How to Use an Automated External Defibrillator (AED)

You’ve probably seen them in public places like the gym, the mall, or the airport, but what are AEDs? Automated external defibrillators (AEDs) are pre-programmed devices that can help you save a life in an emergency. When someone suffers sudden cardiac arrest in public, an AED can help bystanders like you save their life.

Michelle O’Donoghue, MD, MPH, explains what an AED is, when to use an AED, and the step-by-step instructions to use an AED in an emergency.

Dr. O’Donoghue is a Mass General Brigham Heart and Vascular Institute cardiologist and leader in heart disease research who treats patients at Brigham and Women’s Hospital.

What is an automated external defibrillator (AED)?

An AED is a portable version of the defibrillator devices medical professionals use in hospitals and medical offices. If your heart stops, a defibrillator can resuscitate you by delivering a controlled electrical shock. An AED automatically reads the heart rhythm of a person having cardiac arrest and can deliver a shock to restore the heart rhythm.

In public places, AEDs are often stored in white or green boxes attached to the wall. They may look like a first aid kit or a fire extinguisher storage box. You can usually find AEDs in places like:

• Workplaces
• Gyms
• Schools
• Grocery stores
• Restaurants and coffee shops
• Shopping malls and other retail stores
• Airports and airplanes
• Government buildings like courthouses or the post office

You can take a training class and learn how to use an AED. However, you don’t need training to use an AED in an emergency—the device has voice commands and written instructions. Learn the steps now and you can save a life in an emergency.

When to use an AED

Use an AED alongside calling 9-1-1 and doing CPR in a situation where someone has lost consciousness, stopped breathing, or doesn’t have a pulse.

The most common reason for using an AED is sudden cardiac arrest. Cardiac arrest is an emergency condition caused by an electrical problem in the heart. The problem leads to a type of arrhythmia (change in heart rhythm), which causes the heart to stop beating.

Cardiac arrest can look like:

• A sudden collapse
• Jerking movements, similar to a seizure
• Not breathing or agonal breathing, where the person is gasping, breathing irregularly, or taking very short, shallow breaths
• Dizziness or fatigue before a collapse

During a cardiac arrest, a person is unconscious and therefore not responsive.

Cardiac arrest is often caused by ventricular fibrillation—a life-threatening heart rhythm preventing oxygen from reaching the heart—and needs the AED to shock the heart back into a normal rhythm.

Ventricular fibrillation can be caused by:

• Heart attack
• Electrocution
• An injury to the heart
• Heart disease
• Medicines you’re taking
• Weakened heart muscles

Like cardiac arrest, ventricular fibrillation causes a person to collapse and lose consciousness. The collapse or fainting may happen suddenly without warning, or may have symptoms beforehand like:

• Chest pain
• Dizziness
• Nausea
• Heart palpitations (being unpleasantly aware of your heartbeat)
• Shortness of breath

Why you should know how to use an AED

You can provide lifesaving treatment while waiting for emergency services to arrive by knowing how to use an AED.

Every minute is critical:

• Nine out of 10 people who receive an AED shock within the first minute of cardiac arrest live.
• For every minute without CPR during a cardiac emergency, the chance of survival decreases by 10%.
• More than 350,000 people per year have cardiac arrest, and the only way to restore their heart rhythm and blood flow to the heart and other organs is by using an AED. They can’t wait for emergency medical services (EMS) to arrive and need bystanders to help in order to have a chance at survival.

How to use an AED

When you need to use an AED, it’s important to also call 9-1-1 and to do CPR. The combination of CPR and using an AED is what can reduce damage to the heart and even save a life while you wait for EMS.

If you’re by yourself and see someone in cardiac arrest or without a pulse, call 9-1-1 then start CPR. If you don’t know CPR, the emergency dispatcher can talk you through how to do it and can help you use the AED correctly while you wait for the ambulance.

If you’re with several other bystanders, you can work together to help the person affected. Assign a specific person to call 9-1-1 while someone else starts CPR compressions. If any of you are trained in CPR, you can also perform rescue breaths. If no one is trained, don’t worry—doing compressions alone keeps blood flowing to the brain and can increase the chances of survival.

