Health Library Explorer

Overview of Pacemakers and Implantable Cardioverter Defibrillators (ICDs)

What is a pacemaker?

A pacemaker is a small device implanted in the chest. It sends electrical signals to start or regulate a slow heartbeat. It's most often placed in the chest just under the collarbone. A pacemaker may be used if the heart's natural pacemaker (the SA node) is not working properly causing a slow heart rate or rhythm, or if the electrical pathways are blocked.

Illustration of a pacemaker device and the location where it is placed in the chest
Click Image to Enlarge

Another type of pacemaker is called a biventricular pacemaker. It is used for ventricles that don't contract at the same time. This can make heart failure worse. A biventricular pacemaker paces both ventricles at the same time. This increases the amount of blood pumped by the heart. This treatment is called cardiac resynchronization therapy (CRT).

What is an implantable cardioverter defibrillator (ICD)?

An implantable cardioverter defibrillator (ICD) looks similar to a pacemaker, though slightly larger. It works very much like a pacemaker. But the ICD can send a low-energy shock that resets an abnormal heartbeat back to a normal. It can also send a high-energy shock if an arrhythmia becomes so severe that the heart can't pump at all.

Illustration showing the ICD device and the location when placed in the chest
Click Image to Enlarge

Many devices combine a pacemaker and ICD in one unit for people who need both functions. After the shock is delivered, a "back-up" pacing mode is available if needed for a short while.

The ICD has another type of treatment for certain fast rhythms called anti-tachycardia pacing (ATP). This is a fast-pacing impulse sent to correct the rhythm. This can be used instead of shocking the heart in some cases.

What is the reason for getting a pacemaker or an ICD?

Pacemakers are most often used when your heart beats too slowly. ICDs are advised if you are at risk for fatal ventricular arrhythmias. There may be other reasons for your healthcare provider to advise placement of a pacemaker or ICD.

When your heart's natural pacemaker or electrical circuit malfunctions, the signals sent out may become erratic. This signal may be either too slow, too fast, or too irregular (arrhythmia).

Arrhythmias can cause problems with contractions of the heart chambers by:

  • Not letting your heart chambers fill with enough blood because the heart pumps too fast.

  • Not letting enough blood be pumped out to your body because your heart pumps too slowly or too irregularly.

What are the components of a pacemaker/ICD?

A pacemaker or ICD has 3 main parts:

  • A pulse generator with a sealed lithium battery. It makes the electrical signals that make the heart beat. Most can also receive and respond to signals sent by the heart itself.

  • One or more wires (leads). Leads are insulated flexible wires. They conduct electrical signals between the heart and the pulse generator. One end of the lead is attached to the pulse generator and the electrode end of the lead is positioned in the heart. In the case of a biventricular pacemaker, leads are placed on both ventricles.

  • Electrodes. These are found on each lead.

There is also a new leadless pacemaker that combines the pulse generator and the electrode into a single unit. This has its limits and is only used in certain cases.

Pacemakers can "sense" when the heart's natural rate falls below the rate that has been programmed into the pacemaker.


  • Pacemakers that pace both the right atrium and right ventricle of the heart and require 2 pacing leads are called "dual-chamber" pacemakers.


How is a pacemaker/ICD implanted?

X-ray of a single-chamber implanted pacemaker

Your doctor will insert the pacemaker or ICD in the cardiac catheterization, or the electrophysiology lab. You are somewhat awake during the procedure. But your doctor will provide local anesthesia over the incision site. He or she will also sedate you to help you relax during the procedure. You may need to spend the night in the hospital so that your doctor can make sure the device is working properly.

Shown in the photo is a chest X-ray. The large white space in the middle is the heart. The dark spaces on either side are the lungs. The small object in the upper right corner is an implanted pacemaker.

Your doctor will make a small incision just under the collarbone. He or she will insert the pacemaker/ICD lead(s) into the heart through a blood vessel that runs under the collarbone. Once the lead is in place, your doctor can test it to make sure it is in the right place and is working. He or she then attaches the lead to the generator (battery), which is placed just under the skin through the incision made earlier. Your doctor will close the incision with stitches, staples, or a medical glue (adhesive) and apply a dressing. Once the procedure has been completed, you will go through a recovery period or several hours.

You will need to follow certain instructions when you have an implanted pacemaker or ICD. For example, after you get your pacemaker or ICD, you will get an ID card from the manufacturer that includes information about your specific model of pacemaker and the serial number. The card will also show how the device works. Carry this card with you at all times so that the information is always available to any healthcare provider who may examine or treat you.

After you have a pacemaker or an ICD implanted, make sure you ask and understand the instructions from your healthcare provider about:

  • Activity limits

  • Driving

  • Medicines you should be taking

  • Any precautions you should follow with your device

  • How often you should have follow-up appointments

  • How your device will be monitored

  • Symptoms you should report to your healthcare provider after the procedure

Online Medical Reviewer: Lu Cunningham
Online Medical Reviewer: Quinn Goeringer PA-C
Online Medical Reviewer: Steven Kang MD
Date Last Reviewed: 3/1/2019