An implantable cardioverter defibrillator (ICD) is a device that is implanted in the chest to monitor and, if necessary, correct episodes of an abnormal heart rhythm (arrhythmia). The implantable cardioverter defibrillator gets its name from the two functions that it performs.

In the event of an abnormally fast heart rhythm in the ventricles (ventricular tachycardia), the ICD may emit low-energy shocks to convert the heart rhythm to a more normal rate. This is called cardioversion. If the ventricular tachycardia progresses to a very rapid, life-threatening heart rhythm in which the ventricles quiver instead of beat (ventricular fibrillation), the ICD may emit a relatively larger jolt of electricity to “reset” the heart rate. This is called defibrillation. Ventricular fibrillation usually occurs in the presence of underlying heart disease, such as coronary artery disease, and is a leading cause of sudden cardiac death.

Although the main functions of the ICD are cardioversion and defibrillation, it can also be programmed to do the following:

• Tachycardia pacing. When an ICD senses a fast but rhythmic heartbeat (tachycardia), it releases a series of precisely timed low-intensity electrical pulses that gently interrupt the heart and allow it to return to a slower pace. Whereas both cardioversion and defibrillation involve shocks that may feel like a sudden kick in the chest, these low-intensity stimuli from an ICD are generally not felt by the patient.

• Bradycardia pacing. Like a pacemaker, the ICD can sense an abnormally slow heartbeat (bradycardia) and send small electrical signals to pace the heart until it restores and maintains a normal heart rate. ICDs are not used for patients who need constant bradycardia pacing. Rather, they are used for patients who have multiple heart rhythm problems that include bradycardia.

The ICD also records heart activity and can transmit this information to the physician during a routine check, allowing the physician to better diagnose and monitor the underlying conditions causing the patient’s arrhythmia. These checks can be performed in a physician’s office or even over the telephone (transtelephonic monitoring).