What Is an AED Defibrillator and How Does It Work?

An AED, or automated external defibrillator, is a portable electronic device that detects dangerous heart rhythms and delivers a controlled electric shock through the chest wall to restore a normal heartbeat. It is designed specifically for use by ordinary people with no medical training. The device does the diagnostic thinking for you: it reads the heart’s electrical activity through adhesive pads, decides whether a shock is appropriate, and walks you through each step with voice prompts. That combination of automation and simplicity is what makes AEDs so different from the defibrillators you see in hospital dramas, and why they now hang on walls in airports, gyms, and office buildings worldwide.

The Problem an AED Solves

When someone collapses in sudden cardiac arrest, the most common underlying electrical event is ventricular fibrillation. During ventricular fibrillation, the heart’s lower chambers quiver chaotically instead of contracting in the coordinated squeeze that pushes blood forward. The heart rate during this state can exceed 550 disorganized electrical activations per minute, far too fast and too erratic for the muscle to pump anything at all.¹ Blood stops circulating. Within seconds the person loses consciousness, and within minutes, without intervention, the brain begins to suffer irreversible damage.

CPR buys time by manually pushing blood through the body, but it cannot fix the underlying electrical chaos. What the heart needs is a reset: a jolt of current strong enough to simultaneously depolarize a critical mass of heart cells so the organ’s natural pacemaker can reassert a normal rhythm. That is exactly what an AED delivers.

How the Device Works, Step by Step

An AED contains a battery, a capacitor that stores electrical energy, a small computer, and a pair of adhesive electrode pads connected by cables. When you power the device on and stick the pads to the person’s bare chest, the electrodes begin reading the heart’s electrical signals, much like a simplified version of a hospital ECG. The onboard computer runs an algorithm that classifies the rhythm as either “shockable” (ventricular fibrillation or pulseless ventricular tachycardia) or “non-shockable” (everything else, including a flatline). Modern algorithms are remarkably accurate. One study of a newer algorithm that can analyze rhythm even during ongoing chest compressions reported sensitivity of 96% for detecting shockable rhythms and specificity of 98% for correctly identifying non-shockable ones.²

If the algorithm detects a shockable rhythm, the device charges its capacitor and either delivers the shock automatically or tells you to press a clearly marked button. The electrical pulse travels between the two electrode pads, passing through the chest and across the heart. On a cellular level, the shock works by depolarizing heart muscle cells and creating what researchers call “virtual electrodes” throughout the tissue, which help extinguish the chaotic electrical waves sustaining fibrillation.³ If the rhythm is not shockable, the device tells you so and prompts you to continue CPR instead. You cannot accidentally shock someone who does not need it.

Where the Pads Go and Why It Matters

Most AEDs come with diagrams printed directly on the electrode pads showing the standard placement: one pad on the upper right chest below the collarbone and the other on the lower left side of the ribcage. This is sometimes called the anterolateral position. An alternative is the anteroposterior position, with one pad on the front of the chest and the other on the back between the shoulder blades. Research has consistently found that anteroposterior placement produces lower electrical resistance through the chest. One study measured impedance roughly 20% lower in the anteroposterior position compared to anterolateral.⁴ Another found a similar pattern, with anteroposterior impedance averaging about 10 ohms lower.⁵ Lower resistance means more current reaches the heart for a given energy setting.

That said, in an emergency with an untrained rescuer, the anterolateral position shown on most pad diagrams is perfectly acceptable and far better than delaying the shock to figure out a different placement. One older study comparing three recommended positions found all produced acceptable and statistically similar impedance values.⁶ Other practical tips for good pad contact include shaving heavy chest hair if a razor is available and keeping the pads away from breast tissue, both of which help reduce impedance and improve the shock’s effectiveness.⁷

Every Minute Counts

The single biggest factor determining whether an AED shock will work is how quickly it arrives. A large study of over 3,700 patients with witnessed cardiac arrest found that the proportion of successful first-shock termination of ventricular fibrillation dropped from 93% when the shock came within six minutes to 75% when the delay exceeded sixteen minutes. Every additional minute of delay was associated with a 6% higher chance of failing to stop the fibrillation, and a 6% lower chance of surviving to hospital discharge.⁸

This is why public AED programs exist. When bystanders apply an AED before paramedics arrive, survival rates jump dramatically. In a large study across North American communities, overall survival for cardiac arrests treated by emergency medical services alone was about 7%. When a bystander applied an AED and delivered a shock before EMS arrived, survival rose to 38%.⁹ A separate study found that equipping non-medical volunteers with AEDs tripled the survival rate for witnessed sudden cardiac arrest compared to waiting for paramedics.¹⁰

