Does an AED restart a stopped heart with electricity?
― The shock quiets a trembling heart, it doesn't start one
Those red or orange boxes on walls at stations and schools. People often picture the AED inside as "a machine that sends electricity into a stopped heart to get it going again." Actually, it's the opposite. An AED delivers its shock not when the heart has stopped, but when it's trembling out of control. The shock's job is to bring that trembling to a complete halt, all at once.
A hospital operating room in a medical drama. The line on the monitor goes flat, and a long beep sounds. A doctor shouts "Clear!", presses electrode pads to the chest, and sends a jolt of electricity through. The body jumps, and the line starts wobbling again.
That's probably the image most people have of an electric shock. But in real medicine, a shock is never used on a heart whose line has gone flat. It simply wouldn't work.
So when is the shock actually used, and why? The answer lies in the "electrical march" that repeats inside our chest, once every second.
A shock works for two reasons
Most sudden cardiac arrests begin with heart cells contracting out of sync with each other. The heart only quivers; it can't pump blood.
The shock forces every trembling cell into a "rest" state at the same instant. In that sudden quiet, the heart's own pacemaker can set the rhythm again.
So an AED isn't really like a machine that starts an engine. It's closer to a conductor who, when a choir falls into chaos, silences everyone at once. Let's go through this step by step.
The heart runs on an electrical "march"
In the upper right of the heart sits a small region called the sinoatrial node. It fires its own electrical signal about 60 to 80 times a minute. That signal spreads like a wave across the heart's walls, and the cells contract one after another, in order. Because the upper chambers and then the lower chambers contract in sequence, blood gets squeezed out.
Here's the key point: once a cell has fired, it enters a brief "rest" period during which it won't respond to another signal. That rest period is what keeps the wave moving in only one direction, letting the heart beat in a steady rhythm.
But sometimes, triggered by something like a heart attack or a strong blow, the signal wave starts spinning in a loop instead. The loop splits into many smaller loops, and cells all over the heart begin contracting on their own, out of sync. This is ventricular fibrillation. The heart just quivers, barely working as a pump at all. The middle of Figure 1 shows what that looks like on an ECG.
A shock forces everyone into "rest" at the same instant
In a fibrillating heart, one cell is contracting, its neighbor is resting, and the one past that is about to contract. It's complete chaos. Send a strong jolt of electricity through from outside, and most of the heart's cells respond at the very same moment, entering their "rest" period together.
While everyone is resting, the looping signal wave has nowhere left to travel. The loop runs out of room and dies out. Then, once the rest period ends, the first signal to fire comes from the original pacemaker — the sinoatrial node. That's how the heart finds its own rhythm again.
This is why a shock does nothing for a heart whose monitor line has gone flat (cardiac standstill). There's no chaotic signal to erase, so the shock has nothing to work on. The scene in that drama is generally considered dramatic license, for effect.
The odds of survival drop with every passing minute
The longer ventricular fibrillation continues, the more the heart muscle weakens from lack of oxygen. Each minute a shock is delayed, the chances of survival are thought to fall by roughly 7 to 10 percent. But if bystanders keep up chest compressions in the meantime, that decline slows to roughly 3 to 4 percent per minute.
In Japan, it's said to take about 10 minutes on average, nationwide, for an ambulance to arrive after a 119 call. Try moving the slider in Figure 2 out to 10 minutes. The difference between doing nothing and doing compressions plus using an AED turns out to be huge.
An AED reads the heart's electrical activity through its pads and only recommends a shock when the rhythm is one a shock can actually fix, such as ventricular fibrillation. If it judges a shock isn't needed, no electricity flows even if the button is pressed. It's built so the person using it never needs to be able to read an ECG.
Right after being reset, the heart can still only beat weakly. That's why an AED's voice prompts tell you to resume chest compressions immediately after the shock. Keep following the AED's instructions until paramedics take over.
So what should you actually do?
- Shout for help, and get someone to call 119 and bring an AEDPointing directly at a specific person and saying "You, call 119" or "You, get the AED" makes people far more likely to act.
- If they aren't breathing normally, push hard and fast on the center of the chestAim for 100 to 120 compressions a minute, about 5 centimeters deep. If in doubt, go ahead and push.
- When the AED arrives, open the lid and follow its voice promptsOnce the pads are attached, the AED automatically checks the heart's rhythm. If it tells you to deliver a shock, make sure no one is touching the person and step back.
If someone is touching the collapsed person's body the instant the electricity flows, that current can pass into them too. When the AED says "stand clear," everyone should let go and step away. If the chest is wet, dry it off before attaching the pads. For a child who hasn't yet started elementary school, use pediatric pads or a pediatric mode if the AED has one. If you're ever unsure, the 119 dispatcher can walk you through it over the phone.
In short
A collapsed person's heart is often not stopped — it's trembling out of control. An AED's shock wipes out that trembling all at once, clearing a gap for the heart to find its own rhythm again. Every minute before the shock changes the odds, and it's the hands of whoever is nearby that bridge that gap.
An AED doesn't start the heart.
It's a conductor who silences a chaotic choir, just for a moment.
You can read more about how a normal heart runs on electrical signals in "Why does the heart keep beating all on its own?," and about the dangers of electricity passing through the body in "Why is touching an outlet with wet hands so dangerous?"
- On your commute or at work, try to spot three AEDs along your route. Note how long a round trip to each one would take, just in case.
- Press two fingers to your wrist or neck, count your pulse for 15 seconds, and multiply by four. That's roughly how many times per minute your sinoatrial node is firing.
- Put a cushion on the floor and try pressing on it at "100 to 120 times a minute." It's faster than you'd expect, and your arms will be surprisingly tired after just two minutes.
