An ICD (implantable cardioverter defibrillator) is an electronic device that constantly monitors your heart rhythm. When it detects a very fast, abnormal heart rhythm, it delivers energy to the heart muscle. This causes the heart to beat in a normal rhythm again.
Ventricular tachycardia and ventricular fibrillation are two life-threatening heart rhythms that cause the heart to beat very fast. These conditions can be fatal if not treated immediately.