Also known as Automated External Defibrillator (AED), Implantable Cardioverter Defibrillator (ICD), Wearable Cardioverter Defibrillator (WCD)
Defibrillators are devices that restore a normal heartbeat by sending an electric pulse or shock to the heart. They are used to prevent or correct an arrhythmia, a heartbeat that is uneven or that is too slow or too fast. Defibrillators can also restore the heart’s beating if the heart suddenly stops.

Different types of defibrillators work in different ways. Automated external defibrillators (AEDs), which are in many public spaces, were developed to save the lives of people experiencing sudden cardiac arrest. Even untrained bystanders can use these devices in an emergency.

Other defibrillators can prevent sudden death among people who have a high risk of a life-threatening arrhythmia. They include implantable cardioverter defibrillators (ICDs), which are surgically placed inside your body, and wearable cardioverter defibrillators (WCDs), which rest on the body. It can take time and effort to get used to living with a defibrillator, and it is important to be aware of possible risks and complications.

Explore this Health Topic to learn more about defibrillators, our role in research and clinical trials to improve health, and where to find more information.

How They Work - Defibrillators

There are three types of defibrillators: AEDs, ICDs, and WCDs. Each type works by checking for arrhythmias, or irregular heart rhythms. Once detected, each defibrillator will send a shock to restore a normal rhythm. Learn more about how the three types of defibrillators work.

It may also help to understand how the heart works.

How do AEDs work?
- Defibrillators

An AED is a lightweight, battery-operated, portable device that checks the heart’s rhythm and sends a shock to the heart to restore a normal rhythm. The device is used to help people having sudden cardiac arrest.

Sticky pads with sensors, called electrodes, are attached to the chest of someone who is having cardiac arrest. The electrodes send information about the person's heart rhythm to a computer in the AED. The computer analyzes the heart rhythm to find out whether an electric shock is needed. If needed, the electrodes deliver the shock.

Image of an automated external defibrillator in use.
Image of an automated external defibrillator in use. The image shows a typical setup using an automated external defibrillator (AED). The AED has step-by-step instructions and voice prompts that enable an untrained bystander to use the machine correctly.


How do ICDs work?
- Defibrillators

ICDs are placed surgically in the chest or abdomen, where it checks for arrhythmias. Arrhythmias can interrupt the flow of blood from your heart to the rest of your body or cause your heart to stop. The ICD sends a shock to correct the arrhythmia.

An ICD can give off a low-energy shock to speed up or slow down an abnormal heart rate or a high-energy shock, which can correct a fast or irregular heartbeat. If the low-energy shocks do not restore your normal heart rhythm, the device will switch to high-energy shocks for defibrillation. The device also will switch to high-energy shocks if your ventricles start to quiver rather than contract strongly. ICDs are similar to pacemakers, but pacemakers deliver only low-energy electrical shocks.

ICDs have a generator connected to wires to detect your heart’s pulses and deliver a shock when needed. Some models have wires that rest in one or two chambers of the heart. Others do not have wires threaded into the heart chambers but rest on the heart to monitor its rhythm.

The ICD can also record the heart's electrical activity and heart rhythms. The recordings can help your doctor fine-tune the programming of your device so it works better to correct irregular heartbeats. Your device will be programmed to respond to the type of arrhythmia you are most likely to have.

Comparison of an implantable cardioverter defibrillator and a pacemaker.
Comparison of an implantable cardioverter defibrillator and a pacemaker. The image compares an ICD with a pacemaker. Figure A shows the location and general size of an ICD 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 pacemaker in the upper chest. The wires with electrodes on the ends are inserted into the heart through a vein in the upper chest.


How do WCDs work?
- Defibrillators

WCDs have sensors that attach to the skin. They are connected by wires to a unit that checks your heart’s rhythm and delivers shocks when needed. Like an ICD, the WCD can deliver low- and high-energy shocks. The device has a belt attached to a vest and is worn under your clothes. Your doctor will fit the device to your size. The device is programmed to detect a particular heart rhythm.

