Defibrillator: What It Is, Types, How It Works and Recovery
Updated: 6 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026 · How we review
Quick Answer: What is a defibrillator?
A defibrillator is a device that sends an electric shock to the heart to stop two life-threatening rhythms, pulseless ventricular tachycardia and ventricular fibrillation, so a normal rhythm can return. It can be external, wearable or implanted. It can save a life when used within 10 minutes, so call emergency services and start CPR immediately. [1]
What the research says
Public AEDs used by bystanders are linked to strong survival. A systematic review of 41 studies found a median survival to hospital discharge of 40.0% when a public-access AED was used, rising to 53.0% when lay people already at the scene (not sent by emergency dispatch) delivered the shock. (Bækgaard et al., Circulation, 2017) [2]
The closer the AED, the better the odds. Across 16 studies and 55,537 people, one-month survival was 37.1% when the AED was 100 m or less away, compared with 12.8% when it was more than 200 m away. (Ruan et al., Resuscitation, 2021) [3]
Implanted defibrillators lower the risk of death in people at high risk. A meta-analysis of 4 randomized trials in 1,874 people with nonischemic cardiomyopathy found that an implantable cardioverter defibrillator (ICD) cut all-cause death by about 25% (hazard ratio 0.75). (Al-Khatib et al., JAMA Cardiology, 2017) [4]
Key takeaways
A defibrillator treats only two heart rhythms: ventricular tachycardia without a pulse and ventricular fibrillation. [1]
CPR keeps blood moving for a short time; a defibrillator is what can stop the deadly rhythm. [1]
An automated external defibrillator (AED) checks the rhythm, charges itself and gives spoken instructions. [1]
People at high risk may wear a defibrillator vest or have an ICD placed inside the body. [1]
Recovery after cardiac arrest can take months or years, and many survivors need rehabilitation. [1]
What is a defibrillator?
A defibrillator is a medical device that delivers an electric shock to the heart to pull it out of a potentially fatal abnormal heart rhythm (arrhythmia) and back into a normal rhythm. [1]
The two rhythms it treats both start in the ventricles, the heart's lower pumping chambers. When they happen, the heart quivers or races so fast that it stops pumping blood. This is sudden cardiac arrest.
The shock briefly stops all movement of the heart muscle. That pause gives the heart a chance to create its own electrical impulse and restart a normal rhythm. In simple terms, defibrillation resets the heart. [1]
Even with defibrillation, cardiac arrest is still fatal for many people, and survivors often need several types of therapy to recover. [1]
Which heart rhythms does a defibrillator treat?
A defibrillator treats only pulseless ventricular tachycardia and ventricular fibrillation. [1] Other rhythm problems are covered in our heart arrhythmia guide.
Rhythm | Where it starts | What happens | Does a defibrillator treat it? |
Ventricular fibrillation | Lower chambers (ventricles) | The heart quivers instead of pumping | Yes |
Ventricular tachycardia with no pulse | Lower chambers (ventricles) | The heart beats too fast to pump blood | Yes |
Other arrhythmias | Varies | Depends on the rhythm | No. A shock could trigger ventricular fibrillation and cardiac arrest |
What are the types of defibrillators?
There are four main types of defibrillators: automated external, manual, wearable and implantable. [1][7] The first two are used in emergencies. The last two belong to one person who has a high risk of a life-threatening rhythm and need protection with them at all times.
Type | How it works | Who uses it | Where you find it |
Automated external defibrillator (AED) | Analyzes the rhythm, decides if a shock is needed, charges itself and gives voice prompts | Anyone nearby during an emergency | Public places and hospitals |
Manual defibrillator | Paddles or sticky pads; the provider presses the shock button | First responders and healthcare providers | Hospitals and emergency care |
Wearable cardioverter defibrillator (WCD) | A vest worn under clothes; sensors touch the skin and send a shock after sensing a dangerous rhythm | A person at high risk | Worn on the body |
Implantable cardioverter defibrillator (ICD) | An internal "watchdog" that sends the right amount of charge when a fatal rhythm is detected | A person at high risk | Placed inside the body |
An ICD is similar to a pacemaker in that it can help keep the heart beating. The difference is that an ICD can also deliver a shock when it detects a fatal rhythm. [1]
Why is a defibrillator used instead of CPR alone?
A defibrillator is used because CPR only gives temporary support, while a defibrillator can stop the rhythm that caused the cardiac arrest. [1]
CPR pushes blood to the brain and heart by hand. It buys time. The shock is what can end ventricular fibrillation or pulseless ventricular tachycardia.
