Defibrillator: What It Does, How It Works, and When It Is Used

What a Defibrillator Does

A defibrillator is a medical device that delivers a controlled electrical shock to the heart. The goal is to stop a life-threatening rhythm disturbance — most often ventricular fibrillation or pulseless ventricular tachycardia — so the heart can re-establish an effective beat on its own. Without prompt treatment, cardiac arrest can lead to death within minutes.

The word is often misspelled as "defibilator," but the correct term is defibrillator. The name comes from "defibrillation," meaning the removal of fibrillation, a chaotic electrical state in the heart muscle.

How a Defibrillator Works

Every defibrillator has three core parts: a battery, a control unit, and pads or electrodes. The pads sense the heart's rhythm and, if the device determines a shockable rhythm is present, it charges and tells the user to stand clear before delivering energy through the chest.

The shock depolarizes a large portion of the heart muscle at once. This interrupts the chaotic electrical activity and gives the heart's natural pacemaker a chance to resume a normal rhythm. The device does not "restart" a flatline heart; it corrects specific, life-threatening arrhythmias.

Types of Defibrillators

Automated External Defibrillators

AEDs are the devices found in airports, malls, schools, and offices. They are designed for use by bystanders with little or no medical training. The machine analyzes the rhythm automatically and either advises or delivers a shock only when needed. Voice prompts and visual instructions guide the user step by step.

Manual External Defibrillators

These are used by trained professionals — paramedics, nurses, and physicians — in hospitals and ambulances. A provider interprets the rhythm and manually selects the energy level and timing of the shock.

Implantable Cardioverter-Defibrillators

An ICD is a small device placed under the skin, usually near the collarbone, with wires threaded into the heart. It continuously monitors rhythm and can deliver a shock, anti-tachycardia pacing, or cardioversion if it detects a dangerous arrhythmia. People at high risk for sudden cardiac arrest may receive an ICD as a preventive measure.

Wearable Cardioverter-Defibrillators

A WCD is a vest worn outside the body. It is used temporarily — for example, after a heart attack or while waiting for an ICD — and can automatically detect and treat a life-threatening rhythm.

When Defibrillation Is Needed

Defibrillation is needed when the heart is beating ineffectively despite electrical activity. The two main rhythms an AED or manual defibrillator is designed to treat are ventricular fibrillation, where the lower chambers quiver uselessly, and pulseless ventricular tachycardia, where they beat too fast to pump blood.

It is not effective for asystole, a flatline rhythm, or for a person who is conscious and has a pulse. Using a defibrillator on someone who does not need it can be harmful, which is why AEDs are designed to analyze the rhythm first and only advise a shock when appropriate.

Using an AED in an Emergency

If you find someone collapsed and unresponsive, call emergency services immediately and start CPR. If an AED is available, turn it on and follow the voice instructions. Expose the chest, dry any moisture, and place the pads as shown in the diagram. The device will tell you when to stand clear and whether a shock is advised. After a shock, resume CPR until help arrives or the person shows signs of life.

Common Myths About Defibrillators

• Myth: You can accidentally shock a person who does not need it. Fact: AEDs analyze the rhythm and will not deliver a shock unless a shockable rhythm is detected.
• Myth: Only medical professionals can use a defibrillator. Fact: AEDs are designed for bystanders. Minimal training is needed, and the device guides you through every step.
• Myth: Defibrillators are used to restart a stopped heart. Fact: They correct specific chaotic rhythms. A flatline heart needs CPR and medications, not a shock.

Care and Maintenance

AEDs require regular checks. Most devices perform daily self-tests and alert users to any problems. Pads and batteries have expiration dates and must be replaced on schedule. Keeping a defibrillator accessible, well-maintained, and clearly signed increases the chance it will be used quickly in a real emergency.