Implantable cardioverter-defibrillators (ICDs) can stop ventricular arrhythmias, which are caused by abnormal electrical signals that originate within the heart's ventricles (the two chambers in the lower part of the heart). The signals can make the heart beat too fast (ventricular tachycardia), causing too little blood to be pumped with each beat. This can lead to palpitations, light-headedness, dizziness, weakness and fainting, as well as a dangerous drop in blood pressure.
Abnormal signals can also cause the ventricles to twitch rapidly and chaotically (ventricular fibrillation), rendering them unable to pump blood throughout the body. Ventricular fibrillation leads to unconsciousness within seconds. Collapse and cardiac arrest can occur quickly and lead to death within a few minutes without treatment.
Ventricular tachycardia can sometimes progress to ventricular fibrillation.
Source: Prepared by the Editors of The Johns Hopkins Medical Letter: Health After 50