Educational purposes only. This guide reflects information published by the American Heart Association, Cleveland Clinic, and other cardiac emergency resources, combined with field experience. It does not replace professional medical advice or CPR/AED certification training. If you believe someone is in cardiac arrest, call 911 immediately.

I've watched this exact scene play out at a gym, of all places, surrounded by people who spend an hour a day thinking about their heart rate on purpose. Someone went down mid-set, no warning, no stumble, just gone. Somebody yelled for help. Somebody else pointed at the little green cabinet on the wall, the one everybody walks past a hundred times and never touches. And for about fifteen seconds, which felt a lot longer to everyone standing there, nobody moved toward it. Not because they didn't know it was there. Because nobody was sure they were allowed to be the one who used it.

That hesitation is the actual emergency inside the emergency. The device itself is built to be nearly foolproof. The person standing next to it, deciding whether they're qualified to touch it, is the part almost nothing gets fixed.

The Number That Explains the Cabinet on the Wall

Here's the piece most AED guides skip past on their way to the step-by-step instructions. Survival from sudden cardiac arrest doesn't decline gradually while everyone figures out what to do. It falls off fast, and it falls off from the first minute. For every minute that passes without defibrillation, a person's odds of surviving drop sharply, which is exactly why AEDs are stationed in gyms, airports, and office lobbies instead of locked away somewhere that takes five extra minutes to reach. The device isn't there for convenience. It's there because the fifteen seconds of hesitation in that gym were already a meaningful chunk of the window that mattered.

The American Heart Association frames this as the Chain of Survival, six links that all have to hold for someone to make it: recognizing the arrest and calling for help, early CPR focused on chest compressions, rapid defibrillation, advanced care from EMS, post-arrest treatment, and recovery. Miss the early links, and the later ones, no matter how skilled the paramedics or the hospital team are, are working with a much worse starting position. The AED isn't a replacement for any of that chain. It's the third link, and it only works if someone actually walks over and grabs it.

One statistic worth holding onto: people in cardiac arrest who receive a shock from an AED are roughly two and a half times more likely to survive to hospital discharge than those who don't. That's not a marginal edge. That's the difference the cabinet on the wall is designed to make, if someone's willing to open it.

CPR First, Not Instead Of

A mistake I've seen more than once, usually from someone who's genuinely trying to do the right thing, is freezing up to wait for the AED to arrive before starting anything. Don't. The moment you confirm someone isn't responsive and isn't breathing normally, start hands-only CPR immediately, chest compressions, hard and fast, center of the chest. Send someone else to find the AED, and someone else to call 911, while you're already working.

Confirming those first two things, responsiveness and breathing, is where checking a pulse and watching for normal chest rise comes in, and it's worth knowing what you're actually looking for before you're doing it under pressure. Our guide on What Are Vital Signs (And How to Check Them) walks through that. Likewise, knowing the line between a situation that needs an ambulance and one that doesn't matters here too, and

When Should I Call the Ambulance? covers that distinction, though cardiac arrest itself is never a gray area. Call immediately.

Once the AED is in hand, follow its prompts, but don't stop compressions to fumble with the device unless the AED specifically instructs you to. Most units are built to talk you through pad placement while a second person keeps compressions going. If you're alone, pause compressions only as long as it takes to get the pads on and let the device analyze.

What the Device Is Actually Deciding

A lot of the fear around AEDs comes from a misunderstanding of what the machine is doing in that pause after the pads go on. It is not guessing. It is not going to shock someone who doesn't need it. The AED is reading the heart's electrical rhythm and sorting it into one of two categories.

• Shockable rhythms: Ventricular fibrillation, where the heart's lower chambers quiver instead of beating, and ventricular tachycardia, a dangerously fast rhythm. Both prevent the heart from actually pumping blood, and both respond to a shock.
• Non-shockable rhythms: Asystole, where there's no electrical activity at all, and pulseless electrical activity, where there's electrical activity but not enough to pump blood. In both cases, a shock won't fix the problem, and the AED will say "no shock advised" and prompt you to continue CPR instead.

That second category is the one that trips people up. Hearing "no shock advised" can feel like the machine just failed at its one job. It didn't. It made the correct call, and CPR is still the right response, since it's the thing keeping blood moving until a rhythm the device can actually treat shows up, or until EMS arrives with more advanced options.

While the AED is analyzing, the single most important thing to do is nothing. Stop compressions, make sure nobody is touching the person, and say it out loud, everyone clear, so there's no ambiguity. Contact during analysis can throw off the reading entirely, and contact during an actual shock is a real safety risk to whoever's touching the person.

