Defibrillation is a controlled electrical shock delivered to the heart to stop a life-threatening abnormal rhythm and allow the heart’s natural pacemaker to regain control. It is the single most effective treatment for cardiac arrest caused by certain types of irregular heartbeats, and every minute it’s delayed reduces the chance of survival by roughly 7 to 10 percent.
How Defibrillation Works
During a normal heartbeat, a small cluster of cells near the top of the heart generates an electrical signal that travels in an organized wave, causing the heart muscle to contract and pump blood. In certain emergencies, that organized wave breaks down. The heart’s muscle cells start firing randomly and out of sync, so the heart quivers instead of pumping. No blood moves through the body.
A defibrillator delivers a burst of electrical energy through the chest wall and into the heart muscle. That burst depolarizes the vast majority of heart cells at the same instant, essentially resetting them to zero. With all the chaotic signals wiped out, the heart’s natural pacemaker has a window to take over again and restart a coordinated rhythm. Think of it like rebooting a frozen computer: the shock clears the electrical chaos so the normal system can resume.
Which Heart Rhythms Respond to a Shock
Not every cardiac arrest can be treated with defibrillation. Only two rhythms are “shockable”:
- Ventricular fibrillation: the heart’s lower chambers quiver chaotically and pump no blood. This is the most common rhythm found in sudden cardiac arrest and carries higher survival rates than non-shockable rhythms precisely because defibrillation can correct it.
- Pulseless ventricular tachycardia: the lower chambers beat extremely fast but produce no effective pulse. A shock can break this cycle.
Two other cardiac arrest rhythms, called pulseless electrical activity and asystole (flatline), do not respond to defibrillation. They require CPR, medications, and identification of the underlying cause. Correctly distinguishing between shockable and non-shockable rhythms is critical because delivering a shock when it’s not indicated wastes time and doesn’t help.
Defibrillation vs. Cardioversion
You may hear “cardioversion” used alongside defibrillation. They’re related but not identical. Defibrillation delivers a shock at whatever point the heart happens to be in its electrical cycle. It’s used when the rhythm is so disorganized that timing doesn’t matter, like in ventricular fibrillation.
Synchronized cardioversion, by contrast, is timed to a specific part of the heartbeat. The machine tracks the heart’s electrical activity and releases the shock at a precise moment to avoid a vulnerable window during the cycle. If a shock hits that vulnerable window (the tail end of each heartbeat), it can actually trigger ventricular fibrillation rather than fix it. Cardioversion also uses less energy than defibrillation. It’s typically used for abnormal rhythms where the heart is still beating in a somewhat organized pattern, just too fast or irregularly.
Types of Defibrillators
Automated External Defibrillators (AEDs)
These are the portable units you see in airports, gyms, schools, and offices. AEDs are designed so anyone can use them, even without medical training. The device provides step-by-step voice instructions: it tells you where to place the adhesive pads on the person’s bare chest, analyzes the heart rhythm automatically, and advises whether a shock is needed. If a shock is indicated, you simply press a button. The machine will not let you deliver a shock if one isn’t appropriate, which makes AEDs remarkably safe for bystander use.
Manual Defibrillators
These are the machines used by paramedics, nurses, and doctors. Unlike AEDs, manual defibrillators require the operator to read the heart rhythm, decide whether to shock, and choose the energy level. They offer more control but require training to use correctly.
Implantable Cardioverter-Defibrillators (ICDs)
An ICD is a small device surgically placed under the skin, usually near the collarbone, with wires threaded into the heart. It continuously monitors heart rhythm and delivers a shock automatically if it detects a dangerous arrhythmia. ICDs are prescribed for people at high risk of sudden cardiac arrest, such as those with certain inherited heart conditions or a history of dangerous arrhythmias. The shock from an ICD is much smaller than an external defibrillator but is delivered directly to the heart, so less energy is needed.
Energy Levels and Waveforms
Modern external defibrillators come in two waveform types. Biphasic defibrillators send electrical current in two directions through the heart and require between 120 and 200 joules for an adult. Monophasic defibrillators send current in one direction and require a higher dose, typically 360 joules, to achieve the same effect. Most newer machines are biphasic because they achieve similar success rates with less energy, which means less potential for tissue damage. For context, a joule is a unit of energy: 200 joules is roughly equivalent to the energy of a baseball thrown at about 60 miles per hour, concentrated into a fraction of a second.
How to Use an AED
If someone collapses and you suspect cardiac arrest, here’s what to do. First, check whether the person is breathing and has a pulse. If they’re unresponsive with no pulse and no normal breathing, call 911 immediately. If you’re alone, make that call before doing anything else so help is on the way. If others are nearby, one person should call while another retrieves the AED and a third starts chest compressions.
Once you have the AED, turn it on and follow the voice prompts. The device will tell you to place two adhesive pads on the person’s bare chest in specific locations (illustrated on the pads themselves). Once the pads are attached, the AED analyzes the heart rhythm. Do not touch the person during analysis. If a shock is advised, the machine will instruct you to say “stand clear,” confirm no one is touching the person, and press the shock button. Afterward, the AED guides you through CPR. This cycle of analysis, possible shock, and CPR continues until emergency medical services arrive.
Why Speed Matters
The 2025 American Heart Association guidelines emphasize two priorities above all else: high-quality chest compressions and early defibrillation. The heart can only survive in ventricular fibrillation for a limited time before it deteriorates into asystole, a rhythm that cannot be shocked and carries a much worse prognosis. Every minute without defibrillation dramatically narrows the window for a successful outcome. CPR buys time by keeping some blood flowing to the brain and heart, but it rarely restores a normal rhythm on its own. The shock is what actually fixes the electrical problem.
This is why public access AED programs exist. Waiting for an ambulance can take 8 to 12 minutes in many communities. A bystander with an AED can cut that delay to one or two minutes, which is often the difference between someone walking out of the hospital and someone who doesn’t survive.
Risks and Side Effects
Defibrillation is used in situations where the alternative is death, so the risk-benefit calculation is overwhelmingly in favor of shocking. That said, the procedure is not without consequences. Skin burns at the pad sites are the most common side effect, ranging from mild redness to deeper burns depending on the energy used and how well the pads were positioned. Chest soreness after resuscitation is also common, though it’s often hard to separate from the effects of CPR.
In rare cases, the electrical shock itself can cause some damage to the heart muscle, particularly if the energy is excessive or the pads are placed too close together. One documented case involved a patient whose defibrillator pads were positioned in close proximity on the chest, concentrating the electrical current and causing localized tissue injury to the heart wall beneath. Proper pad placement, following the illustrations printed on every set of pads, minimizes this risk. Repeated shocks at high energy levels also increase the chance of heart muscle injury, which is one reason biphasic machines requiring lower energy have become the standard.