What does AED stand for?
AED is short for automated external defibrillator. “Automated” because it analyzes the heart rhythm by itself, “external” because it works through pads on the skin, and “defibrillator” because it can stop a chaotic rhythm with an electric shock.
The American Heart Association describes an AED as a light, portable device that detects certain life-threatening heart rhythms and sends a shock through the chest to try to restore a normal one. The FDA, which regulates AEDs as medical devices, calls it a computerized defibrillator that analyzes the rhythm of a person in cardiac arrest and shocks only when it’s appropriate.
You’ll also hear AEDs called defibrillators or defibs. Hospitals also use manual defibrillators, where a clinician reads the heart rhythm and decides when to shock. An AED makes that decision for you, which is what lets a bystander use one safely.
What does an AED do?
An AED checks whether the heart is in a rhythm that a shock can fix and, if it is, delivers that shock. In between, it tells the rescuer when to do CPR and when to stand clear.
In sudden cardiac arrest the heart often goes into ventricular fibrillation, where the lower chambers quiver instead of pumping, so no blood reaches the brain. An AED can shock only ventricular fibrillation and pulseless ventricular tachycardia, the two rhythms a shock can reset. If the heart is in any other rhythm, or the person isn’t in cardiac arrest, the AED reports that no shock is advised.
Time matters. In a large registry that the AHA cites in its 2025 guidelines, each minute of delay before the first shock lowered the chance of surviving to hospital discharge by 6%. The same data showed the first shock worked 93% of the time when it came within 6 minutes of collapse, against 75% after more than 16 minutes. That is why an AED a few minutes away beats waiting for the ambulance to bring one.
The need is real. About 356,000 adults a year in the US are treated by EMS for a cardiac arrest outside a hospital, according to the AHA’s 2025 guidelines. Its 2026 statistics update reports that about 1 in 10 adults survived to leave the hospital in 2024, that 42% got CPR from a bystander, and that a public AED was used in only about 13% of cases.
How an AED works, step by step
You turn the AED on, put the pads on the person’s bare chest, and follow the voice prompts. The AED analyzes the rhythm, decides whether a shock is needed, and tells you when to restart CPR.
- Turn it on. From then on, the AED speaks each step, and the AHA notes that many also guide you with lights or on-screen text.
- Attach the pads. Bare the chest, peel the pads off their backing and stick them where the picture on the pads shows. Plug the cable into the AED if it isn’t already connected.
- Let it analyze. The AED reads the heart rhythm through the pads. Nobody should touch the person while it does.
- Shock if advised. A semi-automatic AED tells you to press the shock button; a fully automatic one counts down and shocks by itself. Make sure everyone is clear first.
- Restart CPR right away. After a shock, or if no shock is advised, go straight back to chest compressions. The AED checks the rhythm again about every 2 minutes.
For each step in more detail, including what to do before the AED arrives, see our guide on how to use an AED, and for exactly where the pads go, AED pad placement.
Sudden cardiac arrest vs heart attack
The AHA puts it simply: a heart attack is a circulation problem, and sudden cardiac arrest is an electrical problem. An AED is for cardiac arrest, when a person is unresponsive and not breathing normally.
In a heart attack, a blocked artery cuts off blood to part of the heart muscle. The person is usually awake, and the heart usually keeps beating. Call 911 for them, but don’t attach an AED: the Red Cross is clear that an AED is for someone who is unresponsive and not breathing, not for a conscious person with chest pain.
In sudden cardiac arrest, the heart’s electrical system fails and it stops pumping. The person collapses, doesn’t respond and isn’t breathing normally, or is only gasping. Without CPR and a shock, death follows within minutes. The two are linked: a heart attack raises the risk of cardiac arrest, which can happen during the attack or while the person recovers.
Who can use an AED?
Anyone can. AEDs are made for bystanders: they give spoken instructions, and they won’t shock a heart that doesn’t need it. The AHA says there is no minimum age to use one.
