An AED, or automated external defibrillator, is a portable device used to treat sudden cardiac arrest. It analyzes a person’s heart rhythm and, if it detects a life-threatening abnormality, delivers an electrical shock to restore a normal heartbeat. Every minute without that shock reduces the chance of survival by roughly 6%.
How Sudden Cardiac Arrest Works
Sudden cardiac arrest happens when the heart’s electrical system malfunctions and the heart stops pumping blood. This is different from a heart attack, where a blocked artery cuts off blood flow to part of the heart muscle. During cardiac arrest, the heart’s lower chambers (the ventricles) quiver chaotically instead of contracting in a coordinated way. This chaotic quivering is called ventricular fibrillation, and it’s the primary rhythm an AED is designed to correct.
Without blood flow, the brain and organs begin to shut down within minutes. CPR buys time by manually circulating some blood, but it rarely restores a normal heart rhythm on its own. The electrical shock from an AED is what actually resets the heart’s rhythm so it can beat effectively again.
What Happens When You Use an AED
AEDs are built so that someone with no medical training can operate one. The device talks you through every step with voice prompts the moment you turn it on. Here’s the general sequence:
- Call 911 first. Ask someone nearby to call while you grab the AED.
- Power on the AED. Open the case, press the power button, and listen.
- Expose the chest and attach the pads. Remove clothing covering the chest and wipe it dry if needed. Place one adhesive pad on the upper right chest and the other on the lower left side, a few inches below the armpit. Diagrams on the pads show exact placement.
- Let the AED analyze. The device reads the heart rhythm automatically. Make sure no one is touching the person, and say “Clear!” loudly.
- Deliver the shock if prompted. The AED will only advise a shock if it detects a rhythm that can be corrected. Press the shock button when instructed. If no shock is advised, the device will tell you.
- Start CPR immediately after. Whether a shock was delivered or not, begin chest compressions right away and continue until emergency responders arrive.
One of the most important safety features: an AED will not shock a heart that doesn’t need it. The device’s computer analyzes the rhythm first, and if it doesn’t detect ventricular fibrillation or a similar shockable rhythm, it simply won’t allow a shock to be delivered. You cannot accidentally harm someone by using one incorrectly.
Why Minutes Matter
A 2024 study published in Circulation tracked outcomes for patients with witnessed cardiac arrest and found that each additional minute without defibrillation was associated with a 6% lower probability of surviving to hospital discharge. That same delay also reduced the chance of the heart returning to an organized rhythm by 4% per minute. The math is brutal: by 10 minutes without a shock, survival odds have dropped dramatically.
Emergency medical services in most U.S. cities take an average of 7 to 12 minutes to arrive. That gap is exactly why public AEDs exist. A bystander who grabs a nearby AED and uses it before paramedics arrive can bridge the difference between life and death.
Where AEDs Are Located
You’ve likely walked past AEDs without noticing them. They’re typically mounted on walls in bright-colored cases, often near fire extinguishers or first aid stations. As of 2017, 38 states had laws supporting targeted AED placement, with 37 requiring or authorizing specific locations to have one on site. The most common required locations include schools (25 states), fitness centers and athletic facilities (15 states), and state-owned buildings (10 states).
Beyond legal requirements, you’ll commonly find AEDs in airports, casinos, churches, office buildings, shopping malls, and community centers. Some states mandate that the devices be placed in clearly marked, easy-to-access spots. Louisiana, for example, requires college athletic departments to keep an AED in an open-view location within two feet of a phone that can call 911.
Research from the CDC confirms that placing AEDs in public locations where cardiac arrest is likely to occur increases survival rates, improves the chance of the heart restarting on its own, and leads to better neurological outcomes for patients who survive.
Using an AED on Children
AEDs can be used on children, with one modification. For children under 8 years old, the American Heart Association recommends using pediatric pads or a child-dose key if the AED has one. These reduce the energy of the shock to a level appropriate for a smaller body. If pediatric pads aren’t available, use the standard adult pads and adult shock dose. A shock at full power is far better than no shock at all.
For children small enough that the two pads might touch on the chest, place one pad in the center of the chest and the other on the back between the shoulder blades.
Legal Protection for Bystanders
Fear of liability stops some people from acting in an emergency, but federal law provides broad legal immunity to anyone who uses an AED on a person in perceived cardiac arrest. Under U.S. Code, a bystander who uses or attempts to use an AED is immune from civil liability for any resulting harm. The same protection extends to the person or organization that purchased the device, as long as they maintained it properly, notified local emergency responders of its location, and provided reasonable training to employees expected to use it.
This immunity does not cover willful misconduct, gross negligence, or reckless behavior. It also doesn’t apply to licensed healthcare professionals acting within their professional scope, or to hospitals and clinics, since those settings have their own standards of care. But for an ordinary person responding to an emergency in a gym, airport, or office, the law is firmly on your side.
Keeping an AED Ready
An AED that sits in a wall cabinet for years still needs attention. The devices require monthly maintenance checks, and it’s important to document them. Batteries typically last 4 to 5 years depending on the manufacturer, and each battery has an “install by” or expiration date printed on it. The adhesive electrode pads also expire, since the gel that conducts the electrical shock dries out over time. Look for a “use by” date or hourglass symbol on the pad packaging.
Most modern AEDs run automatic self-tests daily or weekly and display a green light or checkmark when they’re functioning properly. A monthly visual check to confirm the indicator light is active, the pads haven’t expired, and the battery date is current takes less than a minute and ensures the device will work when someone’s life depends on it.