The sensors detect when an arrhythmia occurs and notifies you with an alert. You can turn off the alert to prevent a shock if not needed, but if you do not respond, the device will administer a shock to correct the rhythm. Typically, this happens within one minute. The device can deliver repeated shocks during an episode. After each episode, the sensors must be replaced.

The device can also send a record of your heart’s activity to your doctors.

Who Needs Them? - Defibrillators

Defibrillators can be used in children, teens, and adults. AEDs are used to treat sudden cardiac arrest. Your doctor may recommend an ICD or WCD to treat an arrhythmia and prevent new or repeat sudden cardiac arrests.

Who needs an AED?
- Defibrillators

AEDs can save the life of someone having sudden cardiac arrest, when the heart suddenly and unexpectedly stops beating.

AEDs can be used for adults, as well as for children as young as 1 year old. Some devices have pads and cables designed especially for children.

Doing cardiopulmonary resuscitation, or CPR, on someone having sudden cardiac arrest also can improve his or her chance of survival. Learn more about using an AED in an emergency.

Who needs an ICD?
- Defibrillators

ICDs can correct a dangerous arrhythmia or keep an irregular heartbeat from triggering sudden cardiac arrest. Life-threatening arrhythmias can develop for many reasons and can affect people of any age, from newborns to older adults. Your doctor may recommend an ICD if you have a type of arrhythmia that causes your heart’s ventricles to quiver instead of pumping blood. This type of arrhythmia is most likely to cause sudden cardiac arrest.

If you have the following conditions, you may be at risk for a life-threatening arrhythmia and your doctor may recommend an ICD:

  • You survived sudden cardiac arrest.
  • You developed an arrhythmia during or after treatment for a heart attack.
  • You have a genetics condition that causes arrhythmia. This includes having congenital heart disease or an inherited conduction disorder.
  • You have a neuromuscular disorder. For example, the progression of muscular dystrophy can damage the heart and cause unpredictable heart rhythms. This can lead to unexplained fainting and a high risk of death.
  • You have an abnormally slow heart rate or other problem with the heart’s electrical signals.
  • You have cardiac sarcoidosis.
  • You have poor heart function following a procedure to improve blood flow.
  • Your doctor detected an arrhythmia during an electrocardiogram (EKG) or stress test. If this happened several times, you may be at increased risk.

Who needs a WCD?
- Defibrillators

WCDs are used to protect against sudden cardiac arrest in certain circumstances, such as if you are at risk of arrhythmia for just a short time. This might occur under these conditions:

  • You are recovering from a heart attack.
  • You are waiting for a heart transplant.
  • You are fighting an infection.
  • You are removing or waiting to replace your ICD.

Using an AED in an Emergency - Defibrillators

AEDs are found in many public spaces. They may be used in an emergency to help someone who is experiencing sudden cardiac arrest. Learn how to recognize sudden cardiac arrest emergencies—when you might use an AED, how to find an AED if you need one, and how to use an AED until help arrives.

When to use an AED
- Defibrillators

A person whose heart stops from sudden cardiac arrest must get help within 10 minutes to survive. Fainting is usually the first sign of sudden cardiac arrest. If you think someone may be in cardiac arrest, try the following steps:

  • If you see a person faint or if you find a person already unconscious, first confirm that the person cannot respond. The person may not move, or his or her movements may look like a seizure.
  • You can shout at or gently shake the person to make sure he or she is not sleeping, but never shake an infant or young child. Instead, you can gently pinch the child to try to wake him or her up.
  • Check the person’s breathing and pulse. If the person is not breathing and has no pulse or has an irregular heartbeat, prepare to use the AED as soon as possible.

Where to find an AED
- Defibrillators

You often find AEDs in places with large numbers of people, such as shopping malls, golf courses, gyms and swimming pools, businesses, airports, hotels, sports venues, and schools. You can also purchase a home-use AED.