Biphasic Versus Monophasic Shocks

If you read the specs on an AED, you will see it described as delivering either a “biphasic” or “monophasic” waveform. Monophasic devices send current in one direction. Biphasic devices reverse the current partway through the shock, sending it first in one direction and then the other. Almost every modern AED uses biphasic waveforms, and clinical trials of short-duration arrhythmias have shown biphasic shocks to be more effective at lower energy levels.¹¹

The real-world survival difference, however, is less clear-cut. A Cochrane review pooling data from multiple randomized trials found that biphasic shocks did not produce a statistically significant reduction in death before hospital discharge compared to monophasic shocks.¹² A nationwide observational study from Japan reached a similar conclusion, finding no significant difference in one-month survival with good neurological outcomes between the two waveform types.¹³ Biphasic devices still have practical advantages: they tend to be smaller and lighter because they can achieve the same effect at lower energy. But if you encounter an older monophasic AED in an emergency, use it without hesitation. A monophasic shock is far better than no shock.

Using an AED on a Child

AEDs are designed primarily for adults, but they can and should be used on children in cardiac arrest. The concern with pediatric patients is energy dose: an adult-level shock could potentially stress a small heart more than necessary. Most AED manufacturers sell pediatric electrode pads that contain a built-in attenuator, reducing the energy delivered to a child-appropriate level. In an observational study of pediatric pad use, all eight children found in ventricular fibrillation had their arrhythmia terminated, and five of the eight survived to hospital discharge.¹⁴

Current guidance recommends using pediatric pads for children under eight years old, including infants. But the key point every bystander should know is this: if pediatric pads are not available, use the standard adult AED. Delivering an imperfectly dosed shock is far preferable to withholding defibrillation from a child in cardiac arrest.¹⁵ The rhythm analysis algorithms in adult AEDs have also been tested on pediatric rhythms. One study evaluating AED performance during pediatric cardiac arrests found sensitivity of about 89% and specificity above 99% for correctly identifying shockable rhythms in children.¹⁶

Safety for Bystanders

One of the most common fears about AEDs is accidentally shocking yourself or someone else nearby. The risk is real in theory but vanishingly small in practice. A review of the medical literature identified a total of 29 reported adverse events involving bystanders or rescuers across all published accounts. Of those, seven were due to deliberate misuse or accidents unrelated to normal operation, three were from device malfunctions, and four occurred during training or maintenance rather than actual rescues. Among the 15 that occurred during real resuscitations, the typical consequence was a tingling sensation or a minor skin burn. No cases of immediate life-threatening injury or long-term disability to a bystander were found in the literature.

Common-sense precautions still apply. The AED’s voice prompts tell everyone to “stand clear” before the shock is delivered, and you should make sure nobody is touching the person’s body at that moment. The device will not shock unless it detects a shockable rhythm, so it cannot fire unexpectedly. Wet environments do add a wrinkle: a simulation study found that defibrillating a patient lying in a pool of water produced a maximum voltage of about 30 volts at the rescuer’s position, which could cause a mild tingling sensation but fell well within accepted safety limits.¹⁷ The standard advice is to move the person to a dry surface if possible, but if circumstances demand it, using the AED in a wet environment is unlikely to harm the rescuer.

Device Design and How It Helps Untrained Users

AEDs are specifically engineered to be used by people who have never touched one before. But not all devices are equally intuitive. A study testing untrained laypeople on different AED models found that devices with more detailed voice instructions for pad placement achieved correct pad positioning 97% of the time, compared to 68% for devices with less detailed prompts. Devices that provided step-by-step CPR guidance after the shock prompted 84% of users to begin chest compressions, versus 51% for devices that simply said “start CPR.”¹⁸ The takeaway: the quality of the voice guidance matters enormously, and manufacturers who invest in clear, granular instructions produce devices that genuinely perform better in untrained hands.

Most modern AEDs also include visual diagrams on the pads and on the device’s face, flashing lights that indicate the next step, and a metronome that beats at the recommended CPR compression rate. Some newer models begin recording audio when powered on, which helps emergency physicians later reconstruct what happened and how quickly each intervention occurred.

Maintenance Problems That Undermine Public AEDs

An AED mounted on a wall is only useful if it works when someone grabs it. A study auditing registered AEDs found that roughly one in five was not functional when inspected. The most common reasons were expired electrode pads, physical obstacles preventing retrieval of the device, and failed self-diagnostic tests.¹⁹ Electrode pads have adhesive gel that dries out over time, and batteries eventually lose their charge. Most AEDs run daily or weekly self-checks and display a status light, green for ready and red for a problem. The weak link is often the humans responsible for checking that light and replacing consumables on schedule.

