Yes, in a sense, an AED does stop the heart. More precisely, it delivers an electrical shock powerful enough to momentarily stun the heart muscle, wiping out the chaotic electrical activity that’s preventing it from pumping blood. This brief reset gives the heart’s natural pacemaker a chance to take over and restore a normal, organized rhythm. So the goal isn’t to “jump-start” a stopped heart, as movies often suggest. It’s closer to the opposite: forcing a malfunctioning heart to pause so it can restart on its own terms.
What the Heart Is Doing During Cardiac Arrest
Cardiac arrest doesn’t usually mean the heart has gone completely silent. In most cases, the heart is still producing electrical activity, but it’s wildly disorganized. The two rhythms an AED is designed to treat are ventricular fibrillation and rapid ventricular tachycardia (a heart rate above 180 beats per minute). During ventricular fibrillation, the heart’s lower chambers quiver rapidly instead of contracting in a coordinated way. Blood isn’t being pumped. The person has no pulse and is unconscious, but the heart isn’t “stopped” in the way most people imagine.
This distinction matters because it’s exactly why defibrillation works. There’s still electrical energy in the heart muscle. It’s just firing in every direction at once, like an orchestra where every musician is playing a different song. The shock doesn’t add energy to a dead system. It silences the chaos.
How the Shock Resets the Heart
When the AED delivers its shock, the electrical current passes through the chest and into the heart muscle. On a cellular level, this surge forces a massive, simultaneous depolarization of cardiac cells. In plain terms, it pushes the electrical state of nearly every heart muscle cell to the same point at the same time. This makes the tissue temporarily unexcitable, so the disorganized waves of electrical activity that were sustaining the fibrillation have nowhere left to travel. They die out.
Once that slate is wiped clean, the heart’s natural pacemaker, a small cluster of cells in the upper right chamber, has an opportunity to fire on its own and reestablish a normal, coordinated rhythm. Think of it as cutting the power to a glitching computer and letting it reboot cleanly. The shock doesn’t generate the new heartbeat. The heart’s own electrical system does that, if it’s still capable.
Why an AED Won’t Shock a Flatline
This is where the biggest myth comes in. In nearly every dramatic TV scene, someone flatlines, paddles come out, a shock is delivered, and the patient gasps back to life. That’s not how it works. A flatline, known medically as asystole, means there is no electrical or mechanical activity in the heart at all. There’s nothing for the shock to reset. An AED will analyze the rhythm, recognize asystole as a non-shockable rhythm, and refuse to deliver a shock.
AEDs are built with algorithms specifically designed to distinguish shockable rhythms from non-shockable ones. Semi-automated models analyze the rhythm and prompt the user to press a button only when a shock is appropriate. Fully automated models make the decision and deliver the shock without any user input. In both cases, the device will not allow a shock to be delivered to a heart that has flatlined or that is beating normally. This built-in safety feature is one reason bystanders without medical training can use AEDs effectively: the device makes the critical decision for you.
Where the Pads Go and Why It Matters
For the shock to work, the electrical current needs a clear path through the heart. The American Heart Association recommends two pad placements for adults. The most common is anterior-lateral: one pad on the upper right chest below the collarbone, and the other on the lower left side of the chest, aligned with the bottom of the pectoral muscle (or under the breast on women). The alternative is anterior-posterior, with one pad on the front of the chest and one on the back. For children, the front-back placement is preferred because their smaller chests make it more likely the pads could overlap in the side-by-side position.
Incorrect placement or anything that creates resistance between the pads and skin, such as excessive chest hair, wet skin, or medication patches, can reduce the current that reaches the heart and make the shock less effective. Most AED kits include a razor and towel for this reason.
What Happens Right After the Shock
A successful shock doesn’t mean the crisis is over. Even when the chaotic rhythm is eliminated, the heart may not immediately pump blood effectively on its own. That’s why CPR is essential before, during, and after using an AED. Chest compressions manually push oxygenated blood to the brain and vital organs, buying time for the heart to recover its strength.
The standard protocol is to deliver one shock, then immediately resume chest compressions for two minutes before the AED re-analyzes the rhythm. If the dangerous rhythm has returned, another shock is delivered, followed by another two minutes of compressions. This cycle continues until emergency medical services arrive. CPR between shocks isn’t optional. Without it, even a heart that has been successfully defibrillated may not have enough oxygen-rich blood circulating to sustain a normal rhythm.
Why Speed Changes Everything
For every minute that passes during cardiac arrest without defibrillation, the chance of survival drops by roughly 7 to 10 percent. The heart’s chaotic electrical activity gradually deteriorates. Ventricular fibrillation, which is treatable with a shock, eventually degrades into asystole, which is not. This is why public-access AEDs in airports, gyms, and office buildings exist. A bystander who applies an AED within the first few minutes of cardiac arrest dramatically improves the odds of survival compared to waiting for paramedics to arrive.
CPR alone keeps blood moving but cannot fix the underlying electrical problem. The combination of early CPR to maintain circulation and early defibrillation to correct the rhythm is what gives someone the best chance of walking out of the hospital after a cardiac arrest.