An automated external defibrillator (AED) works by analyzing the heart’s electrical rhythm and, if it detects a life-threatening pattern, delivering a controlled electrical shock through the chest to reset that rhythm. The shock doesn’t restart a stopped heart. Instead, it briefly stops all electrical activity in the heart so the organ’s natural pacemaker can take over and restore a normal beat. When used within two minutes of cardiac arrest, survival rates can reach as high as 70%.
What the Shock Actually Does to Your Heart
The most common misunderstanding about defibrillation is that it jump-starts a heart that has flatlined. In reality, the opposite is happening. During cardiac arrest, the heart’s muscle cells are often firing in rapid, chaotic bursts, quivering instead of pumping. The electrical shock forces nearly all of those cells to reset at once, creating a brief moment of electrical silence. If the heart’s natural pacemaker (a small cluster of cells in the upper right chamber) is still viable, it can then fire a clean signal that travels through the heart in the correct sequence, restoring a coordinated pumping rhythm and, with it, a pulse.
Which Rhythms Trigger a Shock
An AED only shocks two specific heart rhythms, both of which involve the lower chambers of the heart (the ventricles) firing incorrectly.
- Ventricular fibrillation (VF): The most common rhythm in sudden cardiac arrest. The ventricles quiver with rapid, disorganized electrical signals instead of contracting. There’s no blood flow. On a heart monitor, it looks like jagged, irregular waves with no recognizable pattern.
- Pulseless ventricular tachycardia (pVT): The ventricles beat dangerously fast, sometimes over 150 beats per minute, but produce no effective pumping. The rhythm appears more organized than VF but still generates no pulse.
If the heart has no electrical activity at all (asystole, or “flatline”), or if it shows organized electrical signals that simply aren’t producing a heartbeat (pulseless electrical activity), the AED will not deliver a shock. Shocking a flatline would serve no purpose because there’s no chaotic rhythm to reset. In these cases, the AED instructs the user to continue CPR.
How the AED Reads Your Heart
When you place the adhesive electrode pads on a person’s chest, the AED begins recording the heart’s electrical signals through the skin, essentially performing a simplified version of an ECG. Built-in software called a shock advice algorithm analyzes the shape, frequency, and amplitude of those signals to classify the rhythm as shockable or non-shockable.
These algorithms are highly accurate. Most AEDs detect shockable rhythms with a sensitivity above 90%, which is the minimum the American Heart Association recommends. Specificity (correctly identifying non-shockable rhythms) is generally strong too, though chest compressions during CPR can introduce electrical noise that mimics heart rhythms. The physical movement creates artifacts that overlap with the heart’s own signals, which is why the AED instructs everyone to stop touching the patient during analysis. Some newer devices use advanced signal processing, including wavelet-based filtering, to read the heart rhythm even while CPR continues.
Step-by-Step: What Happens When You Use One
Every AED provides voice prompts that walk you through the process. While brands differ slightly, the sequence is the same:
You power on the device and it tells you to attach the electrode pads. One pad goes on the upper right chest, below the collarbone. The other goes on the lower left side, under the armpit. Diagrams on the pads show exact placement. Once both pads are attached, the AED analyzes the heart rhythm automatically. This takes a few seconds, during which the device will tell you to stand clear.
If a shockable rhythm is detected, the AED charges and either delivers the shock automatically or tells you to press a button. Most public-access AEDs are semi-automatic, meaning you press a clearly marked shock button when prompted. Fully automatic models deliver the shock on their own after a verbal countdown. Either way, the device then instructs you to resume CPR for about two minutes before it analyzes the rhythm again.
Energy Levels and Pediatric Use
Standard AEDs deliver between 150 and 360 joules per shock when using adult pads. For an average adult weighing 80 to 125 kilograms, that works out to roughly 1 to 4 joules per kilogram of body weight. If the first shock doesn’t work, some devices escalate the energy with each subsequent attempt, up to 360 joules.
Children under eight or weighing less than 25 kilograms need less energy. Pediatric pad and cable systems are designed to reduce the delivered dose to about 50 to 75 joules. They do this by increasing the electrical resistance in the circuit and diverting some current away from the patient. The recommended pediatric dose is around 2 joules per kilogram. If pediatric pads aren’t available, adult pads and energy levels can still be used on a child in cardiac arrest, because the risk of not shocking far outweighs the risk of a higher dose.
Special Situations to Watch For
AEDs are designed to be safe for bystanders and patients, but a few circumstances require extra steps before you attach the pads.
Water is the main concern. It conducts electricity and can create a path for energy to travel from one pad to the other across wet skin, reducing how much reaches the heart. It can also deliver minor shocks or burns to nearby rescuers. If the person is lying in water, drag them to a dry surface first. If their chest is wet, wipe it dry before placing the pads.
Medication patches on the chest can block the shock or cause burns. Peel off any patch in the area where a pad needs to go and wipe the skin. For patients with an implanted pacemaker or defibrillator (you’ll see a visible lump under the skin, usually below the collarbone), place the AED pad at least one inch away from the device to avoid interfering with it. Metal surfaces, despite what many people assume, are not a hazard. They pose no shock risk to the victim or rescuer.
Why Timing Matters So Much
During cardiac arrest, survival drops roughly 7 to 10 percent with every minute that passes without defibrillation. International studies report baseline survival rates between 2% and 11%, largely reflecting how long it takes for someone to get a shock. The dramatic finding, that survival can reach 70% with AED use within two minutes, underscores why these devices are placed in airports, gyms, and office buildings rather than kept only in hospitals. CPR buys time by manually circulating some blood, but it rarely converts a lethal rhythm back to normal on its own. The combination of CPR plus AED use is consistently more effective than CPR alone.
Legal Protection for Bystanders
Every U.S. state has Good Samaritan laws that protect people who use an AED in an emergency. These laws shield you from liability for ordinary negligence, meaning you can’t be sued for trying to help and making a reasonable mistake. The protection applies as long as you act in good faith, don’t expect payment, and aren’t grossly reckless. If the person is unconscious, the law assumes implied consent, so you don’t need anyone’s permission to begin. AEDs are specifically designed so that untrained bystanders can use them safely, and the device itself prevents you from shocking a rhythm that doesn’t need it.
Keeping an AED Ready
An AED that sits in a wall-mounted case for years still needs periodic maintenance. Batteries last two to five years in standby mode depending on the manufacturer, and should be replaced within five to seven years of the manufacture date regardless. Electrode pads dry out over time and typically have a shelf life of two to four years. Some brands, like certain Physio-Control models, recommend pad replacement every two years, while others last longer.
Most modern AEDs run automatic self-checks daily or weekly and display a green indicator light when they’re ready. If the light turns red or starts flashing, the battery or pads likely need replacing. Organizations responsible for a public AED should assign someone to check the status indicator monthly and track expiration dates for both batteries and pads.