You will need a defibrillator if you have a dangerously fast heartbeat that prevents your heart from supplying enough blood to the rest of your body—for example, ventricular tachycardia or ventricular fibrillation. Also, you will need a defibrillator if you are at high risk of such a heart rhythm problem (arrhythmia) typically due to a weak heart muscle; you may require an ICD.