A defibrillator delivers a controlled electrical shock to the heart to stop a life-threatening abnormal rhythm and give the heart a chance to restart with a normal beat. It’s not a “jump start” in the way most people imagine. The shock actually stops the heart’s chaotic electrical activity all at once, creating a brief pause so the heart’s natural pacemaker can take over again.
How the Shock Actually Works
During certain cardiac emergencies, the heart’s electrical signals become completely disorganized. Instead of contracting in a coordinated way to pump blood, the heart muscle quivers or fires too rapidly to fill with blood between beats. Either way, blood stops flowing to the brain and organs.
A defibrillator sends an electrical current through the chest that forces nearly all heart muscle cells to contract at the same instant. This simultaneous reset wipes out the chaotic signals, producing a momentary pause in all cardiac activity. From that clean slate, the heart’s built-in electrical system, specifically a cluster of cells in the upper right chamber that acts as a natural pacemaker, can fire a fresh impulse and restore a normal, organized rhythm. When that happens, the heart begins pumping blood again and the person regains a pulse.
Which Heart Rhythms It Treats
Defibrillators only work on two specific rhythms, both considered “shockable.” The first is ventricular fibrillation, where the lower chambers of the heart quiver chaotically instead of contracting. The second is pulseless ventricular tachycardia, where the lower chambers beat so fast they can’t fill with blood. In both cases, the heart produces no useful blood flow, and the person loses consciousness within seconds.
A defibrillator will not help with every type of cardiac arrest. When the heart has no electrical activity at all (a flatline, known as asystole) or has organized electrical signals that simply aren’t producing a heartbeat, a shock won’t do anything useful. This is why modern defibrillators analyze the heart’s rhythm before delivering a shock. They only fire when they detect one of those two treatable rhythms.
Why Every Minute Counts
The chance of surviving cardiac arrest drops by roughly 10% for every minute without defibrillation. After about 10 minutes, the odds of recovery become minimal. This steep decline is why public access to defibrillators matters so much. Emergency services typically take several minutes to arrive, and the heart can’t wait.
Research published in the New England Journal of Medicine found that when bystanders used a defibrillator before paramedics arrived, patients had significantly lower rates of brain damage, nursing home admission, and death compared to those who received no bystander intervention. CPR alone helps by keeping some blood flowing to the brain, but the defibrillator is what actually corrects the rhythm. The two work together: CPR buys time, and the defibrillator fixes the problem.
Three Types of Defibrillators
Automated External Defibrillators (AEDs)
These are the devices mounted on walls in airports, gyms, offices, and schools. AEDs are designed for anyone to use, even without medical training. They provide spoken step-by-step instructions, automatically analyze the heart rhythm, and will only allow a shock if they detect a shockable rhythm. You can’t accidentally shock someone who doesn’t need it.
Implantable Cardioverter-Defibrillators (ICDs)
An ICD is a small device surgically placed under the skin, usually near the collarbone, with wires running to the heart. It continuously monitors the heart rhythm and delivers a shock automatically if it detects a dangerous arrhythmia. People who survive ventricular fibrillation have a high risk of it happening again, so an ICD acts as a permanent safety net. Some ICDs also function as pacemakers, delivering gentle electrical pulses to keep the heart beating at a steady rate day to day.
Manual Defibrillators
These are the machines used by paramedics and hospital staff. Unlike AEDs, they require a trained provider to read the heart rhythm on a monitor and decide whether to shock, when to shock, and at what energy level. They offer more control but require expertise to operate.
How to Use an AED
If someone collapses and isn’t breathing, call emergency services first, then grab the nearest AED. The device will talk you through every step once you open it, but knowing the basics ahead of time can save precious seconds.
- Expose the chest. Remove or cut away clothing. If the chest is wet, wipe it dry.
- Place the pads. One pad goes on the upper right side of the chest, below the collarbone. The other goes on the lower left side, a few inches below the armpit. Diagrams on the pads show exactly where. If the pads risk touching each other (on a small child, for instance), place one on the center of the chest and one on the back between the shoulder blades.
- Plug in the connector. Some AEDs come pre-connected; others require you to plug the pad cable into the unit.
- Let the AED analyze. Make sure nobody is touching the person. Say “Clear!” loudly so bystanders step back. The device will take a few seconds to read the heart rhythm.
- Deliver the shock if prompted. Again confirm nobody is touching the person, say “Clear!” and press the shock button. Some fully automatic models deliver the shock without a button press.
- Start CPR immediately. Whether the AED delivers a shock or advises that no shock is needed, begin chest compressions right away. The AED will prompt you to pause for another rhythm check every two minutes.
You won’t hurt someone by applying an AED when you’re unsure if they need it. The device makes the decision for you. The far greater risk is hesitating and letting minutes tick by without intervention.
What It Feels Like
People who are in cardiac arrest are unconscious and won’t feel the shock from an AED or manual defibrillator. Those with ICDs, however, are sometimes awake when the device fires. Most describe it as a sudden, sharp jolt or kick in the chest. It’s startling and uncomfortable but typically lasts less than a second. Some people experience anxiety about future shocks after their first one, which is a normal reaction that their care team can help manage.
What a Defibrillator Can’t Do
A defibrillator doesn’t restart a heart that has completely stopped. The flatline scenario shown in movies, where a shock brings someone back from zero electrical activity, is fiction. The device needs some form of electrical chaos to reset. It also doesn’t fix the underlying cause of the arrest. After successful defibrillation, medical teams still need to identify and treat whatever triggered the dangerous rhythm, whether that’s a heart attack, an electrolyte imbalance, or a structural heart problem. The defibrillator’s role is specific and critical: it clears the electrical storm so the heart gets another chance to beat normally.