An AED (automated external defibrillator) should be used whenever someone collapses, is unresponsive, has no pulse, and is not breathing normally. These are the signs of sudden cardiac arrest, and survival drops by 5 to 10 percent with every minute that passes without defibrillation. The device is designed for exactly this emergency, and using one quickly can mean the difference between life and death.
Cardiac Arrest vs. Heart Attack
The most important thing to understand is that an AED is for cardiac arrest, not a heart attack. These two emergencies are often confused, but they involve completely different problems. A heart attack is a circulation issue: a blocked artery prevents blood from reaching part of the heart muscle. The heart keeps beating during a heart attack, and the person is typically conscious and alert, though in severe pain.
Sudden cardiac arrest is an electrical problem. The heart’s electrical system malfunctions, causing it to beat in a rapid, chaotic rhythm or stop entirely. Blood stops flowing to the brain and organs within seconds. The person loses consciousness, stops breathing normally, and has no detectable pulse. Without intervention, death follows within minutes. This is the situation where an AED is needed.
That said, a heart attack can sometimes trigger cardiac arrest. If someone having a heart attack suddenly collapses and becomes unresponsive, the situation has shifted, and an AED becomes appropriate.
How to Recognize the Emergency
You don’t need medical training to identify when an AED is needed. Look for three things:
- Unresponsiveness. The person doesn’t react when you tap their shoulders and shout at them.
- No normal breathing. The chest isn’t rising and falling. Occasional gasping or gurgling doesn’t count as breathing. These irregular gasps, sometimes called agonal breathing, are a sign the brain is losing oxygen and the heart isn’t working.
- No pulse. If you can check for a pulse at the neck and don’t feel one within 10 seconds, proceed as though the heart has stopped.
If all three are present, call emergency services immediately and grab the nearest AED. If you’re unsure whether you feel a pulse or whether the person is breathing, err on the side of action. The AED itself will analyze the heart’s rhythm and only deliver a shock if one is needed. You cannot accidentally shock someone who doesn’t need it.
Why Speed Matters So Much
The heart rhythm most commonly behind sudden cardiac arrest is ventricular fibrillation, where the lower chambers of the heart quiver chaotically instead of pumping blood. Think of it as the heart trembling rather than beating. An AED delivers an electrical shock that resets this chaotic rhythm, giving the heart a chance to resume normal contractions.
But this window is narrow. Survival decreases by roughly 5 to 10 percent for every additional minute between collapse and defibrillation. At 10 minutes without a shock, very few people survive. Average ambulance response times in many areas exceed that threshold, which is why bystander use of public AEDs is so critical. A shock delivered within the first three to five minutes of collapse gives someone the best chance of walking out of the hospital.
What the AED Does (and Doesn’t Do)
An AED analyzes the heart’s electrical activity through adhesive pads placed on the chest. If it detects a rhythm that can be corrected with a shock, like ventricular fibrillation or a dangerously fast heartbeat from the lower chambers, it will instruct you to press the shock button. If the heart has flatlined completely or is beating in a rhythm that a shock won’t fix, the device will tell you “no shock advised” and prompt you to continue CPR.
This built-in analysis is what makes AEDs safe for untrained bystanders. The machine makes the decision. Modern AEDs also provide step-by-step voice prompts, walking you through pad placement, when to stand clear, and when to resume chest compressions. You follow instructions rather than making clinical judgments.
Combining CPR With AED Use
An AED works best alongside CPR, not as a replacement for it. If you witness someone collapse and an AED isn’t immediately within reach, start chest compressions right away. Push hard and fast in the center of the chest, about 100 to 120 compressions per minute. This keeps some blood flowing to the brain and organs while someone else retrieves the AED.
Once the AED arrives, turn it on and follow the voice prompts. You’ll place the pads on the person’s bare chest, and the device will analyze the rhythm. If it delivers a shock, resume CPR immediately afterward, starting with chest compressions. Don’t wait to see if the person wakes up. The AED will prompt you to stop again after about two minutes to reanalyze the rhythm. Continue this cycle of CPR and rhythm checks until emergency medical services arrive or the person starts moving and breathing normally.
Special Situations to Be Aware Of
A few scenarios require minor adjustments, but none of them should stop you from using an AED.
If the person’s chest is wet from rain, sweat, or submersion, quickly wipe the chest dry before placing the pads. Research simulating wet environments has shown that defibrillation doesn’t pose significant risk to the rescuer even in wet conditions, but dry skin helps the pads adhere properly and deliver the shock effectively. If you can’t move the person to a dry area, dry the chest as best you can and proceed.
If the person has a pacemaker or implanted defibrillator, you’ll typically see a visible lump under the skin on the upper chest. Place the AED pad at least an inch away from the device rather than directly over it.
If there’s a medication patch on the chest where a pad needs to go, peel it off and wipe the area clean before applying the pad. Remove any metal jewelry or piercings that would sit between the skin and the electrode pads, as metal can interfere with the shock delivery or cause a burn.
For children between ages 1 and 8, use pediatric pads if the AED has them. These deliver a lower energy dose. If pediatric pads aren’t available, use the standard adult pads. Some AEDs have a pediatric switch or key instead. For infants under 1 year old, a manual defibrillator is preferred, but an AED with pediatric pads can be used if that’s what’s available.
Legal Protections for Bystanders
One reason people hesitate to use an AED is fear of doing something wrong and facing legal consequences. Good Samaritan laws exist in all 50 U.S. states specifically to address this concern. These laws protect individuals who voluntarily provide emergency care without expecting payment. As long as you act in good faith and aren’t reckless, you’re shielded from negligence claims. The legal standard protects against ordinary mistakes made in the stress of an emergency. It does not cover gross negligence or intentional harm, but using an AED on someone in cardiac arrest is exactly the kind of reasonable emergency action these laws were written to encourage.
The greater legal and ethical risk, frankly, is doing nothing. A person in cardiac arrest who doesn’t receive defibrillation within minutes will almost certainly die. The AED guides every step and won’t shock someone who doesn’t need it. There is very little you can do wrong and almost everything to gain by acting.