An AED is needed whenever someone collapses, stops breathing, and has no pulse. These three signs together point to sudden cardiac arrest, and an AED is the only tool outside a hospital that can restore a normal heart rhythm. Survival drops by 7 to 10% for every minute without defibrillation, so the decision to grab an AED should happen within seconds of recognizing the emergency.
Sudden Cardiac Arrest vs. Heart Attack
The confusion between these two events is one of the biggest reasons people hesitate. A heart attack is a circulation problem: a blocked artery cuts off blood flow to part of the heart muscle, but the heart keeps beating. The person is typically conscious, may have chest pain, and does not need an AED.
Sudden cardiac arrest is an electrical problem. The heart’s rhythm becomes chaotic and it stops pumping blood entirely. The person loses consciousness, stops breathing normally, and has no pulse. This is the situation that demands an AED. A heart attack can sometimes trigger cardiac arrest, which is why both emergencies require calling 911 immediately, but the AED only comes into play when the heart has stopped doing its job.
The Three Signs to Look For
Recognizing cardiac arrest is simpler than most people expect. You’re looking for three things happening together:
- Sudden collapse or unresponsiveness. The person doesn’t react when you tap their shoulders and shout.
- No normal breathing. The chest isn’t rising and falling in a regular pattern.
- No pulse. If you’re unsure about checking a pulse, the first two signs are enough to act.
One detail trips up many bystanders: gasping. A person in cardiac arrest often makes occasional snoring, gurgling, or moaning sounds. This is called agonal breathing, and it does not mean the person is recovering or breathing adequately. Research published in Circulation found that gasping is common during cardiac arrest and frequently delays bystanders from starting CPR because they mistake it for real breathing. If someone has collapsed and is only taking occasional, irregular gasps, treat it as cardiac arrest. Start CPR and send someone for an AED.
What the AED Actually Does
An AED analyzes the heart’s electrical activity through adhesive pads placed on the chest. It looks for two specific rhythm problems. The first is ventricular fibrillation, where the heart’s lower chambers quiver uselessly instead of pumping. The second is pulseless ventricular tachycardia, where the heart beats dangerously fast but produces no effective blood flow. These are the two “shockable” rhythms, and ventricular fibrillation is by far the most common one found in witnessed cardiac arrests.
If the AED detects either rhythm, it charges and instructs you to deliver a shock. If the heart has stopped entirely (a flat line, called asystole) or has a rhythm that won’t respond to a shock, the AED will tell you “no shock advised” and prompt you to continue CPR. You cannot accidentally shock someone who doesn’t need it. The device makes the diagnosis for you.
Why Every Minute Counts
Ventricular fibrillation doesn’t last forever. Left untreated, it deteriorates into asystole within a few minutes, at which point a shock can no longer restart a normal rhythm. That’s why survival drops so steeply with each passing minute. If an AED delivers a shock within the first three to five minutes, the odds of survival are dramatically higher than if defibrillation is delayed until paramedics arrive, which in many areas takes eight minutes or longer.
The sequence matters, too. CPR keeps some blood flowing to the brain and heart while you wait for the AED. Once the AED arrives, turn it on, attach the pads, and follow its voice prompts. After a shock is delivered, resume chest compressions immediately rather than waiting to see if the person wakes up. The heart often needs continued support even after a successful shock.
Using an AED on Children
AEDs can be used on children, with some modifications. For children aged 1 to 8 (roughly under 55 pounds), pediatric pads or a pediatric cable system should be used if available. These reduce the energy delivered to a level appropriate for a smaller body. If pediatric pads aren’t available, adult pads are still acceptable. Cardiac arrest in a child is rare, but when it happens, defibrillation is just as time-critical.
For infants under 1 year old, the evidence is less clear, and guidelines have historically not made a strong recommendation for or against AED use in this age group. In practice, if an infant is in cardiac arrest and an AED with pediatric pads is the only option, most emergency guidance favors using it over doing nothing.
Situations That Make People Hesitate
Several common scenarios cause bystanders to pause when they shouldn’t.
Wet surfaces. You can use an AED on someone lying on a wet or metal surface. The key is making sure the skin where the pads attach is dry. Wipe the chest quickly with a towel or clothing before placing the pads, then stand clear when the shock is delivered.
Pacemakers or implanted defibrillators. If you see a visible bulge on the person’s upper chest (usually below the collarbone), that’s likely an implanted device. Standard AED pad placement puts one pad on the upper right chest and one on the left side of the rib cage, which typically avoids the implant. Don’t place a pad directly over the bulge, but don’t let it stop you from using the AED.
Medication patches. If a sticky patch (like a nicotine or pain patch) is on the chest where you need to place an AED pad, peel it off and wipe the area before attaching the pad.
Legal Protection for Bystanders
Federal law in the United States provides Good Samaritan immunity to anyone who uses or attempts to use an AED on a person in a perceived medical emergency. Under 42 U.S.C. ยง238q, you are immune from civil liability for harm resulting from AED use, as long as your actions weren’t reckless or criminally negligent. This protection applies even if you have no formal training. Most states have their own Good Samaritan statutes that reinforce this protection. The legal system is designed to encourage bystanders to act, not punish them for trying to help.