Cardiac defibrillator. Coloured X-ray of the chest of a 55 year old female patient who has been fitted with an implantable cardioverter defibrillator (ICD, right centre). Wires lead from the defibrillator through the subclavian vein and superior vena cava to the patient's heart. Such devices are used to treat abnormal heart rhythm. If ventricular fibrillation occurs, the device delivers an electrical shock that resets the heart rate to a normal rhythm.