An automated external defibrillator (AED) analyzes your heart’s electrical activity and delivers a controlled shock to stop dangerous, chaotic rhythms so the heart can reset itself. The device is designed for anyone to use, even without medical training, because it walks you through every step with voice prompts and will only allow a shock when it detects a rhythm that calls for one.
What Happens Inside the Heart During Cardiac Arrest
To understand what an AED does, it helps to know what’s going wrong. In a normal heartbeat, an electrical signal travels through the heart in an organized wave, causing the muscle to contract and pump blood. During cardiac arrest, that organized signal breaks down into one of several abnormal patterns.
The two rhythms an AED can treat are ventricular fibrillation and pulseless ventricular tachycardia. In ventricular fibrillation, the lower chambers of the heart quiver chaotically instead of pumping. In pulseless ventricular tachycardia, the heart beats dangerously fast but can’t move blood effectively. Both are “shockable” rhythms, meaning an electrical jolt can interrupt them.
Not every cardiac arrest involves a shockable rhythm. In asystole (flatline), there’s no electrical activity at all. In pulseless electrical activity, the heart still produces electrical signals but is too weak to actually pump. Neither of these responds to a shock, which is why an AED analyzes first and won’t deliver energy when it detects these patterns. This built-in safeguard means the device cannot accidentally shock someone who doesn’t need it.
How the Shock Resets the Heart
When ventricular fibrillation occurs, dozens of small electrical circuits are firing at random throughout the heart muscle, each one triggering its own tiny contraction. The result is a quivering mess instead of a coordinated pump. The defibrillation shock sends a large burst of electrical current through the heart that simultaneously interrupts all of those chaotic circuits. It does this by forcing the heart cells into a temporary resting state, essentially hitting a reset button on the entire electrical system.
Once the chaotic signals are wiped out, the heart’s natural pacemaker (a small cluster of cells in the upper right chamber) has a window to take over again and restart a normal, organized rhythm. The shock doesn’t “restart” the heart the way jump-starting a car battery works. It stops the bad rhythm so the heart’s own system can reassert control.
How the AED Analyzes Heart Rhythm
The moment you attach the electrode pads to someone’s chest, the AED begins reading the heart’s electrical signals through the skin, essentially performing a simplified version of the same test a hospital heart monitor runs. A microprocessor inside the device analyzes multiple features of that signal: its frequency, amplitude, and wave shape.
The software is remarkably thorough about filtering out false signals. It checks for interference from radio transmissions, electrical sources like nearby power lines, loose or poorly placed electrodes, and even physical movement of the patient. Some models can detect whether someone is touching or moving the person during analysis, which would distort the reading. All of this happens in seconds. If the device identifies ventricular fibrillation or pulseless ventricular tachycardia, it charges up and prompts the rescuer to deliver a shock. If it detects a non-shockable rhythm, it tells you no shock is advised and to continue CPR.
Energy Levels and Biphasic Technology
Older defibrillators used monophasic waveforms, sending current in one direction through the heart. These required higher energy levels: 200 joules for a first shock, escalating to 300 or 360 joules for subsequent attempts. Modern AEDs use biphasic waveforms, which reverse the direction of the current partway through the shock. This two-phase approach defibrillates more efficiently at lower energy levels.
Biphasic AEDs typically deliver around 150 joules and achieve the same success rates (roughly 86% to 89% for the first shock) as monophasic devices at 200 joules. Lower energy also means less potential damage to heart tissue, which is one reason biphasic technology has become the standard in virtually all AEDs you’ll encounter in airports, gyms, and offices today.
Using an AED Step by Step
Every AED gives spoken instructions the moment you turn it on. The general sequence is the same across brands:
- Power on. Press the power button or open the lid (some models turn on automatically when opened).
- Expose the chest. Remove clothing from the person’s chest. If the skin is wet, wipe it dry.
- Attach the pads. Place one pad on the upper right 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 connect the pad cable to the device.
- Stand clear for analysis. The AED will tell you not to touch the person while it reads the heart rhythm. Say “Clear!” loudly so bystanders step back.
- Shock if advised. If the AED recommends a shock, confirm no one is touching the person, say “Clear!” again, and press the shock button.
After the shock, the AED will prompt you to resume CPR immediately. It will re-analyze the rhythm after about two minutes and advise another shock if needed. If no shock is advised at any point, you continue CPR until emergency medical services arrive.
Pad Placement for Children
For children and infants, some AEDs include pediatric pads or a child-specific key or switch that reduces the energy delivered. If the child’s chest is too small for both pads to sit without touching each other, place one pad on the center of the chest and the other on the back between the shoulder blades. If pediatric pads aren’t available, adult pads can still be used, as delivering a shock is better than not attempting defibrillation at all.
Why Every Minute Counts
Survival chances during cardiac arrest drop by about 10% for every minute that passes without CPR and defibrillation. At five minutes, the odds are roughly half what they were at the moment of collapse. At ten minutes without intervention, very few people survive. This is why AEDs are placed in public spaces where emergency responders might take several minutes to arrive. A bystander who starts CPR and uses an AED in the first few minutes can dramatically change the outcome.
Safety in Wet or Metal Environments
A common concern is whether it’s safe to use an AED near water or on metal surfaces like a boat deck or factory floor. Current research confirms that most AEDs are self-grounded, meaning the electrical current travels between the two pads on the patient’s chest rather than flowing outward into the surroundings. Bystanders standing on a wet or metal surface may feel, at most, a slight tingle during the shock. As long as you aren’t physically touching the person’s chest when the shock is delivered, there is no meaningful risk to the rescuer.
That said, you should move the person away from standing water if possible and dry the chest before applying pads. This isn’t primarily for safety but for effectiveness: wet skin can prevent the pads from adhering properly, and a poor connection reduces how well the shock reaches the heart.
Maintenance and Readiness
AEDs are designed to sit unused for months or years and work immediately when needed, but they do require basic upkeep. Batteries typically last four to five years, depending on the manufacturer, and will have either an “install by” or “expiration” date printed on a sticker. Electrode pads also expire because the adhesive gel dries out over time, so check for a “use by” date on the packaging. Most AEDs run automatic self-checks (daily or weekly) and display a visible status indicator, usually a green light or checkmark, confirming they’re ready. If that indicator shows a problem, the battery or pads likely need replacing.