Defibrillators What are Defibrillators?


Woman using a defibrillator on a man lying on the ground Defibrillators are devices that send an electric pulse or shock to the heart to restore a normal heartbeat. They are used to prevent or correct an arrhythmia, an uneven heartbeat that is too slow or too fast. If the heart suddenly stops, defibrillators can also help it beat again. Different types of defibrillators work in different ways. Automated external defibrillators (AEDs), which are now found in many public spaces, are used to save the lives of people experiencing cardiac arrest. Even untrained bystanders can use these devices in an emergency.

Other defibrillators can prevent sudden death among people who have a high risk of a life-threatening arrhythmia. They include implantable cardioverter defibrillators (ICDs), which are surgically placed inside your body, and wearable cardioverter defibrillators (WCDs), which rest on the body. It can take time and effort to get used to living with a defibrillator, and it is important to be aware of possible complications.

Types and How They Work

There are three types of defibrillators: AEDs, ICDs, and WCDs.

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

How do AEDs work?

An AED is a lightweight, battery-operated, portable device that checks the heart’s rhythm and sends a shock to the heart to restore normal rhythm. The device is used to help people having cardiac arrest.

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 the shock.

Image of an automated external defibrillator in use.
Image of a person using an automated external defibrillator on someone in cardiac arrest. The picture shows the AED machine, the pads with electrodes on the patient’s chest, and information about step-by-step instructions and voice prompts to help untrained bystanders to use the machine correctly. 

How do ICDs work?

ICDs are placed through surgery in the chest or stomach area, where the device can check for arrhythmias. Arrhythmias can interrupt the flow of blood from your heart to the rest of your body or cause your heart to stop. The ICD sends a shock to restore a normal heart rhythm.

An ICD can give off a low-energy shock that speeds up or slows down an abnormal heart rate, or a high-energy shock to correct a fast or irregular heartbeat. If low-energy shocks do not restore your normal heart rhythm, the device may switch to high-energy shocks for defibrillation.

ICDs are similar to pacemakers, but pacemakers deliver only low-energy electrical shocks. ICDs have a generator connected to wires that detect your heart’s beats and deliver a shock when needed. Some ICDs have wires that rest inside one or two chambers of the heart. Others do not have wires going into the heart chambers but instead rest on the heart to monitor its rhythm.

The ICD can also record the heart's electrical activity and heart rhythms. The recordings can help your healthcare provider fine-tune the programming of the device so it works better to correct irregular heartbeats. The device is programmed to respond to the type of arrhythmia you are most likely to have.

Comparison of an implantable cardioverter defibrillator and a pacemaker.
Comparison of an implantable cardioverter defibrillator and a pacemaker. Image A shows the placement of an ICD. Image B (inset on the right) shows the size and placement of a pacemaker. 

How do WCDs work?

WCDs have sensors that attach to the skin. They are connected by wires to a unit that checks your heart’s rhythm and delivers shocks when needed. Like an ICD, the WCD can deliver low- and high-energy shocks. The device has a belt attached to a vest that is worn under your clothes. Your provider fits the device to your size. It is programmed to detect a specific heart rhythm.

The sensors detect when an arrhythmia occurs and gives an alert. You can turn off the alert to prevent a shock if not needed. If you do not respond, the device will give your heart a shock to correct the rhythm. This usually happens within one minute. The device can deliver repeated shocks during an episode. After each episode, the sensors must be replaced.

The device can also send a record of your heart’s activity to your doctors.

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