The machine that shocks your heart is called a defibrillator. It delivers a controlled burst of electricity through the chest to reset the heart’s rhythm when it falls into a dangerous, irregular pattern. Defibrillators come in three main forms: portable units found in public places, hospital machines used by medical teams, and small devices surgically implanted inside the body for people at ongoing risk.
How a Defibrillator Works
During certain cardiac emergencies, the heart’s electrical system misfires. Instead of beating in a coordinated rhythm, the heart muscle quivers chaotically or races so fast it can’t pump blood. This is not the same as the heart “stopping” entirely. It’s more like an electrical storm where thousands of signals fire at once, and the muscle can’t organize itself into a useful contraction.
A defibrillator sends a pulse of electricity through the heart that temporarily makes all the muscle cells unresponsive at the same time. Think of it as a forced reset. Once every cell is briefly stunned into the same state, the heart’s natural pacemaker (a small cluster of cells near the top of the heart) has a chance to take over and restart a normal, organized rhythm. The shock doesn’t “jump-start” a flatlined heart the way movies often show. It interrupts electrical chaos so the heart can find its own beat again.
Which Heart Rhythms Can Be Shocked
Only two cardiac arrest rhythms respond to defibrillation. The first is ventricular fibrillation, where the lower chambers of the heart quiver uselessly instead of pumping. The second is pulseless ventricular tachycardia, where the lower chambers beat so rapidly that no blood actually moves. Both are immediately life-threatening, and both involve disorganized electrical activity that a shock can interrupt.
Two other cardiac arrest rhythms, asystole (a flatline) and pulseless electrical activity, are not shockable. In those cases, CPR and medications are the primary treatments. This is why defibrillators analyze the heart’s rhythm before delivering a shock. If the rhythm won’t benefit from electricity, the machine won’t fire.
Types of Defibrillators
Automated External Defibrillators (AEDs)
These are the portable, wall-mounted units you see in airports, gyms, schools, and offices. They’re designed for anyone to use, even without medical training. When you open the case and power it on, the device talks you through each step with voice prompts: place the adhesive pads on the person’s bare chest, stand clear, and let the machine analyze the heart rhythm. If a shock is needed, you press a single button. If no shockable rhythm is detected, the AED will not allow a shock to be delivered, which makes it essentially impossible to accidentally shock someone who doesn’t need it.
AEDs are remarkably effective. In a study of over 4,600 out-of-hospital cardiac arrests, survival was 24% when a bystander applied an AED before paramedics arrived, compared to 9% with CPR alone. That’s nearly a tripling of survival odds. Every minute without defibrillation during ventricular fibrillation reduces the chance of survival, so having these devices in public spaces closes the gap between collapse and treatment.
Manual Defibrillators
These are the larger machines used by paramedics and hospital teams. Unlike AEDs, they don’t make the rhythm decision automatically. A trained clinician reads the heart rhythm on a monitor and chooses the energy level. This gives medical professionals more control. In hospital settings, manual defibrillators allow slightly shorter pauses in CPR (about 8 seconds shorter per shock cycle compared to AEDs), which matters when every second of chest compressions counts.
Implantable Cardioverter-Defibrillators (ICDs)
An ICD is a small device, roughly the size of a matchbox, surgically placed under the skin near the collarbone. Thin wires run from it directly into the heart. It monitors the heart rhythm continuously, 24 hours a day, and delivers a shock automatically if it detects a dangerous rhythm. Some people describe the shock as feeling like a sudden thump or kick in the chest.
ICDs are typically recommended for people who have survived a cardiac arrest, have a heart that pumps significantly less blood than normal (an ejection fraction below 35%), or carry certain inherited conditions that make the heart’s electrical system unstable, such as long QT syndrome or Brugada syndrome. They’re also used for people with specific types of heart muscle disease, including hypertrophic cardiomyopathy and a condition where the right side of the heart is progressively replaced by scar tissue.
How Much Electricity Is Involved
Defibrillator energy is measured in joules. Modern external defibrillators use biphasic waveforms, meaning the current flows in one direction and then reverses. This design is more efficient, requiring less total energy. A biphasic defibrillator typically delivers between 120 and 200 joules for an adult. Older monophasic models, which push current in only one direction, require 360 joules to achieve the same effect.
For children, the energy is scaled by body weight: 2 joules per kilogram for a first shock, increasing to 4 joules per kilogram for a second, and up to 10 joules per kilogram for subsequent shocks. Pediatric AED pads are smaller and reduce the energy automatically. If pediatric pads aren’t available, adult pads can be used on a child as long as the two pads don’t touch or overlap on the chest.
Using an AED Safely
A few situations require extra steps before applying an AED. If the person is lying in water or their chest is soaking wet, move them to a dry surface or quickly dry the chest before placing the pads. Wet skin can prevent the pads from sticking properly, and the machine may not read the heart rhythm accurately. That said, testing in simulated wet environments has shown the risk of shock to bystanders is minimal even if conditions aren’t perfectly dry.
If the person has a medication patch on their chest where a pad needs to go, peel it off and wipe the area before applying the electrode. If you can see or feel a small lump under the skin of the upper chest (indicating an implanted pacemaker or ICD), place the AED pad at least an inch away from it. These precautions are simple and the AED’s voice prompts will guide you through them.
Keeping an AED Ready
If you’re responsible for an AED at a workplace, school, or community building, maintenance is minimal but important. Batteries last about four years on average. When a battery weakens, most units emit a chirping sound, similar to a smoke detector. Each battery has an “install before” date printed on it, and it’s a good idea to budget for replacement during the fourth year. The adhesive electrode pads also expire, with the date printed on the packaging. Expired pads may not stick well enough to read the heart’s rhythm reliably. After any use, pads need to be replaced immediately since they’re single-use.
Most modern AEDs run automatic self-checks daily or weekly and display a green light or checkmark if everything is functional. A quick visual check of that indicator once a month is all it takes to confirm the device is ready.