An automated external defibrillator (AED) delivers a controlled electrical shock through the chest to reset a heart that has fallen into a chaotic, life-threatening rhythm. The device analyzes the heart’s electrical activity, decides whether a shock is appropriate, and walks the rescuer through every step with voice prompts. No medical training is required to use one, and survival chances drop by about 10% for every minute defibrillation is delayed.
What Happens Inside the Heart During Cardiac Arrest
A normal heartbeat is triggered by a cluster of cells in the upper right chamber that sends a coordinated electrical signal through the heart muscle. During ventricular fibrillation, the most common rhythm in sudden cardiac arrest, that coordination breaks down completely. Instead of one organized wave of electricity, hundreds of tiny, disorganized electrical impulses fire at random across the lower chambers. The heart quivers rather than pumps, and blood flow to the brain and organs stops within seconds.
A less common but equally dangerous rhythm is pulseless ventricular tachycardia, where the lower chambers beat so fast they can’t fill with blood between contractions. The heart is technically still beating in a pattern, but it produces no usable pulse. Both of these rhythms are “shockable,” meaning an electrical shock can potentially correct them.
How the Shock Resets the Heart
The core principle is surprisingly simple: overwhelm the chaos with one massive, uniform electrical pulse. When the shock passes through heart muscle cells, it forces them all to depolarize (fire) at the same time. This makes the entire heart temporarily unexcitable for a brief moment. With every cell in the same “reset” state, the heart’s natural pacemaker cells have a window to regain control and restart a normal, organized rhythm.
At the cellular level, the electric field changes the voltage across each heart muscle cell’s membrane. When that voltage shift reaches a certain threshold, ion channels in the cell open and trigger a full electrical response. Because the shock hits all the cells simultaneously, the competing wavefronts that were sustaining the fibrillation are extinguished at once.
How the AED Analyzes Heart Rhythm
The moment you place the adhesive electrode pads on a person’s chest, the AED begins recording the heart’s electrical signal, essentially performing a simplified version of an electrocardiogram. But raw electrical signals picked up through the skin are messy. Nearby power lines can inject 50 or 60 Hz interference. Muscle tremors add high-frequency noise. Even slight movement of the pads introduces low-frequency drift.
To deal with this, the AED runs the signal through a series of filters. A notch filter strips out power line interference. A low-pass filter set around 20 Hz removes noise from muscle tremors. A high-pass filter at about 2 Hz eliminates the slow baseline wander caused by patient movement or pad shifting. If the signal is still too corrupted after filtering, the device tells the rescuer to stop touching the patient or check the pad placement rather than risk a wrong decision.
Once the signal is clean, the software extracts key features of the rhythm, such as rate, regularity, and waveform shape, and compares them against criteria for shockable rhythms. The entire analysis takes only a few seconds. If the device detects ventricular fibrillation or pulseless ventricular tachycardia, it charges and advises a shock. If it detects any other rhythm, it will not allow a shock to be delivered.
What the AED Cannot Treat
Two cardiac arrest rhythms are not shockable: asystole (no electrical activity at all, the “flatline”) and pulseless electrical activity, where the heart produces organized electrical signals but the muscle doesn’t contract effectively. In both cases, there are no chaotic electrical wavefronts to reset. Shocking a flatline heart does nothing because there is no electrical activity to interrupt and reorganize. For these rhythms, CPR and medications administered by paramedics are the only options.
Biphasic Shocks and Energy Delivery
Nearly all modern AEDs use biphasic waveforms, meaning the electrical current flows in one direction through the heart and then reverses. This is a significant improvement over older monophasic devices, which pushed current in only one direction. Research comparing the two found that biphasic shocks achieved the same or better defibrillation success at roughly half the energy: about 7.5 joules versus 17 joules in direct comparisons. Lower energy means less potential damage to heart tissue while still clearing the abnormal rhythm.
The pads also play a role in calibrating the shock. Before delivering energy, the AED sends a tiny, imperceptible signal through the chest to measure transthoracic impedance, which is essentially how much resistance the current will face passing between the two pads. Factors like body size, chest wall thickness, and pad placement all affect this resistance. Modern devices use that impedance reading to automatically adjust the shock waveform so the right amount of current actually reaches the heart.
What Using an AED Looks Like
AEDs are designed so that someone with zero training can use them effectively. When you open the case and power the device on (some activate automatically when opened), a voice begins giving step-by-step instructions. It tells you to expose the chest, shows you via diagrams on the pads where to place each electrode, and instructs everyone to stand clear while it analyzes the rhythm.
If a shock is advised, the device either charges automatically or asks you to press a clearly marked button. After the shock, the AED prompts the rescuer to begin CPR, typically coaching five cycles of 30 chest compressions before pausing to re-analyze the rhythm. Research from the American Heart Association found that untrained volunteers using AEDs with real-time voice coaching performed CPR at a quality level often comparable to trained providers. The device continues this loop of analyzing, shocking if needed, and coaching CPR until paramedics arrive.
Pediatric Considerations
For children, many AED models come with pediatric pads or an energy attenuator that reduces the shock dose. The American Heart Association defines pediatric patients as infants and children up to 18 years of age for resuscitation purposes, though the exact age and weight thresholds for switching between pediatric and adult settings remain an acknowledged gap in the guidelines. If pediatric pads are available, use them on a child. If they are not, adult pads and standard energy levels are still recommended over withholding defibrillation entirely.
Why Bystander AED Use Matters
The data on bystander-initiated AED use is striking. A large study across nine regional cardiac arrest centers found that patients who received a shock from a bystander-applied AED before paramedics arrived survived to hospital discharge 66.5% of the time, compared to 43% for those who waited for EMS to deliver the first shock. The gap in meaningful recovery was even more dramatic: 57.1% of bystander-shocked patients left the hospital with normal or near-normal brain function, versus 32.7% of those first shocked by paramedics. After accounting for other factors that influence survival, bystander AED use more than doubled the odds of a good outcome. The benefit grew progressively larger as EMS response times increased, reinforcing that the minutes before professional help arrives are the most critical window for intervention.