The Cycle Most Guides Never Actually Spell Out

Almost every AED article tells you to follow the prompts after a shock and leaves it there, which sounds complete right up until you're the one standing there wondering what happens next. Here's the actual sequence, and it's worth knowing before you need it rather than during.

• If a shock is delivered, resume CPR immediately afterward. Don't pause to check for a pulse or reassess right away, the shock alone doesn't guarantee a return of normal rhythm, and lost seconds here matter.
• Continue CPR for roughly two minutes, following the AED's timing prompts, most units will cue you when it's time to stop and reanalyze.
• The AED will reanalyze the rhythm and either advise another shock or tell you to continue CPR. This shock, two minutes of CPR, reanalyze cycle repeats until one of three things happens: the person shows obvious signs of life, EMS arrives and takes over, or you're physically unable to continue.
• Signs of life to watch for include normal breathing returning, movement, coughing, or opening their eyes. If you see these, stop CPR, but leave the AED pads attached and powered on, since the person can go back into cardiac arrest and the device needs to keep monitoring.

If the person does regain a pulse and starts breathing on their own, positioning them properly while you wait for EMS matters. Our guide on How to Move Someone in an Emergency covers safe positioning, including the recovery position, for exactly this kind of situation.

The Button Question

AEDs come in two flavors, and it's worth knowing the difference before you're standing over someone rather than reading about it for the first time in that moment.

Semi-automatic AEDs analyze the rhythm and, if a shock is advised, tell you to press a button to deliver it. Fully automatic AEDs skip that step entirely, they analyze, warn everyone to stand clear with a countdown, and deliver the shock on their own.

The tradeoff isn't really about the technology. It's about what breaks down in a real emergency. Fully automatic units cost a few extra seconds during that countdown, since the machine has to make sure everyone's actually clear before it fires. Semi-automatic units skip that delay, but they hand the actual decision back to a person who might be a stranger, might be someone's coworker, might be someone's spouse, in the worst moment of their day. Research into AED errors has found that when mistakes happen, the large majority trace back to the operator rather than the device, things like hesitating too long, walking away entirely, or accidentally powering the unit off instead of proceeding. A few seconds of automated countdown is a small, predictable delay. Operator hesitation is a much bigger and far less predictable one. That's the actual case for fully automatic units in a lot of public-access settings, not the marginal time difference, but removing the moment where a scared, untrained bystander has to decide whether they're allowed to press the button.

The Unglamorous Part Nobody Rehearses

Every AED training video shows a clean, dry chest and a calm room. Real emergencies rarely offer either. A few things worth knowing before you're the one holding the razor from the AED kit with shaking hands.

• Wet skin: Water conducts electricity and interferes with how well the pads stick and read the rhythm. If the person is wet, from sweat, rain, or having been in a pool, dry the chest thoroughly before applying pads, including the ribs and armpit area, not just the exact spot the pads go.
• Chest hair: Thick hair can stop the pads from making full contact with the skin, which can prevent the AED from getting an accurate reading or delivering the shock effectively. Most AED kits include a small razor for exactly this reason. If there's no razor available, press the pads on as firmly as you can.
• Jewelry and metal: Move necklaces or metal jewelry off to the side, away from the pads, since metal near the pads can cause burns or sparking. A person lying on a metal or wet surface is generally still safe to treat, since the risk comes from direct contact during the shock, not proximity to a conductive surface.
• Medication patches: Remove any visible medication patch before placing a pad over that area, wearing gloves if you have them, since a shock delivered through a patch can cause a burn.
• Pacemakers and implanted defibrillators: These devices show as a small, firm bulge under the skin, usually on the upper chest. Don't place a pad directly over the device. Offset it to the side and continue as normal, the AED will still work.
• Pregnancy: It's safe and recommended to use an AED on a pregnant person in cardiac arrest. The procedure doesn't change, aside from keeping pad placement clear of enlarged breast tissue. Hesitating out of concern for the pregnancy costs time that helps neither person.

That burn risk from a patch or misplaced jewelry is a real, if secondary, injury worth knowing how to treat if it happens. First Aid and Treatment for Burns and Scalds covers the basics, though it's a distant second priority to getting the shock delivered in the first place.

