Defibrillators How Do Defibrillators Work

Defibrillators can detect sudden, dangerous heart rhythms or a cardiac arrest. If a defibrillator detects a cardiac arrest or a dangerous arrhythmia, it can send an electric charge to the heart to try to restore a normal heartbeat or rhythm.

This video explains your heart's electrical system and how an ICD can help prevent death from cardiac arrest and correct dangerous arrhythmias. Source: Nucleus Medical Media. Item ID: ANW10012. Available from https://catalog.nucleusmedicalmedia.com/implantable-cardioverter-defibrillator-icd/view-item?ItemID=70909.

Learn more about the heart’s electrical system at How the Heart Works.

Implanted or wearable cardioverter defibrillators monitor your heartbeat all the time. Some ICDs also act as pacemakers, delivering pacing therapy to help your heart beat at a normal rate and rhythm or help your heart chambers beat in sync. A healthcare provider programs the device for your condition.

During office visits, your provider will “interrogate,” or check, your implanted or wearable device. A remote monitoring box in your home or a smartphone app may allow your healthcare provider to monitor your device and may alert them of shocks or pacing therapy given to your heart.

Defibrillators have batteries that power the device, allowing it to check the heartbeat and send electrical pulses or shocks to treat the heart. Learn more about defibrillator battery life.

How do AEDs work?

An AED is a lightweight, battery-operated, portable device that checks the heart’s rhythm and can send a shock to the heart to restore normal rhythm. The unit tells the operator or bystander what to do. Anyone can use an AED to help someone having cardiac arrest. Experts recommend training, but it is not required.

Sticky pads with sensors, called electrodes, are attached to the chest of someone who is having cardiac arrest. The electrodes send information about the person's heart rhythm to a computer in the AED. The computer analyzes the heart rhythm to find out whether an electric shock is needed. If it is needed, the electrodes deliver one or more shocks. Learn more about using an AED to save a life.

AEDs need regular maintenance to ensure they are ready for emergencies. Some AEDs have rechargeable batteries, but others need new batteries every few years.

Image of an automated external defibrillator in use. — **A person using an AED on someone in cardiac arrest.** This drawing illustrates an AED machine, the pads with electrodes on the patient’s chest, and information about step-by-step instructions and voice prompts for using the machine correctly.

How do ICDs work?

ICDs are implanted (placed) in the upper body through surgery. There are three main types of ICDs. Each works in a slightly different way:

Transvenous (through a vein) ICDs have one or more leads (wires) that go from the device into the heart. They can send a strong, high-energy charge to treat dangerous arrhythmias or cardiac arrest. They can also give low-energy pacing therapy to help your heart beat at a normal rate and rhythm. Most people say they do not feel pacing therapy, but others say that it feels like their heart is fluttering. If low energy charges do not restore your normal heart rhythm, the device may switch to high-energy charges. This is called defibrillation therapy.

A transvenous ICD is surgically placed in the chest and has one or more wires that enter the heart to detect a dangerous arrhythmia or cardiac arrest. The ICD then sends an electrical shock to try to restore a normal heartbeat. This type of ICD can also give low-energy electric pulses like a pacemaker. Credit: Reprinted with permission from Hoevelmann, J., Hähnle, L., Hähnle, J., Sliwa, K., & Viljoen, C. (2020). Detection and management of arrhythmias in peripartum cardiomyopathy. Cardiovascular Diagnosis and Therapy, 10(2), 325–335. https://doi.org/10.21037/cdt.2019.05.03. Creative Commons License BY-NC-ND 4.0.

Subcutaneous ICDs (SICDs) are surgically placed just under the skin on the side of your chest. They do not have leads entering the heart. SICDs can only give high-energy shocks. They do not have a pacemaker function to speed up or slow down heart rhythms.
Cardiac resynchronization therapy defibrillators (CRT-Ds) may help some people who have advanced heart failure that raises their risk of cardiac arrest. CRT-Ds have an extra heart lead so they can give three types of electrical therapy.
- Low-energy pacing keeps the heart beating at a normal rhythm.
- Medium-energy electrical pulses (cardioversion therapy) sync the pumping action of the heart’s ventricles and strengthen the heartbeat.
- High-energy defibrillation therapy treats cardiac arrest or dangerous arrhythmias.

What does a high-energy shock feel like?

Some say a shock feels like a kick in the chest. It might make you faint or fall down. But since people often pass out when a dangerous ventricular arrhythmia or cardiac arrest starts, they probably do not feel the device’s high-energy shocks.

After defibrillation therapy, many people feel reassured that the shock kept them safe, but it can also be an upsetting experience. Most feel better soon, but others need more time to recover. Counseling and support groups can help relieve this stress.

How do WCDs work?

A WCD is like a vest worn under clothing next to the skin. Pads on the vest monitor your heart and can deliver high-energy charges to treat dangerous arrhythmias or cardiac arrest. The device includes a small box that records heart rhythms and contains a rechargeable battery. WCDs do not provide pacing therapy. They can only give high-energy defibrillation charges.

If your WCD detects a life-threatening arrhythmia, it will sound an alert. You can turn off the alert to prevent a shock if not needed. Just before the shock, the electrodes release a gel to ensure good contact with the skin. This all happens in about a minute. The device can give one or more charges to restore a normal heartbeat.

After giving an electrical charge, the WCD’s sensors must be replaced. A WCD can send a record of your heart’s activity to your healthcare provider. Talk with your provider about taking care of your vest, including charging the battery.

It's important to always wear the device unless showering or bathing. Respond to any alerts warning of problems with the vest. Learn more about living with a WCD.

An illustration of a woman wearing a WCD vest — A WCD has built-in pads to deliver shocks when a life-threatening arrhythmia is detected. Wear your WCD correctly and consistently. Credit: Reprinted with permission from Hoevelmann, J., Hähnle, L., Hähnle, J., Sliwa, K., & Viljoen, C. (2020). Detection and management of arrhythmias in peripartum cardiomyopathy. Cardiovascular Diagnosis and Therapy, 10(2), 325–335. https://doi.org/10.21037/cdt.2019.05.03. Creative Commons License BY-NC-ND 4.0.