Automated External Defibrillator (AED): How It Works Step by Step

What Is an Automated External Defibrillator (AED)?

An automated external defibrillator (AED) is a portable medical device that analyzes a person’s heart rhythm and delivers an electric shock to restore a normal heartbeat during sudden cardiac arrest. AEDs are designed for use by bystanders with no medical training, making them one of the most accessible life-saving tools in public spaces, workplaces, and homes.

The AED meaning is straightforward: “automated” because the device handles rhythm analysis on its own, “external” because the pads go on the outside of the chest, and “defibrillator” because it delivers a controlled electrical pulse to correct dangerous heart rhythms.

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Sudden cardiac arrest kills more than 350,000 Americans outside of hospitals each year, according to the American Heart Association. When a bystander uses an AED within the first few minutes, survival rates can increase from under 10% to above 50%. That gap between life and death comes down to one question: does someone nearby know where the AED is and how to turn it on?

This guide breaks down exactly how AEDs work, when they deliver a shock, and how to use one step by step.

How Does an AED Analyze Heart Rhythm?

When you attach the AED electrode pads to a person’s bare chest, the device immediately begins reading the heart’s electrical activity. The built-in algorithm measures several things at once:

• Rhythm rate: How fast or slow the heart is beating
• Rhythm regularity: Whether the electrical signals follow a predictable pattern
• Signal amplitude: The strength of the heart’s electrical output
• Waveform shape: The specific pattern of each heartbeat cycle

The AED’s processor compares these readings against known cardiac rhythm profiles stored in its software. The entire analysis takes about 5 to 15 seconds. During this time, the device instructs everyone to stop touching the patient so body movement does not interfere with the reading.

AEDs are programmed with high sensitivity and specificity. Studies published in the journal Circulation show that modern AEDs correctly identify shockable rhythms more than 95% of the time. The device will not allow a shock to be delivered unless it confirms a rhythm that requires defibrillation.

When Does an AED Deliver a Shock?

An AED only delivers a shock for two specific heart rhythms, both of which are life-threatening and respond to electrical defibrillation:

Ventricular fibrillation (VF): The heart’s lower chambers quiver rapidly and irregularly instead of pumping blood. VF is the most common rhythm found in sudden cardiac arrest and is the primary target for AED therapy. During VF, the heart produces no meaningful blood flow, and the person will be unresponsive and not breathing normally.

Ventricular tachycardia (VT): The heart beats dangerously fast, sometimes above 150 beats per minute, preventing the chambers from filling with blood properly. Pulseless VT looks similar to VF in its effects on the patient: they collapse, lose consciousness, and stop breathing.

For a deeper look at which rhythms respond to defibrillation, read our guide on AED shockable rhythms.

If the AED detects a non-shockable rhythm, such as asystole (flatline) or pulseless electrical activity (PEA), it will announce “no shock advised” and prompt you to continue CPR. This is not a malfunction. These rhythms do not respond to defibrillation, and the correct treatment is chest compressions.

How to Use an AED: Step-by-Step Guide

Using an AED is simpler than most people expect. Every AED provides voice prompts and visual instructions that walk you through each step. Here is the process:

1. Call 911 first: Before reaching for the AED, confirm that someone has called emergency services. If you are alone, call 911 and put the phone on speaker before starting.
2. Turn on the AED: Open the case and press the power button. Some models activate automatically when the lid opens. The device will start giving voice instructions immediately.
3. Expose the chest: Remove or cut away clothing from the person’s chest. The skin must be bare and dry for the pads to stick properly. If the chest is wet, wipe it dry. If there is excessive chest hair, use the razor included in most AED kits to shave the pad areas.
4. Attach the electrode pads: Peel the backing off each pad and place them exactly as shown in the diagram. One pad goes on the upper right chest, below the collarbone. The other goes on the lower left side, below the armpit. Press firmly so the entire pad makes full contact with the skin.
5. Clear the patient: The AED will announce “analyzing rhythm” or a similar phrase. Make sure nobody is touching the person. Say “Clear!” loudly so bystanders step back.
6. Follow the shock prompt: If a shock is advised, the AED will charge and instruct you to press the shock button. Some fully automatic models deliver the shock without a button press. Confirm everyone is clear, then press the button.
7. Begin CPR immediately after: The AED will prompt you to start CPR right away. Perform 30 chest compressions followed by 2 rescue breaths, repeating this cycle for 2 minutes. The AED will then re-analyze the rhythm and advise you whether another shock is needed.

For specific situations like using an AED on children, pregnant women, or people with implanted devices, see our article on special considerations when using an AED.

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Do You Need Training to Use an AED?

No. AEDs are specifically designed so that anyone can use them, even with zero prior training. The voice prompts guide every action, and the device will not deliver a shock unless the heart rhythm requires one. You cannot accidentally harm someone by using an AED incorrectly.