Kids Aren't Just Smaller Adults

If pediatric pads are available in the AED kit, use them for children under about eight years old or under roughly 55 pounds, they're built to deliver a lower energy level appropriate for a smaller body, and they're placed differently, one on the front of the chest and one on the back, rather than the front-and-side placement used for adults. If pediatric pads aren't available, adult pads are still the right call over doing nothing. Use them in the same front-and-back placement to keep the two pads from touching each other on a small chest, and proceed. A shock at adult strength is not without risk, but a heart that's already stopped carries a far greater risk from delay than from an imperfectly sized pad.

CPR itself also changes across age groups in ways worth knowing ahead of time, not just the AED pads. Our guide on CPR Variations: Infant, Child, and Adult covers the compression depth and rate differences that go alongside this.

Being Ready Before the Emergency, Not During It

Almost everything above assumes the AED is charged, the pads haven't expired, and the person reaching for it knows roughly what they're doing. None of that happens by accident. AED batteries and pads both have shelf lives, typically a few years, and an AED that's sat in a cabinet unchecked since it was installed is a real possibility in a lot of workplaces. If you're responsible for a workplace, gym, or community space with an AED on the wall, checking the battery and pad expiration dates on a regular schedule is worth treating as seriously as checking a fire extinguisher's inspection tag, since a dead AED at the moment it's needed is functionally the same as no AED at all.

The gear that lives alongside an AED matters too. A compact CPR mask kept near the unit removes the hesitation some rescuers feel about rescue breaths, and it's worth having ready rather than improvised in the moment. CPR Made Simpler: Choosing and Carrying a Compact CPR Mask walks through what to look for. For workplace or team settings where a bag valve mask might be part of the kit instead,

Bag Valve Mask (BVMs) vs. CPR Mask - What's the Difference? breaks down which fits which setting.

The Question People Are Afraid to Ask Out Loud

Somewhere underneath the hesitation in that gym was probably a quieter fear a lot of people carry without saying it: what happens if I do this wrong. It's a fair question, and the honest answer is that it's already been accounted for.

Good Samaritan laws in all fifty states protect people who act in good faith to help someone in a medical emergency, including AED use, from liability for unintended harm. The device itself won't deliver a shock to a rhythm that doesn't need one, and acting, even imperfectly, carries far less legal or moral risk than standing still. Can I Get in Trouble for Helping Someone in an Emergency? covers this in more detail, and it's worth reading once so it's not a question you're weighing for the first time while someone's life is on the line.

Back to the Gym Floor

Someone did eventually grab that cabinet off the wall. It wasn't the most confident person in the room, or the one with the most training. It was just the first person who decided the fifteen seconds of standing still had gone on long enough. That's really the whole point of how these devices are built, foolproof enough that the decision to act matters more than the decision-maker's qualifications. The machine will tell you what to do. It will not shock someone who doesn't need it. It will not let you get it wrong in the way that matters. The only step it can't do for you is the first one, walking over and taking it off the wall.

AED Use: Frequently Asked Questions

Do I need training to use an AED?

No. AEDs are designed for use by untrained bystanders and will guide you through every step with voice or visual prompts. Training improves speed and confidence, but a lack of it should never stop you from using one in an emergency.

What does "no shock advised" mean?

It means the AED has detected a heart rhythm that won't respond to a shock, such as asystole or pulseless electrical activity. This isn't a malfunction. Continue CPR immediately and let the AED reanalyze periodically as instructed.

What happens after the AED delivers a shock?

Resume CPR immediately for about two minutes, then the AED will reanalyze and advise either another shock or continued CPR. This cycle repeats until the person shows signs of life or EMS takes over.

Is it safe to use an AED on someone who is wet or lying on a metal surface?

Lying on a wet or metal surface is generally safe as long as the person's chest is dried before the pads go on and no one is in direct contact with them when the shock is delivered. Never use an AED while someone is actually submerged in water.

Can you use an AED on someone with a pacemaker?

Yes. Place the pad away from the visible bulge of the device rather than directly over it, and proceed with the AED as normal.

Should I do CPR before using the AED?

Start CPR immediately once you confirm someone is unresponsive and not breathing normally. Don't wait for the AED to arrive to begin. Send someone else to retrieve it while compressions are already underway.

Can I get in trouble for using an AED on someone if it doesn't work?

Good Samaritan laws in every state protect people acting in good faith during a medical emergency, including AED use, from liability for unintended outcomes.

This article reflects information published by the American Heart Association, Cleveland Clinic, and other cardiac emergency resources, combined with general first aid principles drawn from field experience. It is intended for educational purposes and does not replace CPR/AED certification training or professional medical advice. If you believe someone is in cardiac arrest, call 911 immediately.