An AED, or automated external defibrillator, is a portable device that delivers an electrical shock to the heart during sudden cardiac arrest. It’s designed so that anyone, even without medical training, can use it to potentially save a life. The device analyzes the heart’s rhythm on its own, determines whether a shock is needed, and walks the user through each step with voice prompts.
What an AED Actually Does
Sudden cardiac arrest happens when the heart stops beating effectively, usually because its electrical signals go haywire. Instead of pumping blood in a coordinated rhythm, the heart quivers chaotically, a condition called ventricular fibrillation. In that state, no blood reaches the brain or organs. Without intervention, death follows within minutes.
An AED fixes this by delivering a controlled electrical shock through the chest wall. That shock essentially resets the heart’s electrical activity, giving it a chance to resume a normal, organized rhythm. The device’s built-in computer analyzes the heart’s electrical pattern through adhesive pads placed on the chest. If it detects a shockable rhythm, it prompts the user to press a button. If the rhythm isn’t one a shock can fix, it won’t allow a shock to be delivered. This built-in safeguard means you can’t accidentally shock someone who doesn’t need it.
Why Speed Matters So Much
The window for surviving cardiac arrest is narrow. Every minute without defibrillation reduces the chance of survival by roughly 7 to 10 percent. One study found that survival rates reached as high as 70% when an AED was used within two minutes of collapse. That number drops sharply with each passing minute.
Research comparing bystander AED use to waiting for paramedics paints a clear picture. When a bystander used an AED before emergency medical services arrived, 66.5% of patients survived to hospital discharge, compared to 43% when paramedics delivered the first shock. Beyond raw survival, the quality of recovery was also better: 57.1% of bystander-shocked patients left the hospital with good neurological function, versus 32.7% in the group that waited for EMS. The difference comes down to time. Paramedics are typically minutes away, and those minutes matter enormously.
This is why the American Heart Association’s 2025 resuscitation guidelines identify public access defibrillation as one of its top priorities, with a specific goal of increasing the number of cardiac arrest patients who have an AED applied before paramedics arrive.
How to Use an AED
You don’t need certification to use an AED. The device is designed to guide a complete beginner. Here’s what the process looks like:
- Turn it on. Press the power button. The device immediately begins giving voice instructions.
- Expose the chest. Remove all clothing covering the person’s chest. If the skin is wet, wipe it dry.
- Attach the pads. Place one adhesive pad on the upper right side of the chest, below the collarbone. Place the second on the lower left side, a few inches below the armpit. Diagrams on the pads show exactly where they go.
- Plug in the connector. Some models require you to plug the pad cable into the unit.
- Let the AED analyze. Make sure nobody is touching the person. Say “Clear!” loudly. The device will analyze the heart rhythm on its own.
- Deliver the shock if prompted. If a shock is advised, the device will tell you. Confirm no one is touching the person, say “Clear!” again, and press the shock button.
After delivering a shock (or if no shock is advised), the AED will prompt you to begin or continue CPR. Keep following its instructions until paramedics arrive.
Using an AED on Children
AEDs can be used on children, but pad placement differs based on age. For children under 8, many AED models come with pediatric pads that deliver a lower energy dose. If pediatric pads aren’t available, adult pads can still be used.
The key difference is positioning. On small chests, standard pad placement (upper right and lower left) can cause the pads to touch or overlap, which reduces effectiveness. For infants and young children, the recommended approach is to place one pad on the center of the chest and the other on the back between the shoulder blades. For children 8 and older, the standard adult positions work fine: one pad below the right collarbone and the other under the left armpit.
Where AEDs Are Found
AEDs are placed in locations where large numbers of people gather and where cardiac arrest is statistically more likely to be witnessed. You’ll commonly find them in airports, shopping malls, gyms, office buildings, schools, sports arenas, and houses of worship. They’re typically mounted in wall cabinets, often near other emergency equipment, and marked with a green heart symbol or AED signage.
Public access defibrillation programs, supported by the CDC and American Heart Association, have been expanding placement of these devices for over two decades. The goal is simple: get a defibrillator within a few minutes’ reach of any person who collapses in a public space.
How the Shock Works
Modern AEDs use what’s called a biphasic waveform, meaning the electrical current flows in one direction through the heart and then reverses. Older devices used monophasic waveforms, sending current in only one direction. The biphasic approach is more effective at restoring a normal rhythm while using less energy, which allowed manufacturers to make the devices smaller, lighter, and more portable.
A typical biphasic AED delivers between 120 and 200 joules on the first shock, with the option to escalate on subsequent shocks. Older monophasic models used a standard escalating sequence starting at 200 joules and climbing to 360. You don’t need to know any of this to operate one. The device selects the appropriate energy level automatically.
Maintenance and Readiness
AEDs are designed to sit idle for months or years and work perfectly when needed, but they do require basic upkeep. Both the battery and the adhesive electrode pads have shelf lives that typically range from two to five years depending on the manufacturer and model. Most devices run automatic self-checks (daily or weekly) and display a status indicator, usually a green light or checkmark, confirming they’re ready to use.
If you’re responsible for an AED at your workplace or organization, periodic checks are straightforward: confirm the status light is green, verify that the pads and battery haven’t expired, and make sure the device is accessible and clearly marked. Expired pads may not adhere properly to the skin, and a depleted battery can fail to deliver a shock when it matters most.
Legal Protections for Bystanders
Federal law in the United States provides civil liability protection to anyone who uses an AED on a person in a perceived medical emergency. Under this statute, you are immune from lawsuits for harm resulting from your use of the device, as long as you acted in good faith. The only exceptions involve willful misconduct, gross negligence, or reckless indifference to the victim’s safety. In practical terms, a bystander who grabs an AED off the wall and follows its prompts during a cardiac emergency is protected.
Most states have their own Good Samaritan laws that reinforce this protection. The legal framework exists specifically to remove hesitation. Legislators recognized that fear of being sued was a real barrier to bystander action, and the law was written to eliminate that concern.