What Is a Defibrillator? Types, Parts & Uses

Last verified: October 6, 2026. A defibrillator is a medical device that delivers a controlled electric shock to the heart to help stop certain dangerous, irregular rhythms so that the heart has a chance to resume an effective beat. It is one of the best-known pieces of emergency equipment, found in hospitals, ambulances, and increasingly in public spaces such as workplaces, schools, and gyms. This guide explains what a defibrillator is, the main types, how it differs from related devices, and what administrators and facility managers should know.

What Is a Defibrillator?

The heart beats because of coordinated electrical signals. In some emergencies, that signal becomes chaotic. In a rhythm called ventricular fibrillation, the lower chambers of the heart quiver rather than pump, and blood stops circulating effectively. A related rhythm, called pulseless ventricular tachycardia, is a very rapid beat that also fails to produce an effective pulse. A defibrillator delivers a brief, strong electrical pulse through the chest, which can interrupt the disorganized activity and give the heart’s own pacemaker tissue a chance to re-establish a normal rhythm.

A defibrillator does not restart a heart that has stopped completely, which is a common misconception from film and television. It is used for specific rhythms that are considered “shockable.” Telling which rhythm is present is part of the device’s function or the clinician’s role, depending on the type.

Why It Exists: The Problem a Defibrillator Solves

When the heart enters a shockable rhythm, the person collapses and the outcome depends heavily on how quickly the rhythm is corrected. Chest compressions and rescue breathing help maintain some circulation, but they cannot usually correct the rhythm by themselves. A defibrillator is the tool that can. Because time matters so much, a long-standing goal of public-health programs has been to put simple, safe units in more places so that they are close to where people collapse.

Main Parts of a Defibrillator

• Electrodes (pads or paddles) — the contact surfaces that attach to or touch the chest. Most modern emergency units use adhesive pads that stay on the person and also sense the heart’s rhythm. Older and some hospital designs used hand-held paddles.
• Energy storage and delivery circuit — stores electrical charge in a capacitor and releases it in a controlled pulse. Many current devices deliver a waveform that goes in one direction and then the other (biphasic), which is designed to work at lower energy levels than older designs.
• Rhythm analysis system — software and sensors that read the electrical signal from the chest. In automated devices, this decides whether a shock is advised.
• Controls and prompts — buttons and, on public-access models, voice and visual prompts that guide the user step by step.
• Battery — powers the device, and it is a key maintenance item.
• Display and recording — many units show a rhythm or status information, and most store an event record that can be reviewed afterwards.

Who Uses a Defibrillator

Hospital doctors, nurses, and resuscitation teams use manual defibrillators, and so do paramedics and emergency medical technicians. Automated external defibrillators (AEDs) are designed for use by trained responders and, in many places, by laypeople, with the device guiding each step. Coaches, security staff, flight attendants, school staff, and office responders are among the people who may be trained to use them. Research teams running studies of cardiovascular disease, exercise, or resuscitation also handle defibrillators, and their protocols typically cover the training and the equipment readiness.

Types of Defibrillators

• Automated external defibrillator (AED) — a portable unit that analyzes the rhythm itself and tells the user whether a shock is advised. Some are fully automatic, while most are semi-automatic and require the user to press a button to deliver the shock. They are designed so that a shock cannot be delivered when the device does not detect a shockable rhythm.
• Manual external defibrillator — used by trained clinicians, who read the rhythm on a monitor and decide whether and how to deliver a shock. These are often combined with a heart monitor and other functions and are typical of hospitals and ambulances.
• Implantable cardioverter-defibrillator (ICD) — a device placed under the skin of a patient who is at risk of dangerous rhythms. It monitors the heart continuously and delivers treatment automatically from within the body.
• Wearable cardioverter-defibrillator — a vest-style device worn outside the body for a limited period, for people at temporary risk.
• Pediatric-capable devices — some units use special pads or settings for infants and children, which reduce the delivered energy for smaller bodies.
• Training units — practice devices that mimic the prompts of an AED without delivering energy, used in courses.

