A defibrillator, also known as an automated implantable cardioverter-defibrillator (AICD), is a small device implanted under the skin of the chest that monitors the heart's rhythm and delivers an electrical shock to the heart if it detects a life-threatening arrhythmia. The device is typically used to treat ventricular fibrillation and ventricular tachycardia, which can cause sudden cardiac arrest. AICDs have been shown to significantly reduce the risk of sudden cardiac death in people with a high risk of arrhythmias.