Get the AED from its box. An alarm sounds when you open the box—this is normal and intended to help attract attention and help from others if you need it.

Here’s how to use an AED:

1. Turn on the AED. Follow the voice prompts. The AED may turn on automatically when you open the lid.
2. Remove any clothing that covers the chest. Check that the skin is dry; wipe it dry if needed.
3. Remove the backing from two sticky pads with sensors (also called electrodes).
4. Attach the sticky pads to the person’s chest. If needed, insert the pad connector cable into the AED. For anyone over 8 years old or 55 pounds, use adult pads. If the person is smaller or a younger child, the AED may have child-sized pads.
5. Make sure no one is touching the person when you give the shock. The AED checks the heart rhythm. It may let you know that a shock is needed. Or it may give a shock automatically.
6. If no shock is needed, resume CPR.
7. If a shock is needed, press a button to deliver the shock. Resume CPR once the shock is given.

Keep the AED pads on the person—the AED may tell you to pause CPR and check the heart rhythm again.

Continue CPR until:

• You notice obvious signs of life, like a pulse and breathing.
• The AED voice commands tell you to check the heart rhythm again.
• Another bystander is available to take over CPR compressions.
• Emergency services arrive and take over CPR.
• You are alone and too exhausted to continue.
• The area becomes unsafe.

AED FAQs

Here are some commonly asked questions about using an AED.

Can you hurt someone while using an AED?

AEDs are very safe when used correctly. Follow the voice instructions, avoid having anyone—including you—touching the person when a shock is given, and continue CPR until emergency services arrive.

What if I shock someone that still has a pulse?

If the person has a pulse so faint you can’t feel it, the AED will recognize the pulse and won’t deliver a shock. You don’t have to be perfect in this situation—the AED decides if the person needs a shock. Even if you’re wrong, the person only gets a shock if they need it.

“It’s better to use an AED and do CPR imperfectly until help arrives than to do nothing out of fear of doing it wrong,” says Dr. O’Donoghue.

Can you use an AED on a pregnant person?

Yes, use the AED the same way you would for a non-pregnant person. AEDs are safe for pregnant people, and time is critical to ensure the person and the baby continue getting blood flow and oxygen.

Can you use an AED on someone with a pacemaker?

Yes, AEDs are safe to use for people with a pacemaker. The AED shock won’t damage the pacemaker and could save their life.

Can you use an AED on a hairy chest or on someone that has medical patches on their chest?

The AED pads need to make direct contact with skin, so try to avoid hair when placing them. Some AED kits include a safety razor to shave the hair quickly if it’s truly in the way of the device.

If a person has medical patches, don’t place the AED pads on top of them. If you can accurately place the AED pads without removing the patches, do so. If the medical patches are in the way, carefully remove them without touching the underside—you may have a reaction to the medicine on the patch—before attaching the AED pads in the correct places.

Can you use an AED on a child?

If the child is over 8 years old, or over 55 pounds, use the standard adult pads in the AED. If the child is younger or smaller, the AED may have pediatric pads that change the electrical current for their size. If the AED doesn’t have pediatric pads, continue only using CPR until emergency services arrive. Do not use adult pads on a child—the shock will be too strong for them.

Do you need training to use an AED?

No, you don’t need training to use an AED. They are made to be user-friendly, with clear voice commands and written instructions. The machine does most of the work and decides if a shock is needed.

If you want to feel more confident in emergency situations, however, you can take a class on AED use, doing CPR, first aid skills, and recognizing symptoms of cardiac arrest, heart attack, opioid overdose, or other emergency conditions.

Knowing the basics of how to find and use an AED is similar to understanding the basic instructions to use a fire extinguisher. In an emergency, you’ll be glad you know the steps and that you can help save a life.

“The most important thing is being willing to help in an emergency,” Dr. O’Donoghue says. “Bystanders being prepared to find and use an AED, to do CPR, and to call 9-1-1 can make a difference.”