An AED is needed when someone collapses, is unresponsive, and is not breathing normally. These three signs together point to sudden cardiac arrest, where the heart has stopped pumping blood effectively. Every minute without defibrillation reduces the chance of survival by 7 to 10 percent, so recognizing the situation quickly is critical.
The Two Signs That Call for an AED
An AED should only be used on a person who meets both criteria: they are unresponsive and they are not breathing. If someone collapses and you shake their shoulders or shout at them with no reaction, that’s unresponsiveness. If their chest isn’t rising and falling in a normal rhythm, that’s absent breathing. When both are present, the person is likely in cardiac arrest and needs an AED immediately.
There’s one major complication in recognizing this situation: agonal breathing. In the early moments of cardiac arrest, many people make occasional gasping, snorting, or gurgling sounds. These gasps are not real breathing. They’re reflexive movements that can fool bystanders into thinking the person is still breathing on their own. Research shows that no single description reliably captures what agonal breathing looks like, which is why it so often delays rescue efforts. If someone is unconscious and their breathing looks abnormal in any way, treat it as cardiac arrest. Apply the AED.
What the AED Actually Does
During cardiac arrest, the heart’s electrical system typically malfunctions in one of two ways. The heart muscle may quiver chaotically instead of pumping (ventricular fibrillation), or it may beat dangerously fast without moving blood (ventricular tachycardia). Both of these are “shockable” rhythms, meaning an electrical shock can reset the heart’s electrical activity and allow it to resume a normal beat.
The AED handles the diagnosis for you. Once you attach the pads to the person’s bare chest, the device reads the heart’s electrical activity and determines whether a shock is appropriate. If the rhythm isn’t shockable, the AED will not deliver a shock, no matter how many times you press the button. This built-in safety feature means you cannot accidentally harm someone by using an AED on them. If there’s any doubt about whether the situation calls for one, attach it and let the machine decide.
Cardiac Arrest Is Not a Heart Attack
People often confuse these two events, but they require very different responses. A heart attack is a plumbing problem: a blocked artery cuts off blood flow to part of the heart muscle. The person is usually conscious, may have chest pain, and needs emergency medical care. An AED is not used during a heart attack.
Cardiac arrest is an electrical problem: the heart suddenly stops beating effectively. The person loses consciousness within seconds. A heart attack can trigger cardiac arrest because the damage it causes can disrupt the heart’s electrical signals, but the two conditions are distinct. An AED is only for cardiac arrest.
Why Minutes Matter
The survival math is stark. Without any intervention, survival from witnessed cardiac arrest drops by 7 to 10 percent for every minute that passes. If a bystander starts CPR right away, that decline slows to about 3 to 4 percent per minute, buying time until a defibrillator arrives. But CPR alone cannot fix a shockable rhythm. It keeps some blood moving to the brain and organs while you wait for the shock that can actually restart the heart.
This is why the American Heart Association’s Chain of Survival places defibrillation as a core step alongside high-quality CPR. The sequence is: recognize the arrest and call emergency services, begin CPR, use an AED as soon as one is available, then let paramedics provide advanced care. Skipping or delaying the AED step dramatically lowers the odds of survival.
Using an AED on Children
AEDs work on children, but pad selection depends on age and size. For children 8 years old and under, or those weighing 55 pounds (25 kg) or less, pediatric pads are recommended. These deliver a lower energy dose appropriate for a smaller body. For anyone over age 8, standard adult pads are used.
If a child is in cardiac arrest and only adult pads are available, use them. A shock with adult pads is far better than no shock at all. Some AED models have a pediatric mode you can switch to, while others come with separate pediatric pad sets.
Special Situations to Be Aware Of
Wet Surfaces
If the person collapsed in or near water, move them to a dry surface before attaching the AED pads. Dry the chest quickly with a towel or cloth. Standard guidance recommends against using an AED in standing water, though simulation studies have found that even in wet conditions the shock risk to bystanders is minimal. Still, a dry chest ensures the pads stick properly and the electrical current travels through the heart rather than across wet skin.
Pacemakers and Implanted Devices
If you see a scar and a hard bump under the skin of the upper chest, that’s likely an implanted pacemaker or defibrillator. Don’t place the AED pad directly on top of it. Shift the pad at least an inch to the side. The AED will still work normally.
Medication Patches
Transdermal patches (often used for pain relief, nicotine, or hormones) can block the AED pad’s contact with the skin or cause burns. If you see a patch where a pad needs to go, put on gloves, peel off the patch, and wipe the area before applying the pad.
Body Piercings
Metal conducts electricity, so AED pads should not be placed directly over piercings. Move the pad about an inch away from any metal jewelry on the chest. In most cases this small adjustment is enough for the AED to work effectively.
Pregnancy
An AED is safe to use on a pregnant person. The pad placement and procedure are identical to any other adult. AED electrode placement works the same way regardless of breast tissue or body type, and the device poses no additional risk to the fetus. Cardiac arrest is fatal without intervention, so there is no reason to hesitate.
What to Do Step by Step
- Check for responsiveness. Tap the person’s shoulders and shout. If there’s no response, call 911 or ask someone nearby to call.
- Check for breathing. Look at the chest for 5 to 10 seconds. If the person isn’t breathing, or is only gasping, begin CPR immediately.
- Send someone for the AED. If you’re alone, start CPR first. If others are nearby, direct one person to grab the nearest AED while you continue chest compressions.
- Turn on the AED and follow its prompts. The device gives voice instructions. Expose the chest, peel and place the pads where the diagrams show, and let the AED analyze the rhythm.
- Clear the person before the shock. If the AED advises a shock, make sure no one is touching the person, then press the shock button.
- Resume CPR immediately after the shock. Continue until emergency medical services arrive or the person starts breathing normally.
AEDs are designed to be used by anyone, with no medical training required. The voice prompts walk you through every step. The device will not shock a heart that doesn’t need it. In a situation where someone is unresponsive and not breathing, attaching an AED is always the right call.