The AED is in a case about the size of a large first-aid kit. Many AEDs have a heart logo in red or green. Large letters on the case or the wall where it is stored might spell out A–E–D.

How to use an AED
- Defibrillators

Even someone without special training can respond in an emergency by following the instructions relayed by the device. If someone is having sudden cardiac arrest, using an AED and giving CPR can save that person’s life. When using an AED:

  • Call 9-1-1 or have someone else call 9-1-1. If two rescuers are present, one can provide CPR while the other calls 9-1-1 and gets the AED.
  • Make sure the area around the person is clear; touching the person could interfere with the AED’s reading of the person’s heart.
  • If an electric pulse or shock is needed to restore a normal rhythm, the AED uses voice prompts to tell you when and how to give the shock, and electrodes deliver it. Some AEDs can deliver more than one shock with increasing energy.
  • The device may instruct you to start CPR again after delivering the shock.

Surgery for ICDs - Defibrillators

If your doctor recommends an ICD to treat an arrhythmia, your healthcare team will help you prepare for surgery. Learn about what to expect from the surgery and during the early days of your recovery. As with any surgery, there are risks involved.

Before and during surgery
- Defibrillators

You may have questions about your condition and whether an ICD is right for you. Discuss them with your doctor. You may be able to choose between different ICD models with different benefits and risks. Your doctor will help you make the decision that is right for you.

Placing an ICD requires minor surgery, which usually is done in a hospital. Your doctor will discuss the procedure with you. This is a good time to ask questions.

Before the surgery, members of your healthcare team will give you medicines to relax you and to numb the area where the device will go. They may also give you antibiotics to prevent infections.

Typically, the ICD is placed under your breastbone or along your ribs. In infants, it can be placed in the abdomen. With some devices, your doctor may first thread one or two sensor wires through your blood vessels into the chambers of your heart. With others, a single sensor wire is placed along the breastbone. The doctors will use a monitor to guide the wires and put them in the right place.

Once the device is in place, your doctor will test it. Then your doctor will sew up the cut. The entire surgery takes a few hours.

Recovery from surgery
- Defibrillators

You may be able to leave the hospital once the medicines you received for the surgery wear off. You can then continue your recovery at home.

Follow the instructions you receive. Your healthcare team may tell you to take these steps:

  • Check the cut on your chest often and keep the area clean and dry.
  • Call your doctor if any swelling or bleeding occurs or if you develop a fever.
  • Take over-the-counter pain medicines such as acetaminophen if you feel pain. But talk to your doctor first; he or she may tell you to avoid taking ibuprofen or other kinds of pain medicines, for example.
  • Ask your doctor when you can resume taking medicines that you took before the surgery, how soon you can take a shower, and when you can return to work. You will probably have to avoid driving for at least a week while you recover from your surgery. Your doctor may also ask you to avoid high-impact activities and heavy lifting for about a month.

At your next scheduled appointment, ask your doctor about living with an ICD and what to do when you feel an electric pulse or shock from your device.

As with any surgery, there are some risks related to the surgery used to place an ICD. Although they are rare, possible complications include:

  • A bad reaction to the medicine used to make you relax or sleep during the surgery
  • A collapsed lung
  • A defibrillator wire puncturing the heart or a vessel
  • Bleeding from the site where the device was placed
  • Blood vessel, heart, or nerve damage
  • Swelling, bruising, or infection at the area where the device was placed
  • Venous thromboembolism

Some ICD models have a lower risk of clots, puncture, and infection.

Look for
- Defibrillators

Living With - Defibrillators

After sudden cardiac arrest, surgery to implant an ICD, or a fitting for a WCD, you will need regular visits with your doctor to check your condition, the device, or any medicines you are taking. You can get an ID card with information about your device to keep with you. It can be helpful in an emergency, to show airport security, or for medical personnel who need to know more about your device. If you have an ICD, be aware of the signs of complications with your device.