A defibrillator can also be used on someone who already has a pacemaker or an ICD. [1]
How does a defibrillator work, step by step?
A defibrillator works by sending a shock through two pads or paddles placed on the chest, while CPR continues right up to the moment the device is ready. [1] With an AED, the machine checks the heart rhythm itself and tells you whether a shock is needed. [1]
Keep CPR going. Do not stop chest compressions while the defibrillator is being prepared. [1]
Place the pads or paddles. One goes below the right shoulder and the other below the left nipple. [1]
Use conducting material correctly. Sticky pads already contain conducting material to prevent burns. Handheld paddles need conducting material applied to the chest first. [1]
Stand clear. Make sure no one is touching the person or their bed before the shock. [1]
Deliver the shock. With sticky pads, press the button on the machine. With handheld paddles, press the button on each paddle at the same time. [1]
What happens after a defibrillator shock?
After a shock, CPR restarts right away for two minutes, then the pulse and heart rhythm are checked. [1]
If the rhythm is not back to normal: more CPR and another shock are given. [1]
If the first shock did not work: medicines such as epinephrine or amiodarone may be given to help correct the rhythm. [1]
If blood is flowing but the person is not responsive: the medical team carefully controls body temperature between 32 and 37.5 °C (89.6–99.5 °F) and prevents fever to help protect the brain. [1]
Once the person is stable: cardiac catheterization may be done to look at the heart. [1]
Why does timing matter for defibrillation?
Timing matters because a defibrillator can save a life when it is used for the correct rhythm within 10 minutes of that rhythm starting. [1]
Research points the same way. In 5,753 people whose ventricular fibrillation did not stop after the first shock, each one-minute delay before the first shock was linked to about 7% lower odds of a good brain recovery. [5] Distance matters too: survival was almost three times higher when an AED was within 100 m than when it was more than 200 m away. [3]
Distance to nearest AED | One-month survival |
100 m or less | 37.1% |
100–200 m | 22.0% |
More than 200 m | 12.8% |
Source: systematic review of 16 studies, 55,537 people. [3]
What are the risks of a defibrillator?
The main risk of a defibrillator is using it on the wrong rhythm: a shock given for any rhythm other than pulseless ventricular tachycardia or ventricular fibrillation could cause ventricular fibrillation and cardiac arrest. [1]
An AED reviews the heart rhythm first and decides on its own whether a shock is needed. [1]
People who survive may also have injuries from resuscitation, such as a rib fracture from CPR, which can take time to heal. [1]
What does recovery look like after cardiac arrest and defibrillation?
Recovery after cardiac arrest and defibrillation is a long-term process that can take months or years. [1]
Up to 40% of survivors have some impairment that keeps them from returning to work or regular activities after leaving the hospital. [1]
Fatigue is common and can limit daily life. [1]
Some people have trouble walking or have seizures. [1]
Occupational, physical and speech therapy may be needed. [1]
Healing takes patience. Rehabilitation helps, but progress is often gradual. [1]
How do you live with an implantable cardioverter defibrillator (ICD)?
Living with an implantable cardioverter defibrillator (ICD) means regular checkups to make sure it works properly, taking your prescribed medicines and planning for a battery change after about five years. [1]
Why you may get one: after surviving ventricular fibrillation, the risk of it happening again is high. Treating the cause may include an ICD. [1]
What a shock feels like: a shock from a personal defibrillator can be painless or feel like a kick in the chest. [1]
Follow-up: regular visits confirm the device is still working correctly. [1]
Battery: after five years or so, a new ICD battery is needed. [1]
Medicines: keep taking the medicines your doctor prescribed. [1]
When should you call emergency services or see a doctor?
Call emergency services immediately (911 in the US) if you see someone collapse, start CPR and send someone for the nearest AED. [1]
If you have an ICD, or someone used a defibrillator on you, see your doctor regularly to make sure your device and treatment are working as they should. If your ICD gives a shock, contact your heart team the same day; call 911 if you get more than one shock in 24 hours or a shock comes with chest pain, trouble breathing or fainting. [1]
For a deeper clinical overview, see this review of defibrillation [7] and this patient guide to defibrillators [6].
Defibrillators at a glance
Question | Short answer |
What does it do? | Shocks the heart out of a deadly ventricular rhythm |
Which rhythms? | Pulseless ventricular tachycardia and ventricular fibrillation |
Best time window | Within 10 minutes of the rhythm starting |
Pad placement | Below the right shoulder and below the left nipple |
After the shock | 2 minutes of CPR, then a rhythm and pulse check |
ICD battery | Replaced after about five years |
Recovery | Months to years; therapy is often needed |
Conclusion
A defibrillator is the one tool that can stop the deadly rhythms behind sudden cardiac arrest. CPR keeps the brain alive; the shock gives the heart its chance to restart. Minutes and meters both count, which is why knowing where the nearest AED is matters as much as knowing how to use it.
