A heart defibrillator delivers an electrical shock that stops dangerous heart rhythms and gives the heart a chance to reset to its normal beat. It works by depolarizing a large portion of the heart muscle all at once, briefly halting all electrical activity. After that pause, the heart’s natural pacemaker cells can take over and restart a normal, coordinated rhythm that produces a pulse.
There are several types of defibrillators, from the public-access units mounted on walls in airports to surgically implanted devices that monitor your heart around the clock. Each works on the same basic principle, but they serve very different situations.
How the Electrical Shock Actually Works
Your heart beats because of a coordinated wave of electrical signals that travels through the muscle in a precise sequence. In certain life-threatening emergencies, that coordination breaks down. The muscle fibers start firing chaotically, and the heart quivers instead of pumping. This is ventricular fibrillation, the most common cause of sudden cardiac arrest.
A defibrillator sends a pulse of electricity through the chest (or directly through the heart, in the case of an implanted device). That pulse forces nearly all the heart muscle cells to activate at the same instant, wiping the slate clean. With the chaotic signals interrupted, the heart’s built-in pacemaker, a small cluster of cells near the top of the heart, can fire a fresh signal and restore a normal pumping rhythm.
This is not the same as restarting a stopped heart. If the heart has flatlined completely, a condition called asystole, defibrillation will not work. Asystole is classified as a non-shockable rhythm, and no attempt at defibrillation should be made. High-quality CPR is the primary treatment in that scenario. Movies and TV shows routinely get this wrong, showing paddles shocking a flatline back to life.
Which Heart Rhythms It Treats
Defibrillation is effective for two specific 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. Without treatment, either rhythm leads to death within minutes.
Survival odds drop steeply with every passing minute. Without CPR, the chance of surviving decreases by roughly 7 to 10 percent per minute after collapse. When someone performs CPR while waiting for a defibrillator, that decline slows to about 3 to 4 percent per minute. This is why speed matters enormously, and why public-access defibrillators exist in so many buildings.
Types of Defibrillators
Automated External Defibrillators (AEDs)
These are the portable units you see in schools, gyms, offices, and airports. They’re designed for anyone to use, even without medical training. The device analyzes the person’s heart rhythm automatically and will only allow a shock if it detects a rhythm that defibrillation can treat. You cannot accidentally shock someone who doesn’t need it.
Using one is straightforward. You turn it on, place two adhesive pads on the person’s bare chest (one on the upper right side, one below the left armpit), and follow the voice prompts. The device tells you when to stand clear and when to press the shock button. After delivering a shock, or if no shock is advised, you immediately begin CPR starting with chest compressions. The entire process is guided step by step.
Implantable Cardioverter-Defibrillators (ICDs)
An ICD is a small device surgically placed under the skin, usually just below the collarbone, with wires (leads) threaded into the heart. It continuously monitors your heart rhythm and can deliver a shock within seconds if it detects a dangerous arrhythmia. Because it sits right next to the heart, it needs far less energy than an external defibrillator.
ICDs don’t just defibrillate. Most also function as pacemakers, delivering tiny electrical pulses to keep the heart beating at a normal rate during everyday life. Some can also attempt to interrupt a fast rhythm with a series of rapid, low-energy pulses before resorting to a full shock. Patients who receive a shock often describe it as a sudden jolt or kick in the chest. It’s startling and sometimes painful, but it’s over in a fraction of a second.
ICDs are typically recommended for people who have survived a cardiac arrest, who have certain heart conditions that put them at high risk for one, or who have significantly weakened heart muscle that raises the risk of sudden cardiac death.
Wearable Cardioverter-Defibrillators
A wearable defibrillator is a vest worn under clothing that monitors and can shock the heart, similar to an ICD, but without surgery. It fills a specific gap: protecting people whose risk of sudden cardiac death is high but likely temporary.
The most common scenario is a patient whose heart muscle is weak after a heart attack or a new diagnosis of heart failure. Guidelines recommend waiting at least 40 days after a heart attack, and at least 3 months in most other cases of newly diagnosed heart failure, before implanting a permanent ICD. Many patients recover significant heart function during that window with medication and treatment. The wearable vest protects them during this waiting period. It’s also used after acute heart inflammation (myocarditis) or when an infected ICD has been removed and the patient needs protection while the infection clears.
Data shows the risk is highest early on: 75 percent of patients who received a therapeutic shock from a wearable defibrillator got it within the first month of wearing the device.
Modern Defibrillator Technology
Today’s defibrillators use biphasic waveforms, meaning the electrical current flows in one direction and then reverses. Older devices used monophasic waveforms that sent current in only one direction. The difference matters. At the same energy level (200 joules), biphasic shocks reduced the risk of failed defibrillation by 81 percent compared to monophasic shocks. Biphasic devices can also achieve the same success at lower energy settings, around 115 to 130 joules, which causes less damage to the heart muscle after the shock.
Current guidelines from the American Heart Association note that both fixed-energy and escalating-energy approaches are highly effective. For patients who need multiple shocks, escalating the energy with each attempt (for example, 200, then 300, then 360 joules) produces slightly higher rates of restoring an organized rhythm, though overall survival rates between the two approaches are similar. Most modern devices come with manufacturer-recommended settings calibrated to their specific waveform design.
What Happens After a Shock
A successful defibrillation restores an organized electrical rhythm, but it doesn’t fix the underlying problem that caused the cardiac arrest. In an emergency setting, the person still needs advanced medical care to stabilize. For someone with an ICD, a shock means the device did its job, but it also means something triggered a dangerous rhythm. Most cardiologists want to see a patient soon after an ICD shock to review the stored data, check for new heart damage, and adjust medications or device settings if needed.
For people who witness a cardiac arrest and use a public AED, the key thing to remember is that CPR should continue immediately after a shock is delivered, even if the shock was successful. The AED will continue to monitor the rhythm and prompt you if another shock is needed. Keep going until emergency medical services arrive.