What to expect from electric shocks
- Defibrillators

It takes time to get used to having an ICD or WCD. These devices can deliver electrical energy at different strengths to regulate your heart. Read more about how ICDs work.

  • Low-energy shocks. The low-energy electrical shocks your device gives are not painful. You may not notice them, or you may feel a fluttering in your chest.
  • High-energy shocks. The high-energy shocks last only a fraction of a second, but they can be strong or painful. They may feel like thumping or a kick in the chest, depending on their strength. Before a shock, you may feel arrhythmia symptoms. If you feel one or two strong shocks over a short period and the symptoms go away, it may be a sign that the device is working. Notify your doctor’s office and set up an appointment that day or the next day. He or she will want to assess your condition and the device.
  • Unnecessary shocks. During the adjustment period after your surgery, your device may deliver a shock when it is not needed. A damaged wire or a very fast heart rate due to extreme physical activity may trigger unnecessary shocks. These shocks can also occur if you forget to take your medicines. Some people also feel phantom shocks, even when the device does not detect an arrhythmia.

Make sure your WCD is fitted properly
- Defibrillators

You will be wearing it all the time, except when you are taking a shower or bath. Like shocks from ICDs, shocks from WCDs can be painful and sometimes cause burns. Your doctor will explain how to charge and change the batteries in your WCD.

Return to normal daily activities
- Defibrillators

Talk to your doctor about when you can return to normal activities, such as physical activity and driving.

  • Physical activity. An ICD usually will not limit you from taking part in sports and exercise, including strenuous activities. You may need to avoid full-contact sports, such as football. Contact sports can damage your ICD or shake loose the wires in your heart. Ask your doctor how much and what types of physical activity are safe for you.
  • Driving. You probably will be able to resume your typical driving patterns after you recover from surgery. However, if you received an ICD to prevent another sudden cardiac arrest or ventricular arrhythmia, it will probably be several months before your doctor says you are ready to drive again. This is because of the risks of fainting or getting a shock from your device. Your doctor may also suggest driving restrictions based on the activity recorded by your device.

Receive routine follow-up care
- Defibrillators

Once you have an ICD, you will visit your doctor at least every six months. At these visits, your doctor will examine the area where the device was implanted and check information about your heart rhythm that was recorded by the device. In between visits, your doctor can monitor data from your device remotely. If it is needed, he or she can adjust the device’s settings.

At the follow-up visits, your doctor may also take these steps:

  • Prescribe or adjust your medicines to decrease the number of irregular heartbeats you have. Fewer irregular beats will mean fewer high-energy shocks that have to be sent to your heart.
  • Check to make sure the device continues to work properly and that it has not shifted in your body or caused irritation or injury. Over time, your ICD may stop working well because its wires get dislodged or broken, its battery fails, your heart disease progresses, or other devices have disrupted its electrical signaling. Your doctor also may recommend an electrocardiogram (EKG) to check for changes in your heart’s electrical activity.
  • Check to see whether you are at risk of heart failure. If device and medicine adjustments do not reduce your irregular heart rhythms, your doctor may suggest a procedure called ablation to stop excess electrical signals in your heart.
  • Check to see whether the battery needs to be replaced. Batteries in ICDs last between five and seven years. When the batteries in your device run down, you will need surgery to replace them. Replacing the battery is less involved than the original surgery to implant the ICD. Ask your doctor whether the device generator or its wires need to be replaced, too.

Manage devices that can interfere with your ICD
- Defibrillators

Electrical devices and devices with strong magnetic fields can interfere with your ICD. You may not be able to sense when this happens. If something disrupts your ICD, step away from the disruptive device to help restore its normal working. To be safe, keep your ICD at least six inches away from the following devices, or, when necessary, use them only briefly:

  • Cell phones. If you have an ICD on the left side of your chest, hold your cell phone to your right ear.
  • Headphones. Most headphones have a magnetic element in them. Wear them as far away from your ICD as possible, and do not carry your headphones in a chest pocket.
  • Metal detectors, such as those used for airport security. The risk of harm is low, but you can show your ID card and ask for alternative inspection.

