You should use a defibrillator any time someone collapses, is unresponsive, and is not breathing normally. These are the signs of cardiac arrest, and every minute without a shock reduces the chance of survival by 7 to 10 percent when no CPR is being performed. You don’t need medical training to operate one. Automated external defibrillators (AEDs) are designed to walk any bystander through the process with voice prompts, and they will only deliver a shock if the heart actually needs one.
Recognizing Cardiac Arrest
Cardiac arrest looks like this: a person suddenly collapses, loses consciousness, and either stops breathing entirely or begins gasping in an irregular, labored way. They won’t respond to shouting or shaking. This is different from fainting, where someone typically regains consciousness within seconds and begins breathing normally on their own. It’s also different from a seizure, where a person may have rhythmic muscle movements and usually recovers (though they’ll be confused and exhausted afterward).
The critical distinction is responsiveness and breathing. If someone is unconscious and not breathing normally after a collapse, treat it as cardiac arrest. You do not need to check for a pulse. Even trained healthcare providers can struggle to find one quickly, and the seconds spent searching are seconds wasted. Call 911 first, start chest compressions, and send someone nearby to grab the nearest AED.
Why Speed Matters
During cardiac arrest, the heart’s electrical system short-circuits. Instead of beating in a coordinated rhythm, the heart quivers chaotically or races so fast it can’t actually pump blood. The brain starts losing oxygen immediately. With CPR alone, survival odds drop 3 to 4 percent per minute. Without CPR, they drop 7 to 10 percent per minute. After about 10 minutes with no intervention, survival becomes unlikely.
A defibrillator delivers an electrical shock that resets the heart’s rhythm, giving it a chance to resume normal beating. The sooner that shock arrives, the better. In a large South Korean study of over 35,000 witnessed cardiac arrests, people who received a bystander-applied AED shock had a survival-to-discharge rate of 46.6 percent, compared to 23 percent for those who received only CPR. That raw difference is striking, even though other factors like how quickly EMS arrived also played a role.
How an AED Decides Whether to Shock
An AED doesn’t shock every heart rhythm. It analyzes the heart’s electrical activity through the pads you place on the person’s chest and determines whether the rhythm is “shockable.” Two rhythms respond to defibrillation: ventricular fibrillation, where the heart’s electrical signals are chaotic and disorganized, and pulseless ventricular tachycardia, where the heart beats extremely fast (100 to 300 beats per minute) but produces no effective blood flow.
Two other cardiac arrest rhythms are not shockable. One is when the heart has some electrical activity but it isn’t producing a pulse. The other is a complete flatline, with no electrical activity at all. In these cases, the AED will tell you “no shock advised” and prompt you to continue CPR. This is an important safety feature. You cannot accidentally shock someone who doesn’t need it. If the AED says no shock, keep doing compressions until paramedics arrive.
How to Use an AED Step by Step
Turn the device on. Most AEDs have a single power button or activate when you open the case. The machine will begin giving you voice instructions immediately.
Expose the person’s bare chest. Remove all clothing, including anything with metal like an underwire bra. The device comes with two adhesive pads, each labeled with a diagram showing where it goes. One pad typically goes on the upper right chest below the collarbone. The other goes on the lower left side of the ribcage, under the armpit. Press them firmly against the skin.
Once both pads are attached, the AED will analyze the heart rhythm. It will tell you to stop CPR briefly and not touch the person while it reads the signal. If it detects a shockable rhythm, it will either charge and instruct you to press the shock button or deliver the shock automatically, depending on the model. Make sure no one is touching the person when the shock is delivered. After the shock, the AED will prompt you to resume CPR immediately. It will re-analyze the rhythm every two minutes.
Safety Precautions
If the person is lying in water, pull them out before applying the pads. Rain and snow are fine. You can also safely use an AED on someone lying on a metal surface like a grate or bleachers, as long as the electrode pads don’t contact the metal directly and nobody is touching the person during the shock.
Check the chest for a few things before placing pads. If you see a small, hard lump under the skin near the collarbone, that’s likely a pacemaker or implanted defibrillator. Don’t place a pad directly over it. Shift the pad at least an inch away. If the person has a medication patch (common for pain relief or nicotine), peel it off and wipe the skin clean before placing a pad in that spot. Leaving it on can cause a burn when the shock is delivered.
If the person’s chest is excessively hairy, the pads may not stick well. Many AED kits include a razor for exactly this reason. A quick shave of the pad areas ensures good contact.
Using an AED on Children
For children 8 years old or younger, or weighing under 55 pounds, use pediatric pads if the AED has them. These pads deliver a lower energy dose appropriate for a smaller body. If pediatric pads aren’t available, use the adult pads. A shock with adult pads is far better than no shock at all. On very small children or infants, the two pads may need to go on the front and back of the chest rather than both on the front, simply because there isn’t enough room to keep them from overlapping.
Legal Protections for Bystanders
Every U.S. state has some form of Good Samaritan law that protects people who use an AED in good faith during an emergency. These laws generally make you immune from civil liability as long as you act reasonably, call 911, and aren’t grossly negligent. The protection typically extends to the person who trained you, the physician overseeing the AED program, and the owner of the building where the device is located. You don’t need to be certified to use one in an emergency, though the American Heart Association recommends training in CPR and AED use so you feel confident acting quickly when it counts.
When Not to Use a Defibrillator
If the person is conscious, talking, or breathing normally, they are not in cardiac arrest and do not need an AED. Someone who faints but wakes up within a few seconds doesn’t need one. Someone having a seizure doesn’t need one, though you should monitor them afterward because seizures can occasionally trigger dangerous heart rhythms.
If you’re genuinely unsure whether someone is in cardiac arrest, err on the side of turning the AED on and placing the pads. The device will analyze the rhythm and only deliver a shock if one is needed. There is no scenario where applying an AED to an unconscious, non-breathing person causes harm. The worst outcome is the machine says “no shock advised” and you continue CPR until help arrives.