That said, training makes a real difference in an emergency. People who have practiced with an AED respond faster, place the pads more accurately, and feel more confident taking action when seconds count. Without training, hesitation is the biggest barrier. Research from the Resuscitation Council shows that trained bystanders initiate AED use 30% faster than untrained bystanders.

CPR1 offers CPR and AED certification courses through the American Heart Association, American Red Cross, and HSI. Courses are available onsite at your location with certified instructors in all 50 states, or through blended learning options that combine online modules with in-person skills sessions.

AED vs. Manual Defibrillator: What Is the Difference?

Bottom line: AEDs are built for speed and simplicity in public settings. Manual defibrillators give medical professionals full control over energy delivery and rhythm interpretation. Both save lives, but AEDs bridge the gap between cardiac arrest and the arrival of EMS.

The type of electrical waveform matters, too. Most modern AEDs use biphasic technology, which is more effective at lower energy levels. For a detailed comparison, read our guide on monophasic vs. biphasic AED shocks.

Where Are AEDs Required? Public Access and Placement Laws

AED requirements vary by state, but the trend is clear: more locations are required to have them every year. Here is a general breakdown of where AEDs are commonly mandated or recommended:

• Schools and universities: At least 25 states require AEDs in K-12 schools. Many mandate AEDs at athletic events and practice facilities as well.
• Fitness centers and gyms: Several states require health clubs to maintain AEDs on-site and train staff on their use.
• Government buildings: Federal buildings are required to have AED programs under the Federal Occupational Health guidelines.
• Airports, stadiums, and large venues: High-traffic public spaces typically have AED placement programs coordinated with local EMS.
• Corporate offices: While not always legally mandated, OSHA recommends AEDs as part of a workplace first aid program, and many states offer liability protection for employers who maintain them.

Good Samaritan laws in all 50 states provide legal protection for bystanders who use an AED in good faith during an emergency. These laws exist specifically to encourage bystanders to act without fear of lawsuits.

If your organization needs help building a compliant AED program, CPR1 offers AED program management that covers device selection, placement, staff training, and ongoing maintenance tracking through cloud-based software.

Explore CPR1’s full training catalog to find the right certification course for your team or organization.

What to Do After Using an AED

The moments after an AED delivers a shock are just as important as the shock itself. Here is what to focus on:

• Continue CPR: Even if the AED delivered a successful shock, start chest compressions immediately. The heart often needs help maintaining circulation after being defibrillated.
• Leave the pads on: Do not remove the electrode pads. The AED will continue monitoring the heart rhythm and may advise additional shocks.
• Wait for EMS: Keep performing CPR in 2-minute cycles, letting the AED re-analyze between each cycle, until paramedics arrive and take over.
• Report what happened: Tell the arriving EMS team how many shocks were delivered, how long CPR was performed, and any changes you observed. Most AEDs record event data that paramedics can download.

For a complete walkthrough of post-AED care, including emotional recovery for the rescuer, see our guide on what to do after using an AED.

Frequently Asked Questions

Can an AED hurt someone who does not need a shock?

No. An AED will only deliver a shock if it detects a shockable heart rhythm like ventricular fibrillation or pulseless ventricular tachycardia. If the heart rhythm does not require defibrillation, the device will not allow a shock to be delivered, even if you press the button.

How much does an AED cost?

AEDs typically cost between $1,400 and $3,000 depending on the brand, features, and included accessories. Ongoing costs include replacement pads (every 2 years or after each use) and batteries (every 3 to 5 years). CPR1’s sister company, AED Leader, offers a range of AEDs for home and workplace use.

How long does an AED battery last?

Most AED batteries last between 3 and 5 years in standby mode. The device runs a daily or weekly self-check and will display a warning light or beep when the battery is running low. Replacing the battery before it expires is part of basic AED maintenance.

Can you use an AED on a child?

Yes. Many AEDs include pediatric pads or a pediatric key/switch that reduces the energy level for children under 8 years old or weighing less than 55 pounds. If pediatric pads are not available, adult pads can be used on a child. Saving the child’s life takes priority over having the exact right equipment.

Do all AEDs work the same way?

All AEDs follow the same basic principle: analyze the heart rhythm, advise whether a shock is needed, and deliver the shock. The main differences between brands are the user interface, voice prompt style, energy levels, and whether they are semi-automatic (you press a button) or fully automatic (the device delivers the shock on its own after a countdown).

AEDs Save Lives When People Act

An automated external defibrillator is only as useful as the person willing to open the case and turn it on. The device does the hard work of reading the heart and deciding when to shock. Your job is to act fast, follow the voice prompts, and keep doing CPR until help arrives.

Every minute without defibrillation during sudden cardiac arrest reduces the chance of survival by 7% to 10%. That is why AED access and training matter so much, not just for medical professionals, but for teachers, coaches, office managers, and everyday people who might be the first person on the scene.

Get your team CPR and AED certified with CPR1. Call 858-665-8444 or book online to schedule a training session at your location.