How a Defibrillator Differs from Related Devices

• Defibrillator vs. AED — an AED is one type of defibrillator, the automated, public-access kind. “Defibrillator” also covers manual and implanted devices.
• Defibrillation vs. cardioversion — cardioversion uses a timed shock to correct certain fast rhythms in a person who still has a pulse, while defibrillation addresses rhythms without an effective pulse. Many devices can do both, and the choice is a clinical one.
• Defibrillator vs. pacemaker — a pacemaker sends small, regular pulses to keep a slow heart on pace. A defibrillator delivers a strong shock to correct a dangerous fast rhythm. Some implantable devices combine both functions.
• Defibrillator vs. stethoscope or pulse oximeter — those are assessment tools that listen to the heart or estimate blood oxygen. They do not treat an abnormal rhythm.
• Defibrillator vs. crash cart — a crash cart is a mobile cabinet of emergency supplies, and a defibrillator is often one item on it.

Practical Relevance for Research Administration and Procurement

For anyone responsible for a facility, a defibrillator is not a purchase-and-forget item. The device needs a program around it.

• Readiness checks. Batteries and pads have expiration dates and need replacing on the schedule the manufacturer states. Many devices run self-tests and show a status indicator, but someone should be assigned to check it regularly. The CASRAI guide to AED program management, registration, and maintenance goes into this in more detail.
• Selection. The right model depends on who will use it, whether children are common users, the environment, and the support available. See the AED buying guide for facility selection.
• Placement. Units are generally placed where they can be reached quickly and are clearly signed, and staff should know where they are.
• Training. Even though AEDs are designed for lay use, training improves confidence and speed. Facilities usually combine AED placement with CPR training.
• Rules vary. Requirements for placement, registration, medical oversight, and liability protection differ by jurisdiction and by type of facility. Confirm local requirements with a qualified source rather than assuming a general rule.
• Research settings. Laboratories and research centers with exercise testing, large staff, or public access often include defibrillators in their emergency plans and equipment inventories.

Common Misconceptions

Because defibrillators are so often shown on screen, several myths persist. One is that a defibrillator can be used to “jump-start” any stopped heart. As noted above, it is meant for particular rhythms and is guided by the analysis of the device or the clinician. Another is that a lay user can do serious harm by using an AED on the wrong person. Public-access AEDs are designed to analyze the rhythm and withhold a shock when one is not appropriate, which is a key reason that they are made available to untrained bystanders. A third is that using a defibrillator replaces CPR. In practice the two are used together, with the device’s prompts guiding the pauses.

There is also a misconception that a defibrillator, once bought, is good indefinitely. As covered earlier, consumables expire, and a device that has been ignored for years may not be ready when it is needed. Programs that work well tend to name a responsible person, set a check schedule, keep a log, and review the device after any use. For research and training environments, it is also worth keeping a practice unit so that staff can rehearse without using the live device.

The Short Version

A defibrillator delivers a controlled electric shock to correct certain dangerous heart rhythms. AEDs are automated and built for public use, manual units are used by trained clinicians, and implanted or wearable devices protect people at ongoing risk. It does not restart a stopped heart, and it does not replace CPR. Owning one means maintaining it: batteries, pads, checks, and trained people.

This page offers general information only. It is not clinical or safety training. Always follow your institution’s procedures, current resuscitation training, and the manufacturer’s instructions for any specific device.

Frequently Asked Questions

What does a defibrillator do?

It delivers a brief electrical shock to the heart to interrupt certain dangerous, disorganized rhythms so the heart may return to an organized beat.

Is an AED the same as a defibrillator?

An AED is a type of defibrillator. It analyzes the heart rhythm itself and prompts the user, so it is designed for public and lay use. Manual and implantable defibrillators are other types.

Can a defibrillator restart a stopped heart?

Generally no. Defibrillators are intended for specific shockable rhythms, not a heart with no electrical activity at all. This is a common misunderstanding.

What is the difference between a defibrillator and a pacemaker?

A pacemaker sends small, regular signals to keep a slow heart at an appropriate pace. A defibrillator delivers a strong shock to correct a dangerous fast rhythm. Some implanted devices include both functions.

Do defibrillators need maintenance?

Yes. Batteries and electrode pads expire, and the device should be checked regularly. Most units show a status indicator, and the manufacturer’s instructions specify replacement timing.

Can a defibrillator be used on children?

Many devices offer pediatric pads or a pediatric mode that reduces the delivered energy. Whether and how to use it is covered in the device instructions and in training.