A defibrillator is a device that delivers an electrical shock to the heart to restore a normal rhythm during cardiac arrest. When the heart’s electrical system malfunctions and the muscle begins quivering chaotically instead of pumping blood, a defibrillator sends a jolt of energy through the chest that essentially resets the heart’s electrical activity. More than 350,000 out-of-hospital cardiac arrests happen in the United States each year, and the overall survival rate sits at just 8% to 10%. When a defibrillator is used immediately on-site, survival jumps to 50% to 74%.
How a Defibrillator Resets the Heart
During cardiac arrest, the heart’s muscle cells fire in a disorganized frenzy. Instead of contracting together in a coordinated squeeze that pushes blood through the body, different patches of muscle twitch independently. The heart quivers but doesn’t pump. Within minutes, the brain and organs begin to starve for oxygen.
A defibrillator works by sending an electrical charge strong enough to temporarily stun the vast majority of heart muscle cells at once. This makes them all briefly unresponsive, wiping out the chaotic electrical signals bouncing through the tissue. With the slate cleared, the heart’s natural pacemaker (a small cluster of cells in the upper right chamber) has the opportunity to take over again and restart a coordinated rhythm. Think of it like silencing a room full of people shouting over each other so one voice can be heard clearly.
The shock doesn’t restart a heart that has flatlined, despite what movies suggest. It only works on specific types of electrical chaos, primarily ventricular fibrillation (where the lower chambers quiver uselessly) and pulseless ventricular tachycardia (where they beat dangerously fast without producing a pulse). If the heart has no electrical activity at all, a shock won’t help.
Types of Defibrillators
Automated External Defibrillators (AEDs)
AEDs are the portable, battery-powered units found in airports, gyms, schools, and offices. They’re designed so that anyone, even without training, can use them in an emergency. The device talks the user through every step and automatically analyzes the heart’s rhythm to determine whether a shock is appropriate. If the rhythm isn’t shockable, the AED won’t deliver one, which makes it very difficult to use incorrectly. Some doctors recommend keeping one at home if a household member is at high risk for cardiac arrest.
Implantable Cardioverter Defibrillators (ICDs)
ICDs are small devices surgically placed inside the chest for people who are at ongoing risk of dangerous heart rhythms. They continuously monitor the heart and automatically deliver a shock if they detect a life-threatening arrhythmia. Some ICDs also function as pacemakers, sending gentle electrical pulses to keep the heart beating at a steady rate between emergencies. Others can coordinate the timing of the heart’s lower chambers to improve pumping efficiency.
A newer type called a subcutaneous ICD places the wire just beneath the skin along the breastbone rather than threading it through a vein into the heart. This simplifies the procedure, reduces the risk of blood infections, and avoids the mechanical wear that can cause traditional wires to fracture over time. Roughly 55% of people who need an ICD are candidates for this subcutaneous version, though it isn’t suitable for patients who also need continuous pacing.
Wearable Cardioverter Defibrillators (WCDs)
WCDs are vests worn over the torso with a rechargeable battery pack. Like an ICD, they detect dangerous rhythms and deliver a shock automatically. They’re typically a short-term solution, used during a temporary high-risk period, such as while waiting for an ICD implant or recovering from a heart procedure.
Manual Defibrillators
These are the machines used by paramedics and hospital staff. Unlike AEDs, they require a trained operator to read the heart rhythm and choose the energy level. A modern biphasic manual defibrillator typically delivers between 120 and 200 joules per shock. Older monophasic models use a fixed 360 joules.
How to Use an AED
Using an AED takes less than a minute to set up. After turning the device on, you place two sticky electrode pads on the person’s bare chest. One 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. For small children where the pads might touch or overlap, place one pad on the center of the chest and the other on the back between the shoulder blades.
Once the pads are in place, the AED analyzes the heart rhythm automatically. If a shock is needed, it will tell you to press a button (or, in some fully automatic models, deliver the shock itself). The device will instruct you to make sure no one is touching the person before the shock is delivered. Afterward, the AED will guide you to continue CPR until emergency services arrive or the device prompts another analysis. The standard AED steps apply to non-breathing adults and children aged 8 or older who weigh more than 55 pounds. Pediatric pads with reduced energy are available for younger or smaller children.
Survival Impact and Timing
Speed is everything. For cardiac arrests where the rhythm is shockable, the 30-day survival rate is 64% when a bystander uses an AED before the ambulance arrives, compared to 47% when no AED is used before paramedics get there. Even when bystanders used an AED without performing CPR, survival was 17%, nearly double the 9% survival rate when no intervention happened at all. Every minute without defibrillation reduces the chance of survival, which is why public-access AEDs are placed in high-traffic areas where a device can reach someone within a few minutes of collapse.
Maintenance and Readiness
AEDs require very little upkeep, but they do need periodic checks. The electrode pads have a shelf life of about two to two and a half years after purchase, though some models use pads rated for four or five years. Batteries generally last four to five years from the date they’re installed. Most AEDs run a self-check and display a status indicator (usually a green light or checkmark) to confirm they’re ready. If you’re responsible for an AED in a workplace or public space, checking that indicator regularly and tracking expiration dates is the main task.
Legal Protection for Bystanders
All 50 U.S. states and Washington, D.C. have Good Samaritan laws that protect people who voluntarily help during an emergency. These laws shield bystanders from liability for ordinary negligence when using an AED on someone in cardiac arrest. If the person is unconscious, the law assumes they consent to being helped. The protection applies as long as you’re acting in good faith and not expecting payment. It does not cover willful misconduct or gross negligence, but using an AED as instructed by the device falls well within protected territory. Commercial aircraft are also required to carry AEDs on board for in-flight emergencies.