An automated external defibrillator (AED) is a portable device that analyzes a person’s heart rhythm and delivers an electric shock only when it detects a specific type of life-threatening cardiac arrest. It will not shock a normal heartbeat, a stopped heart with no electrical activity, or any rhythm that doesn’t match the two conditions it’s programmed to treat. That built-in safety feature is one of the most important things to understand about how AEDs work.
How an AED Analyzes Heart Rhythm
When you place the sticky electrode pads on someone’s bare chest, sensors in the pads pick up the heart’s electrical signals and send them to a small computer inside the device. That computer runs an algorithm to determine whether the rhythm is “shockable” or “non-shockable.” The entire analysis takes only a few seconds.
The two shockable rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pulseless VT). In ventricular fibrillation, the heart’s lower chambers quiver chaotically instead of pumping blood. In pulseless VT, those chambers beat extremely fast but fail to circulate blood effectively. Both are fatal without intervention, and both respond to an electrical shock that resets the heart’s electrical system so a normal rhythm can resume.
If the AED detects any other rhythm, including a normal heartbeat or a flatline (asystole), it will announce “no shock advised” and will not allow a shock to be delivered. You cannot accidentally shock someone who doesn’t need it.
Step-by-Step Voice Guidance
AEDs are designed so that someone with no medical training can use one. The device walks you through the entire process with spoken prompts and visual indicators. After you power it on, it will instruct you to stay calm, check if the person is responsive, call emergency services, and attach the electrode pads to the person’s bare chest. If a pad isn’t making good contact, the device will tell you to check the connection.
Once the pads are on, the AED will say something like “don’t touch patient, analyzing.” After the analysis, it either advises a shock or tells you no shock is needed. If a shock is advised, the device charges itself and prompts you to press a clearly marked, flashing button. Some models then coach you through CPR, even providing real-time feedback like “push harder” or “good compressions” to help you maintain effective chest compressions between analysis cycles.
Why Every Minute Counts
For every minute a person’s heart remains in cardiac arrest without treatment, their chance of survival drops by 7 to 10 percent. That math is stark: after 10 minutes with no intervention, survival odds are extremely low. Untreated ventricular fibrillation quickly deteriorates into asystole, a flatline rhythm from which recovery rates are dismal.
Using an AED before emergency medical services arrive nearly doubles a person’s odds of surviving. A large study published in the Journal of the American College of Cardiology found that overall survival to hospital discharge was just 9 percent when bystanders performed CPR alone. When an AED was applied, survival rose to 24 percent. When the AED analyzed the rhythm and delivered a shock, survival reached 38 percent. That gap illustrates why public access to AEDs matters so much: the device bridges the critical minutes between cardiac arrest and the arrival of paramedics.
Who Can Legally Use an AED
You do not need medical training to use an AED in most of the United States. All 50 states and Washington, D.C., have enacted laws that limit civil liability for lay bystanders who use an AED in good faith. In 48 states, even untrained rescuers are protected. Only three states (Rhode Island, Illinois, and New York) require AED training as a condition of that legal protection. These Good Samaritan protections exist specifically to encourage bystanders to act without fear of a lawsuit.
Safety Precautions During Use
A few situations call for extra attention. If the person’s chest is wet from rain, sweat, or a pool, you should dry the skin around where the pads will go so they stick properly and conduct the electrical signal clearly. While best practice is to move someone to a dry area before defibrillation, research simulating wet environments has not shown significant shock risk to the rescuer if circumstances make moving the person impossible.
If the person has a visible implanted device like a pacemaker (usually a small lump under the skin near the collarbone), avoid placing a pad directly over it. You can use an anterior-posterior placement instead: one pad on the front of the chest and one on the back, directly behind the first. This prevents the shock energy from being diverted by the implant.
Other practical steps: remove any medication patches from the chest before applying pads, and make sure no one is touching the person when the AED analyzes or delivers a shock. The device will remind you of this with a “don’t touch patient” prompt.
Maintenance and Shelf Life
An AED sitting in a wall-mounted case at a gym or airport isn’t maintenance-free. Both the battery and the electrode pads have expiration dates, and letting them lapse means the device may not work when someone needs it.
Most AED batteries last two to five years in standby mode, depending on the manufacturer. Some models push that to seven years. Each battery is printed with a manufacture date, an install-by date, and an expiration date. Electrode pads typically last two to four years before the conductive gel that helps them stick to skin and conduct electricity dries out. Pads must also be replaced after every use, even if a shock was never delivered, because the sealed packaging has been opened.
Most modern AEDs run daily or weekly self-checks and display a green light or status indicator to confirm they’re ready. If the light turns red or starts flashing, the battery or pads likely need replacing. Organizations that maintain AEDs should assign someone to check this indicator regularly and keep spare pads and a backup battery on hand.
What an AED Cannot Do
An AED treats two specific electrical malfunctions of the heart. It does not restart a heart that has completely stopped producing electrical activity (asystole, or flatline). It does not treat heart attacks, which involve blocked blood flow rather than a rhythm problem, though a heart attack can trigger the kind of cardiac arrest an AED does treat. And it will not shock someone who still has a pulse, even if their heart rhythm is abnormal. The device’s internal algorithm ensures that a shock is delivered only when it will help.
CPR remains essential alongside AED use. Chest compressions keep some blood flowing to the brain and organs during the minutes between cardiac arrest and defibrillation. The best outcomes happen when a bystander starts CPR immediately and uses an AED as soon as one is available.