because it can lead to a dreaded condition, ventricular fibrillation.

In ventricular fibrillation, the ventricles beat rapidly in a chaotic, purposeless fashion. The heart cannot pump blood effectively to the body. If untreated, ventricular fibrillation can be fatal within minutes, or even seconds. An estimated 250,000 Americans die in this way each year.

Ventricular tachycardia is most commonly associated with heart attacks or scarring of the heart muscle from a previous heart attack.