How does an implantable cardioverter defibrillator (ICD) work?

An implantable cardioverter defibrillator (ICD) is a small, electronic device that continuously monitors the electrical activity of the heart. It is about the size of a stop watch and is usually inserted under the skin in the upper chest. It consists of a pulse generator and wires, called leads. The pulse generator contains the battery and a tiny computer. One or more lead wires connect the pulse generator to specific locations in the heart. Although an ICD is a small device, it has many life-saving functions. Four key functions are anti-tachycardia pacing (ATP), cardioversion (light shock), defibrillation (stronger shock), and backup pacing.

ATP involves rapid regular pacing impulses delivered in order to correct and restore normal rhythm by overtaking a fast irregular rhythm. Most people are unaware when this therapy is being delivered.

Cardioversion (light shock) impulses are the next level of therapy. This light shock can generally be felt but not strongly. Defibrillation, which is described by many as feeling like a "kick in the chest," is the strongest therapy that an ICD can be programmed to deliver.

The ICD can also be programmed to function as a basic pacemaker as needed. Sometimes after a shock is delivered, the heart may beat too slowly. The ICD has a back-up pacemaker, which can stimulate the heart to beat faster until the normal heart rhythm returns. Additionally, an ICD can act as a pacemaker not only after a shock is delivered but also any time the heart rate drops below a pre-programmed rate.

These therapies (ATP, cardioversion, defibrillation, and basic pacing) are programmed by your physician. Generally, the mildest form of treatment (ATP) is delivered first, and then additional treatments (cardioversion or defibrillation) are added if the others do not restore the normal heart rhythm. Your physician will program your ICD to your specific needs.
Once the implantable cardioverter defibrillator (ICD) senses that you’re having a heart rhythm problem, the pulse generator sends a signal to correct it.
  • Pacing signals correct a heartbeat that is a bit too fast or too slow. You will probably not notice these signals.
  • Cardioversion is a mild shock used if ventricular tachycardia (fast heartbeat) continues even after the pacing signals are sent. This can feel like a sudden thump in the chest that lasts for just a moment.
  • Defibrillation is a strong shock that is sent if your heart goes into ventricular fibrillation. This sudden shock can feel like being punched in the chest. The shock may be surprising or briefly painful, but it is intended to save your life.
The ICD can also record your heart’s electrical activity, so that the doctor can adjust its settings during follow-up appointments.
Indrajit Choudhuri, MD
Cardiology (Cardiovascular Disease)
During your electrophysiology (EP) study, or on an electrocardiogram (ECG) or other type of rhythm strip, your doctor has learned which type of arrhythmia you have and whether there is a fast heart rate (tachycardia). When your cardioverter defibrillator is implanted, it will be programmed to detect this rate. When it does, it will try to stop the rhythm in one of three ways. Your electrophysiologist will choose which methods are best for your rhythm problem.

The three ways are:
  • Pacing (called antitachycardia or fast pacing) -- The device sends
           small, rapid electrical impulses to your heart. These impulses
           will try to stop your abnormal rhythm and let the normal heart
           rhythm return. You will probably not feel these pacing
  • Cardioversion -- The device sends a low-energy shock. One or
           more of these shocks can be sent to convert your fast heart
           rhythm to a more normal rhythm. You will be aware of the
           shock even though it is at a low energy level. It may feel like
           a jolt in the chest.
  • Defibrillation -- The device may send a high-energy shock to your
           heart. This shock is needed if the low-energy shock is not
           successful or if your heart goes into ventricular fibrillation
           (VF). You will feel this shock -- it feels like a kick or a jolt in
           the chest
Piedmont Heart Institute
An implantable cardioverter defibrillator (ICD) has wires with electrodes on the ends that connect to one or more of your heart's chambers. These wires carry the electrical signals from your heart to a computer in the ICD. The computer monitors your heart rhythm.
If the ICD detects an irregular rhythm, it sends low-energy electrical pulses to prompt your heart to beat at a normal rate. If the low-energy pulses restore your heart's normal rhythm, you may avoid the high-energy pulses or shocks of the defibrillator (which can be painful).
Single-chamber ICDs have a wire that connects to either the right atrium or right ventricle. The wire senses electrical activity and corrects faulty electrical signaling within that chamber.
Dual-chamber ICDs have wires that connect to both an atrium and a ventricle. These ICDs provide low-energy pulses to either or both chambers. Some dual-chamber ICDs have three wires. They connect to an atrium and both ventricles.
The wires on an ICD connect to a small metal box implanted in your chest or abdomen. The box contains a battery, pulse generator, and computer. When the computer detects irregular heartbeats, it triggers the ICD's pulse generator to send electrical pulses. Wires carry these pulses to the heart.
The ICD also can record the heart's electrical activity and heart rhythms. The recordings can help your doctor fine-tune the programming of your ICD so it works better to correct irregular heartbeats.
This answer from the National Heart, Lung and Blood Institute has been reviewed and/or edited by Dr. William D. Knopf.

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