How Does a Defibrillator Work?

Have you ever walked past a building that has a defibrillator attached to the wall and wondered how it works? This article explores the science behind a defibrillator and why they are essential in preventing a cardiac arrest.

What is a Defibrillator?

A defibrillator, specifically an Automated External Defibrillator (AED), is a machine that delivers a shock to a heart that is experiencing a cardiac arrest in order to help it reset back to a regular rhythm.

The earliest examples of defibrillation date all the way back to 1899 when scientists in Switzerland discovered that electric shocks can be used to affect the heart's rhythm. By the early 1960's scientists had created a portable machine that use pads to detect the heart's rhythm and deliver a shock when needed. With the first paramedics being introduced in the UK from the 1990's a portable defibrillator became an essential part of emergency response.

How Do They Work?

First, the Automated External Defibrillator is turned on and the voice prompt will talk the user through every step of the process. It will ask you to prepare the casualty's chest by removing clothing, chest hair, and any moisture from their skin. Diagrams will help to show the user how to prepare the casualty.

Once the pads are placed correctly it will begin analysing the heart's rhythm. It is detecting an irregular heart rhythm known as ventricular fibrillation (VF), which can lead to a cardiac arrest meaning the heart stops beating entirely. This process is done electronically and is extremely accurate.

Ventricular Fibrillation (VF).

If the machine detects VF it will then advise a shock. This shock is produced from the machine's battery filling a high-capacity capacitor and is delivered through electrodes which are attached to the casualty's chest via adhesive pads. Most machines will automatically set the amount of electricity needed to stop the heart from fibrillating - this can be up to 360 joules depending on the make of defibrillator. A current will be passed from one pad to the other within a fraction of a second to stop the heart from fibrillating.

If the machine is fully automatic, it will instruct everybody to stand clear of the casualty and count down before delivering a shock. However most AEDs will require the user to press an orange 'shock' button to deliver the shock. You must ensure nobody is touching the casualty when the shock is being delivered as it can severely injure them.

This shock will momentarily stop the casualty's heart from beating, allowing the pacemaker of the heart (the heart's internal electrical system) to reset and begin beating again. This is a lot like pressing 'restart' on a computer - switching it off and on again to reset the system, which normally resolves the issue. If this is successful, the heart returns back to its normal sinus rhythm and a cardiac arrest has been prevented.

If the casualty does not immediately wake up the machine will instruct the user to begin CPR for two minutes, or until they see signs of life. If there are no signs of life after two minutes, the machine will analyse the heart rhythm again and repeat. The heart may still be going through VF which means it may need another shock.

If the defibrillator delivers a shock within 3-5 minutes of the casualty collapsing, along with continuous CPR, it can produce survival rates of up to 74%. However if only CPR is performed, the chance of survival can be as low as 6%. It is important to know where your nearest defibrillators are in case of an emergency...

Where Are They Found?

Outside of hospitals, Public Access Defibrillators (PADs) can be found in all sorts of places. This can include railway and bus stations, shopping centres, supermarkets, gyms, community centres, and more! Look out for the green sign below which will indicate a defibrillator is close by...

Green Defibrillator Sign.

In an emergency you can also call 999/112 - they will direct you to the nearest Public Access Defibrillator and give you instructions on how to open the cabinet.

How Do I Obtain a Public Access Defibrillator?

If your workplace does not have a defibrillator on-site you can easily grab you should find the next nearest defibrillator, which may be attached to a shop around the corner for example. To open the cabinet and take the defibrillator...

1. Call the number written on the cabinet - this is usually 999.

2. Tell the call handler where you are - there is often a location code written on the cabinet.

3. Listen carefully to their instructions - they will give you the code to open the cabinet.

4. Take the defibrillator and any extra equipment you may find in the cabinet away with you.

Once you have finished using the defibrillator it should be placed back in its original location - the emergency services will assist you in this.

Do I Need to be First Aid Trained to Use a Defibrillator?

Whilst it is recommended to undertake some form of training, you do not have to be trained to use a Public Access Defibrillator. Any member of the public can call 999/112 and they will talk you through how to obtain and use the defibrillator.

If you want hands-on training on using defibrillators and performing CPR, you can book one of our Basic Life Support or Defibrillator Awareness courses. We can provide a trainer to visit your workplace, school, or community to give you and those around you confidence and experience in using our training defibrillators.