A defibrillator delivers a controlled electrical shock to the heart to stop a life-threatening irregular rhythm and give the heart a chance to reset to its normal beat. It is primarily used during cardiac arrest, when the heart’s electrical system malfunctions and the heart can no longer pump blood effectively. Defibrillators come in several forms: public-access devices designed for bystanders, hospital equipment operated by medical teams, wearable vests for high-risk patients, and surgically implanted devices for long-term protection.
How a Defibrillator Actually Works
Movies and TV often portray defibrillators as devices that “jump-start” a stopped heart. That’s not what happens. A defibrillator sends an electrical current through the heart muscle that forces nearly all the cardiac cells to depolarize, or discharge, at the same time. This effectively causes a brief moment of total electrical silence in the heart.
Once that chaotic rhythm has been wiped clean, the heart’s natural pacemaker (a cluster of cells in the upper right chamber) has a window to reassert control and fire off a normal, coordinated electrical signal. If it succeeds, the heartbeat resumes in an organized pattern, blood starts flowing again, and the person regains a pulse. The shock doesn’t restart anything. It clears the slate so the heart’s own system can take over.
Which Heart Rhythms a Defibrillator Can Treat
Not every cardiac arrest responds to a shock. Defibrillators work on what are called “shockable rhythms,” the two most important being ventricular fibrillation and ventricular tachycardia. In ventricular fibrillation, the heart’s lower chambers quiver chaotically instead of pumping. In ventricular tachycardia, those same chambers beat dangerously fast, sometimes too fast to move blood. Both are electrical storms that a shock can interrupt.
What a defibrillator cannot treat is a flatline, known medically as asystole. Despite what you see in hospital dramas, shocking a flatline does nothing, because there is no electrical activity left to reset. Asystole requires CPR and medications to attempt to restart any heart rhythm at all. This distinction matters: automated defibrillators are designed to analyze the heart’s rhythm before delivering a shock, and they will refuse to fire if the rhythm isn’t shockable.
Automated External Defibrillators (AEDs)
AEDs are the devices you see mounted on walls in airports, gyms, schools, and office buildings. They’re built so that anyone, with zero medical training, can use one during a cardiac emergency. The device walks you through every step with voice prompts the moment you turn it on.
The basic steps, outlined by the American Red Cross, are straightforward. After calling 911 and confirming the person isn’t breathing normally, you turn on the AED and follow the audio instructions. You remove clothing from the chest, wipe it dry if needed, and place one adhesive pad on the upper right chest and the other on the lower left side, a few inches below the armpit. The device then analyzes the heart rhythm on its own. If a shock is needed, you make sure nobody is touching the person, say “clear,” and press the shock button. If no shock is needed, the AED tells you to continue CPR.
Speed is critical. Survival rates during cardiac arrest drop rapidly with every passing minute without defibrillation. That’s why public-access AEDs exist: they put a life-saving device within reach while paramedics are still on the way.
Manual Defibrillators in Hospitals
In emergency rooms and intensive care units, medical teams use manual defibrillators. Unlike AEDs, these require a trained operator to read the heart rhythm on a monitor, choose the energy level, and decide when to shock. The tradeoff is speed: a study comparing the two types during in-hospital cardiac arrests found that manual defibrillators allowed for chest compression pauses that were about 8 seconds shorter around each shock, compared to AEDs. Those seconds add up when blood isn’t circulating. The accuracy of rhythm analysis was similar between both types.
Modern defibrillators use what’s called a biphasic waveform, meaning the electrical current reverses direction during the shock. This design is more efficient than older monophasic technology, particularly at lower energy levels. Biphasic devices typically deliver effective shocks at 150 to 200 joules, while older monophasic units often needed 200 to 360 joules to achieve the same result. In animal studies, biphasic shocks at just 70 joules succeeded 80% of the time, compared to 32% for monophasic shocks at the same energy. Higher efficiency means less electrical energy passes through the heart, which reduces the risk of tissue damage.
Implantable Cardioverter-Defibrillators (ICDs)
For people with ongoing risk of sudden cardiac arrest, a small defibrillator can be surgically placed inside the body. An implantable cardioverter-defibrillator, or ICD, sits just under the skin near the collarbone, with wires threaded into the heart. It continuously monitors heart rhythm and delivers a shock within seconds if it detects a dangerous arrhythmia.
ICDs are typically considered for people whose heart doesn’t pump strongly enough, measured by a number called ejection fraction (the percentage of blood pushed out with each beat). A healthy heart ejects about 55% or more. People with significantly reduced ejection fraction, certain genetic heart conditions, or a history of surviving dangerous arrhythmias are the most common candidates. The decision involves a combination of factors: heart function, symptoms, electrical test results, and the underlying cause of the heart problem. These criteria were updated in 2025 guidelines from the American College of Cardiology and several partner organizations.
Living with an ICD means carrying a constant safety net. Most people go months or years without receiving a shock. When the device does fire, patients typically describe it as a sudden jolt or kick in the chest. It’s startling and sometimes painful, but it can be the difference between a brief scare and sudden death.
Wearable Defibrillator Vests
Some patients fall into a gap: they’re at high risk for sudden cardiac arrest, but they aren’t ready for an implanted device yet. This might be someone recovering from a heart attack, waiting for heart function to improve, or scheduled for ICD surgery that hasn’t happened yet. For these situations, a wearable cardioverter-defibrillator fills the bridge.
The most widely used version is the LifeVest, a garment worn under clothing that continuously monitors heart rhythm. If it detects a life-threatening arrhythmia, it alerts the patient first (in case they’re conscious and the reading is a false alarm), then delivers a shock automatically if no response comes. Guidelines recommend wearing the device around the clock, removing it only for showering or bathing. In the FDA study that led to its approval, 289 patients wore the device for an average of 20 hours a day over three months. Some patients wear it for just a couple of weeks before transitioning to an ICD, while others use it for several months during recovery.
What Defibrillators Do Not Do
A defibrillator is not a treatment for a heart attack. A heart attack happens when blood flow to part of the heart is blocked, like a clogged pipe. The heart may still be beating normally during a heart attack. A defibrillator only becomes relevant if the heart attack triggers a dangerous rhythm change, which sometimes happens but isn’t guaranteed.
Defibrillators also don’t replace CPR. During cardiac arrest, chest compressions keep blood moving to the brain and organs while the defibrillator addresses the electrical problem. The two work together. Starting CPR immediately and applying a defibrillator as soon as one is available gives someone in cardiac arrest the strongest chance of survival.