You should use an AED as soon as one is available during CPR. There is no reason to wait, finish a cycle of compressions, or confirm a diagnosis first. The moment someone brings an AED to the scene, turn it on and apply the pads while another person continues chest compressions. Survival chances drop by about 10% for every minute that CPR and defibrillation are delayed, so speed matters more than almost anything else in cardiac arrest.
Recognizing When an AED Is Needed
An AED is appropriate any time you suspect cardiac arrest. The signs are straightforward: the person collapses suddenly, is unconscious, and is either not breathing or only gasping. They won’t respond to shouting, tapping, or shaking. You do not need to check for a pulse before starting. Finding a pulse on an unresponsive person is unreliable even for trained medical professionals, and the time spent trying is time wasted. If someone is unresponsive and not breathing normally, call 911, begin chest compressions, and send someone to grab the nearest AED.
Where the AED Fits in the CPR Sequence
The standard sequence is: recognize the emergency, call 911, start compressions, then use the AED the moment it arrives. If you’re alone, call 911 first (put the phone on speaker), start CPR, and retrieve an AED yourself only if one is very close by. If someone else is present, one person starts compressions while the other goes for the AED. Compressions should not stop for more than a few seconds at a time.
Once the AED is at the person’s side, open it and power it on. Every modern AED gives step-by-step voice prompts, so you don’t need to memorize a protocol. The device will tell you to attach the pads, stand clear while it reads the heart rhythm, and press the shock button if a shock is indicated. The entire analysis takes roughly 10 to 15 seconds.
What the AED Actually Does
An AED analyzes the heart’s electrical activity and determines whether a shock can help. During cardiac arrest, the heart is typically in one of two broad states. In the first, the heart’s electrical signals are firing chaotically or racing out of control. These are the “shockable” rhythms, and a jolt of electricity can reset the heart so it starts beating normally again. In the second state, there is either no electrical activity at all or electrical signals that aren’t producing a heartbeat. No shock will fix those rhythms.
The critical point: you don’t have to figure out which state the person is in. The AED does that automatically. If it detects a shockable rhythm, it will charge and instruct you to press the shock button. If it doesn’t detect one, it will tell you no shock is advised and to continue CPR. You cannot accidentally shock someone who doesn’t need it.
What to Do Immediately After a Shock
After the AED delivers a shock, resume chest compressions right away. Don’t stop to check for a pulse or wait to see if the person wakes up. Current resuscitation guidelines recommend continuing compressions for two full minutes (about five cycles of 30 compressions and 2 breaths) before pausing for the AED to reanalyze. Even when a shock successfully resets the heart rhythm, most people remain pulseless for a period afterward, and compressions keep blood flowing to the brain during that window.
The AED will prompt you when it’s time to stop and reanalyze. Leave the pads on and the device powered on until emergency medical services take over.
Pad Placement for Adults and Children
For adults and children over eight years old (or over 55 pounds), use the standard adult pads. Place one pad on the upper right chest, above the breast and below the collarbone. Place the second on the lower left side of the chest, below the armpit. Most pads have diagrams printed directly on them showing the correct position.
For children under eight or under 55 pounds, use pediatric pads if the AED has them. These deliver a lower energy dose. If pediatric pads aren’t available, adult pads are acceptable. On infants, place one pad on the center of the chest and the other on the back, between the shoulder blades. This front-and-back placement prevents the pads from overlapping on a small torso.
Safety Concerns That Shouldn’t Slow You Down
People often hesitate to use an AED because the scene feels unsafe, especially if the person is lying on a wet surface or a metal floor. Both are safe for defibrillation. As long as the pads are stuck firmly to the skin and nobody is touching the person when the shock is delivered, there’s no pathway for the electricity to reach you. If the person’s chest is wet, wipe it dry quickly so the pads adhere properly, then proceed.
If the person has a medication patch on their chest where a pad needs to go, peel it off and wipe the area before placing the pad. If you can see or feel a lump under the skin suggesting an implanted pacemaker or defibrillator, place the pad at least an inch away from it. Neither situation is a reason to skip the AED.
You’re Protected by Law
Every U.S. state has some form of Good Samaritan law that protects bystanders who use an AED in an emergency. The details vary by state, but the principle is consistent: if you act in good faith, don’t seek compensation, and don’t behave recklessly, you won’t be held liable for trying to help. AEDs are specifically designed for untrained bystanders, and the legal framework reflects that. A person in cardiac arrest will die without intervention, so the risk of doing nothing far outweighs any concern about doing something wrong.
Why Every Minute Counts
The heart rhythm most likely to respond to a shock degrades over time. With each passing minute, the chaotic electrical activity in the heart becomes weaker and less organized, eventually flattening into a rhythm that no shock can fix. That’s the biological reason behind the 10%-per-minute drop in survival. CPR alone keeps some blood moving, which buys time, but it rarely restarts the heart on its own. The combination of immediate compressions plus early defibrillation is what gives someone the best chance of walking out of the hospital. If an AED is mounted on a wall 30 seconds away, retrieving it is one of the most important things a bystander can do.