Sudden cardiac arrest happens when the heart stops pumping blood effectively, usually because of a chaotic electrical rhythm. It is not the same as a heart attack, which is a blockage of blood flow to the heart muscle. An automated external defibrillator (AED) can analyze the heart's rhythm and, if appropriate, deliver a shock that lets the heart reset. This guide shows how to use an AED step by step on an adult, from recognizing the emergency to handing care to EMS. You need no training to follow it and no equipment beyond the AED itself. A hands-on CPR and AED course makes each step faster and calmer when it counts.
Step 1: Confirm the Emergency and Call 911
Check the person quickly. Tap them firmly on the shoulder and shout, "Are you okay?" If there is no response, look at the chest for no more than about 10 seconds. If they are not breathing, or only gasping, treat it as cardiac arrest. Gasping can look like breathing, but it is an abnormal reflex and not a sign of effective breathing.
Then get help moving:
- Call 911 yourself, or point to a specific person and say, "You, call 911 and come back." Vague requests to a crowd often go unanswered.
- Send another person to find the nearest AED and bring it back.
- Put the phone on speakerphone and place it next to you so the dispatcher can coach you through compressions while your hands stay on the chest.
The most common mistake is waiting to be certain. Bystanders hesitate because the person might be sleeping, fainted, or having a seizure. If someone is unresponsive and not breathing normally, act immediately. Starting CPR on someone who turns out not to need it carries far less risk than delay does for someone who does.
If you are alone and no one is nearby, call 911 on speaker first, then move to the next step. Do not leave the person to search for help unless the dispatcher tells you to.
Step 2: Start CPR While the AED Is Retrieved
Compressions keep oxygen-carrying blood flowing to the brain and heart until the AED is ready. Kneel beside the person on a firm surface. Place the heel of one hand on the center of the chest, on the lower half of the breastbone, and put your other hand on top. Lock your elbows and position your shoulders over your hands.
Following American Heart Association (AHA) guidance for adults, push hard and fast:
- Rate: 100 to 120 compressions per minute.
- Depth: at least 2 inches (about 5 cm), but not more than about 2.4 inches.
- Recoil: let the chest rise fully between compressions without leaning on it.
- Pauses: keep interruptions as short as possible.
Check the AHA's current guidelines periodically, since recommendations are updated, and a certified course will teach the version in effect. If you are trained and willing to give rescue breaths, use the ratio of 30 compressions to 2 breaths. If you are untrained or unsure, hands-only CPR with continuous compressions is far better than doing nothing.
If you are alone
Decide based on distance. If the AED is in sight, such as on the wall of the same room, get it, bring it back, and begin. If you do not know where one is or it is far away, stay and do compressions until help arrives.
Where to look for an AED
AEDs are commonly mounted in lobbies, gyms, schools, airports, offices, malls, and community centers. Look for a cabinet or sign with a heart and a lightning bolt. Shout the location to whoever is fetching it if you can see it.
Step 3: Turn On the AED and Prepare the Chest
As soon as the AED arrives, place it beside the person's head or shoulder, on the side where you are working, and keep compressing until you are ready to attach the pads. If a second person is there, they can set up the device while you continue.
Open the case and press the power button. Some models start automatically when you lift the lid. From this point the device talks you through each action, so listen to the prompts. AED models differ slightly in layout and wording, and your device's instructions take priority over any general guide.
Next, expose the chest. Pull or cut the clothing away, including any bra with metal underwire in the pad area. Most kits include trauma shears for this. The pads must stick to bare skin.
A quick check of the skin helps the pads make good contact:
- Wet chest: wipe it dry with a towel or the person's shirt. Pads will not stick to wet skin.
- Medication patches: if one sits where a pad needs to go, remove it and wipe the area clean.
- Chest hair: if hair is thick enough to block contact, press the pad down firmly first. Shave only if a razor is in the kit and it takes seconds. If the first pad pulls off a lot of hair, you can use a second set of pads, if available, but do not spend time hunting for one.
- Jewelry: remove a metal necklace if it lies directly in the pad area. Small jewelry elsewhere can stay on.
The goal is speed. Every minute without defibrillation reduces the chance of survival, so aim for good enough, not perfect.
Step 4: Place the Pads Correctly
Open the pad package and look at the diagram on each pad. For an adult, the standard placement is:
- One pad on the upper right chest, just below the collarbone.
- The other pad on the left side of the chest, a few inches below the armpit, along the ribs.
Peel the backing off each pad, press it down firmly, and smooth it from one edge to the other so that the whole pad sticks. If the pad cable is not already connected to the AED, plug in the connector as the prompts direct.
