Both ICDs and pacemakers are small electrical devices, implanted into the chest, that help manage heart arrythmias. A pacemaker uses steady, low-energy shocks to maintain a normal heart rhythm. Pacemakers can detect if your heartbeat is abnormal and deliver shocks when required. Much like external defibrillators, an ICD delivers a high-energy electric shock if the device detects a life-threatening arrhythmia. They are often implanted in people who have previously had a heart attack.