Some medical procedures, including magnetic resonance imaging (MRI), can damage ICDs. This depends on the procedure and your device model. For example, some people with newer models can undergo MRI. Tell your dentist or doctor that you have an ICD and discuss your options with them. You can show them the card with information about your device.

Learn the warning signs of complications and make a plan
- Defibrillators

It is possible for a wire in an ICD to fire at the wrong time or become infected. Call your doctor if you have signs of symptoms that concern you, and if you have these signs in particular:

  • Fainting
  • Dizziness or feeling out of breath
  • Fever
  • Heart palpitations or chest pain

Go to a hospital emergency room if you feel many strong shocks from your device in a short time.

Occasionally an ICD does not work properly and cannot be fixed with changes in programming or medicine. If this happens, your doctor might have to replace the ICD. When you visit the doctor, ask whether the manufacturer has announced any problems with your device.

Tend to your emotional health
- Defibrillators

If you have survived sudden cardiac arrest, you may worry about whether it will happen again. After surgery to implant an ICD or fitting for a WCD, adjusting to the uncertainty of life with the device may cause fear, anxiety, and stress. When your device sends a strong shock, it may startle or distress you or cause you pain. This can make some people stop doing the things they used to do. After having sudden cardiac arrest or getting an ICD, some people report anxiety, depression, or other more serious emotional health concerns.

If you have survived cardiac arrest or are living with an ICD or WCD and experience anxiety or depression, tell your doctor about your concerns.

- Defibrillators

  • Return to Who Needs Them? to review the health conditions that increase your risk of needing a defibrillator.

Research for Your Health

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discovery to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders, including defibrillators. Learn about the current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research
- Defibrillators

Learn about the following ways NHLBI continues to translate research and science into improved health for people who need defibrillators. Research on this topic is part of the NHLBI’s broader commitment to advancing heart and vascular disease scientific discovery.

  • Advancing training in emergency care. The NHLBI helped establish the first trans-NIH training program in emergency care at four sites. This program will support young investigators who are committed to research careers in emergency cardiovascular medicine.
  • Funding advances in emergency response. The Resuscitation Outcomes Consortium (ROC) is a clinical trial network that tests treatments to address high rates of injury and death from out-of-hospital cardiac arrest and severe traumatic injury. Researchers are comparing how emergency response teams transport patients to the hospital to look for ways to improve outcomes. A registry of sudden cardiac arrests that ROC established has helped track important information about these events. In 2018, ROC data helped show that more patients survive sudden cardiac arrest in public spaces when bystanders use an AED while waiting for a standard emergency response. In addition, patient outcomes were better when bystanders used an AED.
  • Promoting a clinical trial network to address emergency medicine. The trans-NIH Network for Emergency Care Clinical Trials: Strategies to Innovate EmeRgENcy Care Clinical Trials Network (SIREN), in partnership with the Department of Defense, will conduct clinical trials to improve outcomes in emergency departments and pre-hospital settings.
  • Helping show how AEDs can save lives. Our Public Access Defibrillation trial helped show the value of having AEDs in public spaces by showing they could increase survival among people experiencing sudden cardiac arrest. Researchers found that more people experiencing sudden cardiac arrest survived when teams of volunteers trained in CPR and AED use responded, compared with volunteer responders trained in CPR only.
  • Informing guidelines with evidence from the Sudden Cardiac Death in Heart Failure Trial. We funded research that showed that for patients with mild or moderate congestive heart failure and a weakly pumping heart, conventional heart failure treatment paired with a simple ICD therapy is more effective than the conventional treatment alone. Survival rates were 23 percent higher among patients with an ICD.
  • Supporting heart failure research collaboration. The NHLBI-supported Heart Failure Clinical Research Network (HFN) conducts clinical trials to evaluate treatments for acute and chronic heart failure. The HFN brings together nine Regional Coordinating Centers and additional clinical sites in the United States to form a collaborative platform to research strategies that address the increasing public health burden of heart failure.
  • Assessing optimal use of AEDs. An NHLBI-supported clinical trial investigated the use of AEDs placed in the homes of patients who had experienced a heart attack. Although the findings showed that having access to an AED in the home reassured participants and their spouses or companions, making the AED, CPR, and Emergency Medical Services (EMS) available did not improve survival among those who experienced sudden cardiac arrest at home, compared with CPR and EMS alone. In many cases, the patient’s spouse or companion did not witness the cardiac arrest.

