An AED, or automated external defibrillator, delivers a controlled electrical shock to the heart during cardiac arrest. Its purpose is to stop a dangerous, chaotic heart rhythm so the heart’s natural pacemaker can restart a normal beat. Contrary to what movies suggest, an AED doesn’t jump-start a stopped heart. It resets one that’s beating out of control.
How an AED Corrects a Dangerous Rhythm
During certain types of cardiac arrest, the heart’s lower chambers (the ventricles) either quiver chaotically or fire electrical signals so fast they can’t pump blood. The heart is technically still active, but it’s producing no useful circulation. Within minutes, the brain and organs begin to suffer irreversible damage.
An AED stops this by sending an electrical current through the heart muscle, forcing all the cells to depolarize at once. This briefly halts all electrical activity, like clearing a jammed system. If the heart’s natural pacemaker (a small cluster of cells in the upper right chamber) is still viable, it can then resume sending organized electrical signals, restoring a coordinated heartbeat and blood flow.
Which Heart Rhythms It Can Treat
An AED only works on two specific rhythms: ventricular fibrillation, where the ventricles quiver uselessly, and pulseless ventricular tachycardia, where the ventricles fire extremely fast but produce no actual blood flow. These are called “shockable” rhythms. If the heart has flatlined completely (asystole) or has another type of non-shockable rhythm, a shock won’t help, and the AED will not deliver one. In those situations, CPR is the only option until paramedics arrive.
This distinction matters because the device makes the decision for you. You don’t need to know what rhythm the person is in.
How the Device Guides You Step by Step
Modern AEDs are designed so someone with zero medical training can use one effectively. When you open the case and power it on, the device begins issuing voice prompts and visual instructions that walk you through the entire process.
First, you’ll be told to place two adhesive pads on the person’s bare chest. One goes just below the right collarbone, to the right of the breastbone. The other goes below and to the left of the left nipple, along the side of the chest. Most pads have diagrams printed on them showing exact placement. Once the pads are attached, the AED reads the heart’s electrical activity and analyzes the rhythm. During this step, it will tell you not to touch the person so movement doesn’t interfere with the reading.
If it detects a shockable rhythm, it announces “shock advised” and either delivers the shock automatically or tells you to press a button. If the rhythm isn’t shockable, it says “no shock advised” and prompts you to continue CPR. Some models go further: they play a metronome to help you compress at the right rate (100 per minute), monitor how deep your compressions are, and coach you with prompts like “push harder” or “good compressions.” Every two minutes, the device pauses CPR to reanalyze the rhythm and decide whether another shock is needed.
Built-In Safety Features
One of the most common concerns people have is accidentally shocking someone who doesn’t need it. AEDs are engineered to prevent this. The device analyzes the heart rhythm before every shock and will not deliver electricity to a normal heartbeat or a non-shockable rhythm. In clinical testing, AEDs correctly withheld shocks for normal sinus rhythm 100% of the time, and their accuracy for other non-shockable rhythms exceeded 99.5%.
That said, the AED cannot detect whether someone has a pulse. This is why checking for a pulse (or at least confirming the person is unresponsive and not breathing normally) before applying the pads is important. Ventricular tachycardia sometimes occurs with a pulse, meaning the person is not in cardiac arrest. Shocking them in that scenario could cause harm.
Why Speed Matters So Much
For every minute a person’s heart is in cardiac arrest without defibrillation, their chance of survival drops by 7 to 10 percent. That math gets grim fast. At five minutes, survival may already be cut in half.
The numbers tell the story clearly. In a large study of cardiac arrests occurring before paramedics arrived, survival to hospital discharge was 9% when bystanders performed CPR alone. When an AED was applied before EMS arrival, survival jumped to 24%. And when the AED analyzed the rhythm and actually delivered a shock, survival reached 38%. That’s a fourfold improvement over CPR alone. The difference is almost entirely about time: the faster a shockable rhythm gets defibrillated, the more likely the heart’s natural pacemaker can successfully take over again.
Pediatric Use
AEDs can be used on children. For infants and children under 8 years old or weighing less than 55 pounds, the American Heart Association recommends using pediatric electrode pads, which deliver a lower energy dose. Many AED models include a pediatric setting or separate pediatric pads. If pediatric pads aren’t available during an emergency, adult pads can still be used. Some manufacturers recommend placing one pad on the chest and one on the back for smaller children to avoid the pads touching each other.
Legal Protections for Bystanders
All 50 U.S. states have some form of Good Samaritan law designed to protect people who use an AED in an emergency. These laws shield you from civil liability as long as you acted reasonably and in good faith. You don’t need formal training or certification to be protected. The laws exist specifically because bystander action is so critical to survival, and legislators didn’t want fear of lawsuits to stop someone from grabbing a nearby AED.
The protection does have limits. It doesn’t cover someone who acts with gross negligence or willfully disregards the person’s safety. But using an AED as directed on someone you genuinely believe is in cardiac arrest falls well within the definition of reasonable, good-faith action.
Keeping an AED Ready
AEDs require minimal maintenance, but they do have consumable parts that expire. Electrode pads typically last about two years before the adhesive gel dries out and they need replacing. Batteries last two to five years depending on the model. Both components have expiration dates printed on them. Most AEDs run automatic self-checks (daily or weekly) and display a status indicator, usually a green checkmark or a flashing light, so you can confirm at a glance that the device is ready to use. If the indicator shows a problem, it typically means the battery is low or the pads have expired.
If your workplace, school, or home has an AED, checking that status light periodically and replacing expired components is the only routine maintenance involved. The device itself can last a decade or more.