An implantable cardioverter defibrillator (ICD) is a device that is implanted in the chest to constantly monitor and correct abnormal heart rhythms (arrhythmias). The devices were developed originally to correct heart rhythms that are too fast, but recent technological advances have increased the pool of possible patients who may benefit from an ICD. 

ICDs are mainly used to treat two forms of abnormal heart rhythms, both of which occur in the ventricles, or lower pumping chambers of the heart. If the ventricles begin to beat too quickly (ventricular tachycardia), the device may emit low-energy electrical pulses that allow the heart to regain its normal rhythm. If the tachycardia progresses to a very rapid, life-threatening rhythm that causes the ventricles to quiver rather than beat (ventricular fibrillation), the device may deliver a relatively stronger jolt to reset the heart rate (defibrillation).

In recent years, ICDs have also been developed that can treat additional heart rhythms. For example, if the heart rhythm becomes too slow (bradycardia), the ICD may function like a traditional pacemaker, emitting pulses that will increase the heart rate. ICDs can also pace the heart in the event of a mildly elevated heart rate. The ICDs response to any situation depends on how it has been programmed. Finally, modern ICDs can record abnormal heart rates and keep a record of any activity. This record can be viewed by the physician to help diagnose underlying heart problems.

Most ICDs are implanted in patients who have suffered from sustained ventricular tachycardia, ventricular fibrillation and/or sudden cardiac death, which occurs when the heart stops beating. If constant pacing is needed, the physician will likely implant a traditional pacemaker. ICDs are not designed to provide constant pacing and doing so will cause their batteries to wear down faster. In turn, the device will have to replaced earlier.

An appropriate ICD is chosen and programmed by the physician according to the patient's needs. The device is implanted into the chest of the patient during a minor surgical procedure (not open-heart surgery). A short stay in the hospital is usually required, and some patients may need to take medications that help the heart maintain a normal heart rhythm (antiarrhythmics). Once the ICD is in place, it runs on batteries for about four to seven years, depending on how often an electric shock is discharged. ICD batteries will not run out unexpectedly. During a routine office visit, physicians can detect when the battery is running low.