An Implantable Cardioverter-Defibrillator (ICD) is a small, battery-powered device implanted in the chest that constantly monitors heart rhythm and delivers electrical shocks or pacing to correct dangerously fast or irregular heartbeats (arrhythmias) like ventricular tachycardia or fibrillation, preventing sudden cardiac death by restoring a normal rhythm, essentially acting as a 24/7 heart guardian. It consists of a pulse generator (battery & circuits) and wires (leads) connected to the heart.