Neither the AHA nor the Red Cross requires training before you use an AED in an emergency, and both encourage untrained people to go ahead and follow the prompts. Training still helps: the AHA recommends it because it builds the confidence to act quickly. The AHA’s 2025 guidelines use the term “lay rescuer” for anyone without medical training who steps in, and they recommend public access defibrillation programs in communities at high risk.
Organizations that keep AEDs usually train a group of employees or volunteers anyway, because some states make training a condition of their AED laws. See our guide on CPR and AED certification for how long cards last, and the AED laws by state for what yours requires.
Semi-automatic vs fully automatic AEDs
A semi-automatic AED prompts you to push its shock button. A fully automatic AED shocks by itself after warning everyone to stand clear. Both analyze the rhythm the same way.
The FDA’s definition covers both kinds. Many popular models come in either version, for example the ZOLL AED Plus, the LIFEPAK CR2 and the HeartSine samaritan PAD 350P and 360P. Some AEDs are made for professional rescuers, with a screen that shows the heart rhythm or a manual override; the FDA calls the rest public access AEDs.
Which is better is a buying decision, not a safety one. A fully automatic AED removes the hesitation of pressing the button; a semi-automatic one lets a trained rescuer time the shock. Our AED models pages list each model’s versions, pads and batteries.
Where you find AEDs
AEDs are kept where people gather and where cardiac arrests are seen: schools, gyms, offices, airports, stadiums, churches and government buildings. Many states require them in some of those places.
The AHA’s 2026 statistics update found that 71% of adult out-of-hospital cardiac arrests happen at home and 18% in public places. Survival is higher in public: the AHA’s 2025 guidelines report survival to discharge of 21.1% for arrests in public against 8.9% at home.
Look for the AED sign, often a heart with a lightning bolt. AEDs usually hang in a wall cabinet near a main hallway, lobby or reception desk. The Red Cross’s 2025 guidelines say public AEDs should sit in unlocked cabinets, or come with clear and quick instructions if they must be locked. Our AED placement guide covers how many a building needs and where they go.
No federal law requires most organizations to have an AED, but state laws name places such as schools and health clubs. The FDA regulates the devices themselves: since 2015 every AED and its pads and batteries need the FDA’s premarket approval. Most AEDs are sold with a prescription; a few, such as the Philips HeartStart OnSite and Home, are approved for sale without one.
Keeping an AED ready
An AED is only useful if it works when someone collapses, so it needs regular checks: a status light that says it’s ready, pads and a battery within their dates, and supplies on hand.
Most AEDs test themselves daily, weekly or monthly and show the result with a green light or check mark. Pads expire after about 2 to 5 years depending on the model, and batteries usually need replacing every 3 to 7 years. Someone has to look at the AED, replace parts before their dates and record it. Our AED inspection checklist covers the monthly check.
That job is easy to lose track of when AEDs are spread across buildings. AED Log keeps every AED, its pads and battery dates and its inspections in one place, and sends a reminder before anything expires. See AED management for how it works.
What is an AED FAQ
What does AED stand for?
It’s short for automated external defibrillator: a portable device that checks the heart rhythm of someone in sudden cardiac arrest and, if a shock can help, delivers one through pads on the chest.
Is an AED the same as a defibrillator?
An AED is one kind of defibrillator. It reads the heart rhythm and decides on the shock by itself, so a bystander can use it. Manual defibrillators, used by paramedics and hospitals, leave that decision to a trained clinician.
Can an AED shock someone who doesn’t need it?
No. It shocks just two rhythms, ventricular fibrillation and pulseless ventricular tachycardia. For anything else, or a person who isn’t in cardiac arrest, it reports that no shock is advised and tells you to continue CPR.
Do you need training to use an AED?
No. The AED gives spoken instructions for every step, and the AHA and the Red Cross both encourage untrained bystanders to use one. A CPR and AED course still builds the confidence to act quickly.
Do you need a prescription to buy an AED?
For most models, yes: the FDA classes most AEDs as prescription devices, and sellers usually include the prescription. A few, such as the Philips HeartStart OnSite and Home, are approved for sale without one.