What Is an Implantable Cardioverter Defibrillator?

An implantable cardioverter defibrillator (ICD) is a small device that's placed in your chest or abdomen. This device uses electrical pulses or shocks to help control life-threatening, irregular heartbeats, especially those that could lead the heart to suddenly stop beating (sudden cardiac arrest). If the heart stops beating, blood stops flowing to the brain and other vital organs. This usually causes death if it's not treated in minutes.

The Heart's Electrical System

Your heart has its own internal electrical system that controls the speed and rhythm of your heartbeat. With each heartbeat, an electrical signal spreads from the top of the heart to the bottom. As it travels, the electrical signal causes the heart to contract and pump blood.

Electrical signals normally begin in a group of cells called the sinus node. As these signals spread from the top to the bottom of the heart, they coordinate the timing of heart cell activity. First, the two upper chambers of the heart, called atria (AY-tree-uh), contract. This contraction squeezes blood into the lower chambers of the heart, which are called ventricles (VEN-trih-kuls). The ventricles then contract and send blood to the rest of the body. The combined contraction of the atria and ventricles is a heartbeat.

For more information about the heart's electrical system and how a healthy heart works, see the Diseases and Conditions Index article on How the Heart Works.

Overview

A problem with any part of the electrical system process can cause irregular heartbeats called arrhythmias (ah-RITH-me-ahs). During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm. Faulty electrical signaling in the heart causes arrhythmias.

ICDs use electrical pulses or shocks to treat life-threatening arrhythmias that occur in the ventricles (lower chambers of the heart).

When these ventricular arrhythmias occur, the heart can't effectively pump blood, and a person will pass out within seconds and die within minutes if not treated. To prevent death, the condition must be treated right away with an electric shock to the heart. This treatment is called defibrillation.

An ICD has wires with electrodes on the ends that connect to the chambers of your heart. The ICD will continually monitor your heart rhythm. When the device detects that you're having an irregular rhythm in your ventricles, the ICD will use low-energy electrical pulses to restore a normal rhythm. If this doesn't restore your normal heart rhythm, or if your ventricles start to quiver rather than contract strongly, the ICD will switch to high-energy electrical pulses for defibrillation.

Doctors also treat irregular heartbeats with another device called a pacemaker. An ICD is similar to a pacemaker, but there are some differences. Pacemakers can only give off low-energy electrical pulses. They are often used to treat less dangerous heart rhythms, such as those that occur in the upper chambers of your heart. Most new ICDs can act as both pacemakers and ICDs.

Comparison of an Implantable Cardioverter Defibrillator
and a Pacemaker

The illustration compares an implantable cardioverter defibrillator and a pacemaker. Figure A shows the location and general size of an implantable cardiac defibrillator in the upper chest. The wires with electrodes on the ends are inserted into the heart through a vein in the upper chest. Figure B shows the location and general size of a double-electrode pacemaker in the upper chest. The wires with electrodes on the ends are inserted into the heart through a vein in the upper chest.