Advancing research for improved health
- Defibrillators

In support of our mission, we are committed to advancing research for people who can benefit from defibrillators in part through the following ways.

  • We perform research. Our Division of Intramural Research (DIR) and its Cardiovascular Branch conduct research on diseases that affect the heart and blood vessels. Specific projects aim to answer clinically relevant questions in diagnostics, therapeutics, and interventions.
  • We fund research. Our Division of Cardiovascular Sciences (DCVS) supports research on arrhythmias and the science of resuscitation as part of the Adult and Pediatric Cardiac Research Program. The Heart Failure and Arrhythmia Branch within the DCVS supports research to advance our understanding of and interventions for pediatric and adult cardiovascular diseases. We also support the development of innovative technologies to diagnose, prevent, and treat heart and vascular diseases. The Center for Translation Research and Implementation Science supports research to translate these discoveries into clinical practice. Search the NIH RePORTer to learn about research that the NHLBI is funding on defibrillators.
  • We stimulate high-impact research. Our Trans-Omics for Precision Medicine (TOPMed) program includes participants with atrial fibrillation, which is a common type of arrhythmia that can be treated with an ICD. Findings from TOPMed may help us understand how genes contribute to the development of atrial fibrillation—for example, in women and in patients who have no underlying heart disease. The NHLBI Strategic Vision highlights ways we may support research over the next decade.

Learn more about exciting research areas the NHLBI is exploring on various heart treatments, including ICDs.

Participate in NHLBI Clinical Trials

We lead or sponsor many studies relevant to defibrillator use. See whether you or someone you know is eligible to participate in our clinical trials.

Are you considering getting or replacing an implantable cardioverter defibrillator?

Implantable cardioverter defibrillators (ICDs) can present patients with many stressful decisions, and this study tests the effectiveness of educational videos and handouts to inform patients about ICDs and help them assess their options. To participate in this study, your doctor must have given you the option of implanting or replacing an ICD. The study is located in Denver, Colorado.

Are you an adult living in Beijing, China, with a heart conduction disorder?

This study will investigate health outcomes among people with a heart conduction disorder that is sometimes treated with an implantable cardioverter defibrillator. To participate in this study, you must be between 18 and 60 years old and have complete left bundle branch block. The study takes place in Beijing, China.

Do you have heart disease that requires an ICD to prevent sudden cardiac arrest?

This study aims to learn what signs distinguish patients who have a higher risk of an arrhythmia that leads to sudden cardiac arrest. Researchers plan to track patients with a recent ICD replacement for 10 years. To participate in this study, you must be between 18 and 85 years old. This study is located in Washington, D.C.; Baltimore, Maryland; and Richmond, Virginia.

Do you have a ventricular arrhythmia?

This study aims to evaluate the safety and effectiveness of low-energy shocks for the treatment of ventricular tachycardia and ventricular fibrillation. This approach will be tested during a catheter ablation or placement of an implantable cardioverter defibrillator (ICD). To participate in this study, you must be between 18 and 75 years old. This study is located in Minneapolis, Minnesota, and Austin, Texas.

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