For people at high risk, a wearable vest or an ICD brings that protection everywhere. And for survivors, recovery is a long road that goes best with steady follow-up and rehabilitation.
Take the next step: learn CPR, find the AEDs at your workplace, school or gym, and share this guide with family. Keep reading about your heart:
FAQ
What is a defibrillator used for?
A defibrillator is used to stop pulseless ventricular tachycardia or ventricular fibrillation, the rhythms behind sudden cardiac arrest. The shock lets the heart return to a normal rhythm. [1]
Can anyone use an AED?
An AED is designed to guide the person using it. It checks the heart rhythm, decides if a shock is needed, charges itself and gives spoken instructions. [1] Keep CPR going until it is ready.
Can a defibrillator be used on someone with a pacemaker or ICD?
Yes. A defibrillator can be used on someone who already has a pacemaker or an implantable cardioverter defibrillator. [1]
Does a shock from an ICD hurt?
A shock from an ICD or wearable defibrillator can be painless or feel like a kick in the chest. [1]
How long does an ICD battery last?
An ICD battery usually needs replacing after about five years. Regular checkups make sure the device keeps working properly. [1]
What happens if a defibrillator is used on the wrong heart rhythm?
A shock given for the wrong rhythm could cause ventricular fibrillation and cardiac arrest. That is why a defibrillator is used only for pulseless ventricular tachycardia or ventricular fibrillation. [1]
How long does recovery take after cardiac arrest and defibrillation?
Recovery can take months or years. Up to 40% of survivors have some impairment that keeps them from returning to work or regular activities. [1]
What should I do if I see someone collapse?
Call emergency services right away, start CPR and get the nearest AED. Keep giving CPR until the AED is ready to use. [1]
What is the difference between an AED and an ICD?
An AED is a portable device found in public places that anyone can use on a person in cardiac arrest. An ICD is a small device placed under the skin that watches your heart rhythm and shocks it automatically if needed.
What is a wearable defibrillator?
A wearable defibrillator is a vest worn under clothing that watches your heart rhythm and can deliver a shock if a deadly rhythm starts. It is often used for a short time while doctors decide whether you need an ICD.
Related Reading
Subcutaneous ICD (S-ICD): How It Works, Procedure and Recovery explains the subcutaneous ICD.
References
Cleveland Clinic. Defibrillator: Types, Uses and Purpose. Cleveland Clinic. Last updated May 10, 2022. https://my.clevelandclinic.org/health/treatments/23020-defibrillator
Bækgaard JS, Viereck S, Møller TP, et al. The Effects of Public Access Defibrillation on Survival After Out-of-Hospital Cardiac Arrest: A Systematic Review of Observational Studies. Circulation. 2017;136(10):954-965. PMID 28687709. https://doi.org/10.1161/CIRCULATIONAHA.117.029067
Ruan Y, Sun G, Li C, et al. Accessibility of automatic external defibrillators and survival rate of people with out-of-hospital cardiac arrest: A systematic review of real-world studies. Resuscitation. 2021;167:200-208. PMID 34453997. https://doi.org/10.1016/j.resuscitation.2021.08.035
Al-Khatib SM, Fonarow GC, Joglar JA, et al. Primary Prevention Implantable Cardioverter Defibrillators in Patients With Nonischemic Cardiomyopathy: A Meta-analysis. JAMA Cardiology. 2017;2(6):685-688. PMID 28355432. https://doi.org/10.1001/jamacardio.2017.0630
Lee SGW, Park JH, Ro YS, et al. Time to first defibrillation and survival outcomes of out-of-hospital cardiac arrest with refractory ventricular fibrillation. The American Journal of Emergency Medicine. 2021;40:96-102. PMID 33360396. https://doi.org/10.1016/j.ajem.2020.12.019
National Heart, Lung, and Blood Institute. Defibrillators: What are defibrillators? NHLBI Health Topics. Last updated June 6, 2023. https://www.nhlbi.nih.gov/health-topics/defibrillators
Goyal A, Chhabra L, Singh B, et al. Defibrillation. StatPearls. Last updated March 23, 2025. https://www.ncbi.nlm.nih.gov/books/NBK499899/
Disclaimer
This article is for general education and does not replace advice, diagnosis or treatment from a qualified healthcare professional. If you think someone is in cardiac arrest, call your local emergency number immediately.
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