An AED, or automated external defibrillator, is a portable device that analyzes a person’s heart rhythm and delivers an electric shock to restore a normal heartbeat during sudden cardiac arrest. It’s designed so that anyone, even without medical training, can use it in an emergency. AEDs are the lightweight, wall-mounted devices you see in airports, gyms, schools, and offices, and they exist because the faster a person in cardiac arrest receives a shock, the more likely they are to survive.
What an AED Actually Does
When someone goes into sudden cardiac arrest, their heart doesn’t just stop. It typically starts quivering chaotically instead of pumping blood. This chaotic rhythm, called ventricular fibrillation, means the heart is firing electrical signals randomly and can’t push blood to the brain or organs. A less common but equally dangerous pattern is when the heart beats so fast it can’t fill with blood between beats.
An AED fixes this by delivering a controlled electrical shock through adhesive pads placed on the chest. The shock essentially resets the heart’s electrical system, giving it a chance to resume a normal, coordinated rhythm. Think of it like rebooting a frozen computer. The device has a built-in computer that reads the heart’s electrical activity through the pads and determines whether a shock is appropriate. If the heart’s rhythm isn’t one that a shock can fix, the AED will not deliver one, no matter how many times you press the button. This built-in safety feature is a major reason AEDs are considered safe for untrained bystanders to use.
Cardiac Arrest vs. Heart Attack
These two terms get used interchangeably, but they describe different emergencies. A heart attack is a circulation problem: a blocked artery cuts off blood flow to part of the heart muscle, which starts to die. The person is usually conscious, often experiencing chest pain, and their heart is still beating. An AED is not designed for heart attacks.
Sudden cardiac arrest is an electrical problem. The heart’s rhythm goes haywire and it stops pumping entirely. The person collapses, loses consciousness, and stops breathing normally within seconds. Without intervention, they die within minutes. This is when an AED is needed. That said, a heart attack can trigger cardiac arrest, which is part of why the two are so often confused.
Why Every Minute Matters
A 2024 study published in the American Heart Association’s journal Circulation tracked how quickly survival odds drop when defibrillation is delayed. Every additional minute without a shock was associated with a 6% lower probability of surviving to hospital discharge. That compounds fast. After five minutes, a person’s chances are dramatically worse than at one minute. Emergency medical services typically take 8 to 12 minutes to arrive, which is why publicly accessible AEDs and bystander action fill a gap that ambulances alone cannot.
How to Use One
AEDs are built to walk you through every step with voice prompts and visual instructions. You do not need certification to operate one. Here’s what the process looks like:
- Turn it on. Open the case and press the power button. The device immediately begins giving you audio instructions.
- Expose the chest. Remove or cut away clothing covering the person’s chest. If the skin is wet, wipe it dry.
- Attach the pads. Peel the adhesive pads from their packaging. Place one on the upper right side of the chest, below the collarbone. Place the other on the lower left side, a few inches below the armpit. Diagrams on the pads themselves show you exactly where they go.
- Let the AED analyze. Once the pads are connected, the device reads the heart rhythm. Make sure nobody is touching the person. Say “Clear!” loudly so bystanders step back.
- Deliver the shock if prompted. If the AED determines a shock is needed, it will tell you to press the shock button. Again, confirm no one is touching the person before pressing it. If no shock is advised, the AED will tell you to continue CPR.
After delivering a shock, the AED will instruct you to resume chest compressions. It will re-analyze the rhythm every two minutes and advise another shock if necessary. Keep following the prompts until emergency medical services arrive.
Pad Placement for Children
For children younger than 8 or weighing less than 55 pounds, many AEDs come with smaller pediatric pads that deliver a lower energy dose. If only adult pads are available, 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 touching or overlapping on a small body. For children 8 and older, standard adult pads are used in the normal positions.
Legal Protections for Bystanders
All 50 U.S. states and the District of Columbia have some form of Good Samaritan law covering AED use. In 39 states and D.C., these protections apply to anyone who uses an AED in good faith during an emergency, regardless of training. Twelve states limit liability protection to users who have received training from a recognized organization. Congress also passed the Cardiac Arrest Survival Act in 2000, providing federal-level Good Samaritan coverage for AED use in public settings.
The practical takeaway: using an AED on someone in cardiac arrest carries essentially no legal risk for a bystander acting in good faith. Not using one, on the other hand, means the person has almost no chance of surviving without advanced medical care arriving in time.
Where AEDs Are Found
Public AEDs are typically mounted in wall cabinets with clear signage, often near elevators, lobbies, or high-traffic areas. You’ll find them in airports, shopping malls, sports arenas, fitness centers, schools, office buildings, and government facilities. Many are housed in alarmed cabinets so that opening the case alerts building staff or security, but the alarm is not a deterrent. You’re meant to grab it.
Home AEDs are also available without a prescription. They use the same technology as public units and are worth considering for households where someone has a known heart condition or is at elevated risk for cardiac arrest.
Keeping an AED Ready
An AED that isn’t maintained can fail when it’s needed most. Batteries typically last two to five years, and each battery pack has a manufacture date, install-by date, and expiration date printed on it. Electrode pads also expire, generally within two to five years, because the adhesive gel dries out over time. Expired pads may not stick properly or conduct the shock effectively.
Most modern AEDs run automatic self-checks on a weekly basis. If something is wrong, the device signals the problem with a beeping sound, a red indicator light, or a message on the display screen. If you’re responsible for an AED in your workplace or building, checking that status indicator regularly and replacing batteries and pads before they expire is the extent of the maintenance involved.