A defibrillator delivers a controlled electrical shock through the heart to stop a dangerous, chaotic rhythm and give the heart a chance to restart with a normal beat. It doesn’t “jump-start” a flatlined heart the way movies suggest. Instead, it works by simultaneously resetting the electrical activity of heart muscle cells so the heart’s natural pacemaker can regain control.
What the Shock Actually Does
Your heart beats because electrical signals travel through it in an organized wave, causing the muscle to contract in a coordinated squeeze that pumps blood. In life-threatening rhythms like ventricular fibrillation, that organized wave breaks down into hundreds of tiny, competing electrical impulses. The heart quivers uselessly instead of pumping. Blood stops flowing. Brain damage begins within minutes.
The defibrillator shock sends a burst of electrical energy through the chest and across the heart, depolarizing the heart muscle cells all at once. Think of it like hitting a reset button: by forcing every cell into the same electrical state simultaneously, the chaotic signals are wiped out. This creates a brief pause, a blank slate, that allows the heart’s natural pacemaker (a small cluster of cells in the upper right chamber) to fire again and re-establish a normal, organized rhythm.
This is why defibrillation doesn’t work on a heart that has completely stopped producing electrical activity, a condition called asystole. There’s no chaotic rhythm to reset. CPR is the treatment for that situation, not a shock.
Which Heart Rhythms Can Be Shocked
Defibrillators treat two specific life-threatening rhythms: ventricular fibrillation and pulseless ventricular tachycardia. Both involve the heart’s lower chambers (ventricles) firing so rapidly or chaotically that they can’t pump blood. These are the rhythms most associated with sudden cardiac arrest, and rapid defibrillation is the single most effective intervention for them.
A related but different procedure called cardioversion uses a lower-energy, precisely timed shock to treat less immediately dangerous fast rhythms. These include atrial fibrillation (where the upper chambers quiver), atrial flutter, and certain types of rapid heartbeats originating above the ventricles. The key difference is that cardioversion synchronizes the shock with the heart’s remaining electrical activity to avoid triggering ventricular fibrillation, while defibrillation delivers an unsynchronized shock because the rhythm is already so disorganized that timing doesn’t matter.
Types of Defibrillators
Automated External Defibrillators (AEDs)
AEDs are the portable devices found in airports, gyms, schools, and offices. They’re designed so anyone can use them without medical training. The device analyzes the heart’s rhythm automatically and determines whether a shock is appropriate. It gives voice prompts through every step, and it will not allow a shock to be delivered unless it detects a rhythm that defibrillation can treat. This built-in safety feature makes it nearly impossible for a bystander to accidentally shock someone who doesn’t need it.
Implantable Cardioverter-Defibrillators (ICDs)
ICDs are small devices surgically placed under the skin, typically below the collarbone, with wire leads threaded into the heart. They continuously monitor the heart’s rhythm by reading the electrical signals directly from inside the chambers. The device tracks both the rate and duration of abnormal rhythms using programmed thresholds. When it detects a dangerous rhythm, it can respond in two ways: first by trying to pace the heart back to normal with a series of rapid, low-energy electrical pulses, and if that fails, by delivering a full defibrillation shock internally. If the first shock doesn’t work, the ICD will detect the ongoing arrhythmia and escalate to progressively stronger shocks. ICDs remain the most reliable method for treating sudden cardiac arrest outside a hospital.
Wearable Defibrillators
A wearable defibrillator, sometimes called a life vest, is worn externally under clothing. It provides continuous monitoring and can deliver a shock in under one minute after detecting a life-threatening arrhythmia, without needing anyone else to intervene. These are typically prescribed for people at temporary risk of sudden cardiac arrest, such as those waiting for an ICD implant or recovering from a heart attack, who need protection but aren’t yet candidates for a permanent device.
How Energy Levels Differ
Modern defibrillators use one of two waveform types. Biphasic defibrillators send current in one direction and then reverse it, which turns out to be more efficient at resetting the heart. They require 120 to 200 joules for an adult shock. Older monophasic defibrillators send current in only one direction and need a higher energy level of 360 joules to achieve the same effect. Most devices in use today are biphasic.
For children, the energy is scaled to body weight, starting at 2 joules per kilogram and increasing to 4 joules per kilogram or higher for subsequent shocks, up to a maximum of 10 joules per kilogram. Pediatric AED pads deliver a reduced dose automatically.
How to Use an AED
If someone collapses and isn’t breathing, the steps for using an AED are straightforward. After calling 911 (or asking someone nearby to call), turn the AED on. It will begin giving voice instructions immediately.
- Expose the chest. Remove clothing and wipe the chest dry if it’s wet.
- Attach the pads. Place one pad on the upper right chest, below the collarbone. Place the second pad on the lower left side, a few inches below the armpit. The pads have diagrams showing exactly where they go. If the person is small enough that the pads might touch each other, place one on the center of the chest and one on the back between the shoulder blades.
- Let the AED analyze. Make sure no one is touching the person. Say “Clear!” loudly.
- Deliver the shock if advised. Press the shock button when the AED instructs you to. Again, make sure no one is touching the person.
- Start CPR immediately after. Begin chest compressions right after the shock, or if the AED advises no shock is needed.
The entire process from opening the AED to delivering a shock typically takes under two minutes. Speed matters enormously: survival rates drop with every minute of delay in defibrillation.
Pad Placement and Why It Matters
The two standard pad positions are anterior-lateral (one pad on the front upper right chest, one on the left side) and anterior-posterior (one pad on the front of the chest, one on the back). Both are effective. The European Resuscitation Council recommends the anterior-lateral position as the default, while the American Heart Association considers either position reasonable.
The goal is to position the pads so the electrical current passes through as much of the heart as possible. If the first several shocks in either position fail to restore a normal rhythm, switching to the alternative position (called a vector change) can sometimes succeed by sending the current through the heart along a different path.
Safety Around Defibrillators
The most important safety rule is simple: don’t touch the person while the shock is being delivered. As long as no one is in physical contact with the patient during the discharge, there is no risk of shock to bystanders, even on wet or metal surfaces. Testing has confirmed that rescuers are not exposed to hazardous voltages on wet surfaces when this precaution is followed.
The American Heart Association does recommend removing a person from freestanding water (a pool, puddle, or lake) and drying the chest before attaching AED pads. This isn’t because of electrocution risk to bystanders, but because water on the chest can create a path for current to travel across the skin instead of through the heart, reducing the shock’s effectiveness. Metal surfaces like helicopter floors or stretchers pose no additional hazard to rescuers.
If the person has a pacemaker or ICD (visible as a small bump under the skin near the collarbone), the AED pads should be placed at least an inch away from the device to avoid damaging it. Medication patches on the chest should be removed and the area wiped clean before pads are applied, since they can block energy delivery or cause skin burns.