Fire departments and the Japanese Red Cross Society, among others, run courses where you can practice using an AED and performing chest compressions. Going through the motions once makes it much easier to act when it actually matters.
Want to go deeper? ― terms, formulas, and how this connects to the curriculumFrom middle-school science to university-level specialist courses, each level is labeled
- MSCovered in middle-school science
- HSCovered in high-school physics or biology
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (electrophysiology, medical engineering)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Ventricular fibrillation: a state where cells in the heart's lower chambers (ventricles) contract out of sync, so the heart only quivers and can't pump blood.
- Defibrillation: using an electric shock to remove fibrillation. "AED" stands for "automated external defibrillator."
- Sinoatrial node: a cluster of cells in the upper right of the heart (the right atrium) that fires its own rhythm signal. The body's natural pacemaker.
- Refractory period: the brief interval after a cell fires during which it won't respond to another stimulus. This is the "rest" referred to in this article.
MSHSCheck the numbers: how much energy is in one shock?
An AED stores electricity from its battery in a capacitor (a component that stores charge), then releases it all at once. Using the formula for energy stored in a capacitor, we can estimate the size of a single shock. The capacitance and voltage here are illustrative example values for the calculation.
| In symbols | E ＝ ½ × C × V² |
| In words | stored energy ＝ ½ × capacitance × voltage squared |
| Where it comes from | Each bit of charge added has to work against the charge already stored. Adding up all that work gives the energy left stored (conservation of energy) |
| Symbol | Meaning and unit |
| E | Energy stored in the capacitor (joules) |
| C | Capacitance of the capacitor (farads) |
| V | Voltage across the capacitor (volts) |
| Capacitance (example: 100 microfarads) | 0.0001 farads |
| Voltage (example) | 1700 volts |
| Discharge time (approximate) | about 0.01 seconds (10 milliseconds) |
| Gravitational acceleration | 9.8 meters per second squared |
| Voltage squared | 1700 × 1700 ＝ 2890000 |
| Multiply by capacitance | 2890000 × 0.0001 ＝ 289 |
| Halve it (energy) | 289 ÷ 2 ＝ 144.5 joules |
| Convert to power (per second) | 144.5 ÷ 0.01 ＝ 14450 watts |
| Equivalent in 1000-watt microwaves | 14450 ÷ 1000 ≒ 14 units |
| Equivalent work: lifting 1 kg how many meters | 144.5 ÷ 9.8 ≒ 15 meters |
A single shock is around 150 joules, almost exactly matching the 150–200 joules most real AEDs actually use. As a quantity, that's no more than the work of lifting a 1-kilogram object 15 meters. But because it's all packed into a hundredth of a second, that instant carries the punch of more than a dozen microwave ovens. It's that speed, not the raw amount, that lets it snap every heart cell into line at once.
HSHS+Cell "electricity," and the shape of the shock
HSHeart cells normally sit with their inside about 0.09 volts lower than the outside (the resting potential). When a signal arrives, sodium ions flow in and the inside flips positive (the action potential), which triggers the cell to contract. During the action potential, the cell won't respond to another stimulus. This is the refractory period.
HS+Older defibrillators sent current in only one direction (a monophasic waveform). Most AEDs today use a biphasic waveform, which reverses the current's direction partway through. This achieves the same effect with less energy, and is thought to put less strain on the heart muscle.
UnivWhy loops form, and why they die out
Heart muscle is a type of what electrophysiology calls an "excitable medium" — under the right conditions, a wave of excitation can keep spinning in a loop. This is called a spiral wave (or reentry), and ventricular fibrillation is thought to be a state where this loop has split into many smaller loops. Among the proposed explanations for why defibrillation works is the "critical mass hypothesis," which holds that a loop can't keep going once enough of the heart muscle is made to fire together.
📖 For the full derivation and further reading: Defibrillation (Japanese Wikipedia) ／ Ventricular fibrillation (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Where fibrillation starts, and what sustains it. Whether a small number of large "parent" loops drive it, or many small loops keep replacing each other, is still debated.
- What's the best way to deliver the shock? The optimal combination of waveform, energy, and pad placement is still being compared in clinical research.
- How to get an AED there faster. Countries are testing things like delivering AEDs by drone, and systems that alert nearby volunteers' smartphones.
In other words, even this article reflects only "what's understood so far." Still, countless records confirm one thing clearly: the sooner compressions start and the sooner a shock is delivered, the more lives are saved.
How this maps to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Year 2 science: "electric current," "blood circulation" | How the heart works, electricity flowing |
| HS | Physics: "capacitors"; Biology: "conduction of excitation" | The energy formula, resting potential and refractory period |
| HS+ | Advanced physics/biology | Biphasic waveform |
| Univ | Electrophysiology, medical engineering | Spiral waves, critical mass hypothesis |
| Research | Cardiology, resuscitation science | How fibrillation arises, getting AEDs there faster |
| ― | Everyday connection | Calling 119, chest compressions, and AEDs when someone collapses |
- Fire and Disaster Management Agency (消防庁), "Current State of Emergency Medical Services and Rescue" and related materials
- Japan Resuscitation Council (日本蘇生協議会), JRC Resuscitation Guidelines 2020, Igaku-Shoin (医学書院)
- American Heart Association, Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care
- Japan Foundation for Emergency Medicine (日本救急医療財団), "Guidelines on Appropriate AED Placement"
※ This article is a general-audience science explainer. The figures given are approximations meant to illustrate how things work. For actual emergency care, follow the AED's voice prompts and the instructions of the 119 dispatcher, fire department, and local authorities. Taking a hands-on training course is recommended.