An AED (automated external defibrillator) delivers a controlled electrical shock that forces the heart’s muscle cells to reset at the same time, stopping the chaotic electrical activity that prevents the heart from pumping blood. It does not “restart” a stopped heart. Instead, it interrupts a specific type of electrical malfunction so the heart’s natural pacemaker can take over and restore a normal rhythm.
The Electrical Problem an AED Fixes
A healthy heartbeat starts with a tiny cluster of cells at the top of the heart called the sinus node. This natural pacemaker fires an electrical signal that spreads in an orderly wave, causing the heart’s chambers to squeeze in sequence and push blood out to the body. During cardiac arrest, that orderly wave breaks apart into hundreds of disorganized, competing electrical impulses. The heart quivers instead of pumping. This is ventricular fibrillation (VF), and it’s the most common rhythm an AED is designed to treat.
A second treatable rhythm is pulseless ventricular tachycardia (pVT), where the lower chambers fire so rapidly that the heart can’t fill with blood between beats. Though it looks different on a monitor than fibrillation, the result is the same: no meaningful blood flow to the brain and organs. Both of these are called “shockable rhythms” because an electrical shock can interrupt them.
How the Shock Resets the Heart
Every heart muscle cell has a tiny electrical charge across its membrane. During fibrillation, different patches of cells are in different stages of their electrical cycle, firing and recovering at random. The AED’s shock sends a surge of current through the chest that forces a large majority of those cells into the same electrical state simultaneously. For a brief moment, the entire heart muscle is effectively “stunned” and electrically quiet.
With the chaotic signals extinguished, the sinus node gets a clean slate. If it’s healthy enough, it fires its next impulse and a normal, organized rhythm resumes. The pause between the shock and that first natural beat is typically very short, though a diseased sinus node may take longer to recover. This is why CPR immediately after a shock is so important: it keeps blood moving to the brain during those critical seconds while the heart’s pacemaker re-establishes control.
What an AED Cannot Do
Television and movies often show defibrillators jolting a flatlined patient back to life. In reality, an AED will not shock a flatline. A flatline, called asystole, means there is no electrical activity in the heart at all. There’s nothing to reset. The AED analyzes the rhythm and simply will not deliver a shock if it detects asystole, because doing so would be pointless and potentially harmful.
For asystole, high-quality CPR is the primary treatment and the most important predictor of whether the person survives. An AED only helps when there is disorganized electrical activity that can be interrupted.
How the Device Reads Your Heart
An AED isn’t just a button that delivers electricity. It contains a computer that analyzes the heart’s electrical activity through adhesive pads placed on the chest. One pad goes to the right of the breastbone, just below the collarbone. The other goes on the left side of the chest, roughly under the armpit. This positioning directs current across the heart’s lower chambers, where the dangerous rhythms originate.
The device’s algorithm classifies what it detects into categories: ventricular fibrillation, rapid ventricular tachycardia, normal sinus rhythm, other organized rhythms (like atrial fibrillation or heart blocks), and asystole. It measures signal amplitude and heart rate to distinguish between these. Fine fibrillation with very small electrical signals can sometimes be harder for the algorithm to classify, but modern AEDs are highly accurate. The device will only advise a shock for VF or rapid ventricular tachycardia. For everything else, it tells you to continue CPR.
Some newer AEDs can even analyze the heart’s rhythm while chest compressions are happening. These systems run a first analysis during CPR, and if they detect a shockable rhythm, they trigger a brief pause for a confirmation check before advising a shock. This reduces the total time CPR is interrupted, which matters because every second without chest compressions means less blood reaching the brain.
Why Every Minute Counts
Survival rates during cardiac arrest drop by roughly 10% for every minute defibrillation is delayed. After 10 minutes of ventricular fibrillation without a shock, the chances of successful resuscitation approach zero. This steep decline is why AEDs are placed in airports, gyms, schools, and offices. A bystander who applies an AED within the first few minutes of cardiac arrest dramatically improves the odds compared to waiting for paramedics.
The energy an AED delivers ranges from 150 to 360 joules in adults. For children under 8 (roughly under 25 kg or 55 pounds), pediatric pads with a built-in dose attenuator reduce the delivered energy to about 50 to 75 joules. These special pads raise electrical resistance and divert some current away from the child’s smaller body. If pediatric pads aren’t available, adult pads can still be used on a child, because the risk of untreated cardiac arrest is far greater than the risk of a higher-energy shock.
What Happens After the Shock
A single shock doesn’t always work. If the first shock fails to restore a normal rhythm, the AED will re-analyze and may advise another. Fibrillation that persists through three or more shocks is considered “persistent VF.” In some cases, the rhythm is successfully terminated but then returns within seconds, a pattern called recurrent fibrillation. Most refibrillation events happen within the first minute after a shock, which is one reason CPR should resume immediately rather than stopping to check for a pulse.
Even when the shock successfully eliminates fibrillation, the heart may not immediately pump effectively. The muscle can be temporarily stunned from both the arrest itself and the electrical shock. CPR bridges that gap, maintaining circulation until the heart regains enough strength to pump on its own. This is why AED instructions always alternate between shock delivery and CPR cycles, typically in two-minute intervals. The device guides you through this process with voice prompts, re-analyzing the rhythm at each pause to decide whether another shock is needed or whether a normal rhythm has returned.