Special situations
- Implanted devices: a pacemaker or defibrillator often shows as a hard lump under the skin on the upper chest, usually with a small scar. Place the pad at least 1 inch away from it, not directly on top.
- Breast tissue: place the pad on the chest wall, not over or under a skin fold. Lift the breast aside if needed.
- Children under 8: use pediatric pads or a pediatric key or switch if the AED has one. If only adult pads are available, use them with one pad in the center of the chest and the other in the center of the back, making sure the pads do not touch each other.
Never delay a shock to hunt for ideal conditions. If you are unsure about a placement, put the pads on as close to the diagram as you can and let the AED proceed. The device will tell you if contact is poor, with a message such as "Check pads," and you can press them down again.
Step 5: Let the AED Analyze and Clear the Area
Once the pads are attached, the AED will say it is analyzing the heart rhythm. Some models ask you to press an "Analyze" button first. Take your hands off the person. Motion, including compressions, can interfere with the reading, so do not continue CPR while the device is analyzing.
While it works, do a quick safety sweep:
- Say "Clear!" loudly, or "Everyone clear!"
- Look at the person from head to toe to confirm that no one is touching them, including whoever has been helping with compressions.
- Check that the person is not in contact with standing water. If they are lying in a puddle, move them to a dry spot and wipe the chest dry before continuing. Wet surroundings from rain or a pool deck are fine once the person and the pad area are dry.
You do not need to read heart rhythms or decide whether a shock is needed. The AED makes that judgment. It will advise a shock only for rhythms that a shock can correct, and it will not deliver one if the person does not need it. That design is why untrained bystanders can use these devices safely, and it addresses the common fear of hurting someone by mistake.
Analysis usually takes only a few seconds. Stay quiet and still so the device can finish.
Step 6: Deliver the Shock or Resume CPR as Prompted
The AED will give one of two messages.
If it says "Shock advised"
Confirm again that no one is touching the person, then press the flashing shock button. Fully automatic models deliver the shock themselves after a countdown, so make sure everyone stays clear. The person's body may twitch or jump briefly. That is normal.
Immediately after the shock, restart compressions. Do not stop to check for a pulse or to see whether the shock worked, unless the AED tells you to. The heart often needs several minutes of CPR to rebuild circulation after a shock.
If it says "No shock advised"
Resume CPR right away. This message does not mean the person is fine. It means the rhythm is one a shock cannot correct, or that there is no rhythm to shock, and the person still needs compressions to circulate blood.
The cycle that follows
In either case, the AED will typically prompt another analysis after about 2 minutes of CPR. When it does, stop, clear the area, and let it analyze again. Follow the prompts each time, whether that leads to another shock or more compressions. Do not skip a cycle because the person looks the same or seems to be improving, and do not interrupt the sequence to ask questions.
Step 7: Continue Until Help Takes Over and Follow Up
Leave the pads on the chest and keep the AED powered on until EMS arrives. Stopping early is a common error. Only stop if the person clearly begins breathing normally, moves purposefully, or opens their eyes, or if trained responders tell you to. Even then, keep the pads attached in case the heart stops again, and watch their breathing closely.
If another person is available, switch compressors every 2 minutes, ideally during the AED's analysis pause so that no extra time is lost. Compression quality drops with fatigue, often within a couple of minutes, even when the rescuer feels fine.
When paramedics arrive, give them a brief report:
- What you saw: when the person collapsed and whether it was witnessed.
- How long CPR has been going.
- Whether the AED advised a shock, how many were delivered, and roughly when.
Then let them take over. Many AEDs store event data that EMS or your program coordinator can download.
After the event
Replace the used pads and check the battery before returning the AED to its cabinet. Report the use to your site's AED program coordinator, who can arrange data review, restocking, and any required documentation. Experiences like this are stressful, so it is reasonable to talk with a colleague or counselor afterward.
If you have never practiced these steps on a manikin, a CPR and AED class from Midwest CPR Center lets you handle real trainer devices and build muscle memory for the sequence.
Practice These Steps Before You Need Them
AEDs are built for people with no medical background. The device gives voice prompts, analyzes the rhythm, and shocks only when it is appropriate, so your job is to start early and follow it. Early CPR combined with defibrillation gives a person in cardiac arrest the best chance of survival, and each minute of delay lowers that chance.
Good Samaritan protections for people who help in emergencies vary by state, so if you have questions about your situation, ask your employer or a qualified legal professional. Whatever the answer, do not let uncertainty keep you from calling 911 and starting CPR.
Don't wait until an emergency happens. Equip yourself and your team with life-saving skills today. Schedule your CPR certification class now and gain the confidence to respond when every second counts. Midwest CPR Center offers CPR/AED and BLS courses for community members, childcare providers, workplace responders, and healthcare professionals.