If you manage a workplace or public space with an AED, the most important maintenance tasks are simple: check the status light regularly, replace pads before their printed expiration date, and swap the battery on the manufacturer’s recommended schedule, usually every three to five years. Keeping the AED in a visible, unlocked, easily accessible location is equally critical. A device locked in a supply closet or buried behind equipment might as well not exist during an emergency.

Legal Protections for Bystander Rescuers

Fear of being sued is one of the most commonly cited reasons people hesitate to use an AED on a stranger. In the United States, legal protections for bystander rescuers are broad. A review of state-level laws found that in 41 of 51 jurisdictions (the 50 states plus the District of Columbia), untrained lay rescuers are protected from civil liability when they use an AED in good faith. Seven additional jurisdictions provide protection if certain conditions are met, such as calling 911 first. Only three states at the time of the review required CPR training as a condition for civil immunity.²⁰ The practical reality is that there are essentially no published cases of a bystander being successfully sued for using an AED during a genuine cardiac emergency.

What Happens to the Heart After a Shock

A successful shock stops the chaotic electrical activity, but the heart does not always snap back to a strong, steady beat immediately. A phenomenon known as myocardial stunning can follow defibrillation, where the heart muscle temporarily contracts weakly despite having restored an organized electrical rhythm.²¹ This is one reason paramedics continue monitoring and providing supportive care even after a successful AED shock. Animal research has also shown that higher-energy shocks tend to produce more severe post-resuscitation heart dysfunction than lower-energy ones, which is part of the rationale for biphasic waveforms that achieve defibrillation at lower energy levels.²² For the bystander, the practical lesson is straightforward: even if the AED says “shock delivered” and the person shows signs of recovery, call 911 if you have not already, and keep the pads in place until paramedics take over.

The Growing Impact of Public AED Programs

The large randomized PAD trial published in the New England Journal of Medicine demonstrated that equipping trained community volunteers with AEDs doubled the number of survivors to hospital discharge compared to communities where volunteers were trained only in CPR.²³ Two decades of accumulated data since then have only strengthened the case. A 23-year analysis of an Italian public-access defibrillation program found that survival from ventricular fibrillation among patients reached by community volunteers rose from 22% in the program’s first decade to 73% in its second, coinciding with a dramatic increase in the number of AEDs placed in public locations.²⁴ The volunteers in that program arrived roughly four and a half minutes sooner than ambulance crews on average, and that time gap was one of the strongest independent predictors of whether the patient survived with a good neurological outcome.

Drone Delivery and What Comes Next

One persistent limitation of public AEDs is geographic coverage. A defibrillator in an airport terminal does nothing for someone who collapses on a suburban sidewalk three miles away. Researchers in Scandinavia have been testing a potential solution: dispatching AED-equipped drones to suspected cardiac arrest locations. The idea is that a drone can fly directly to the scene faster than a person can run to and from the nearest wall-mounted AED. Early observational work has suggested that combining drone-delivered AEDs with mobile-phone-dispatched volunteer first responders could eliminate the time currently lost to AED retrieval, shortening the interval before the first shock.²⁵ The technology is still in its early stages, and regulatory hurdles around autonomous drone flights in populated areas remain significant. But given how steeply survival drops with each passing minute, even shaving two or three minutes off the response time could meaningfully change outcomes.

How AEDs Compare to Implantable Defibrillators

People sometimes confuse AEDs with implantable cardioverter-defibrillators, or ICDs, which are small devices surgically placed inside the chest of patients known to be at high risk for dangerous heart rhythms. The two serve the same ultimate purpose but occupy very different roles. An ICD monitors the heart continuously and can deliver a shock within seconds of detecting fibrillation, without requiring any bystander action. An AED, by contrast, depends on someone being nearby, recognizing the emergency, and applying the device. ICDs are more effective for individual patients but vastly more expensive and invasive, requiring surgery and ongoing medical follow-up. One economic analysis concluded that if budget constraints forced a choice, providing in-home AEDs to a larger number of at-risk people might save more total lives than implanting ICDs in fewer patients.²⁶ In practice, the two technologies complement each other: ICDs protect identified high-risk individuals around the clock, while public AEDs serve as a safety net for the much larger population of people whose first symptom of heart disease is sudden cardiac arrest itself.