How does a defibrillator (AED) work?

Defibrillators are one of the best life-saving tools available to the public. They work by delivering an electric shock of up to 3000 volts to the heart of a patient suffering from cardiac arrest using advanced technology.

This piece covers AEDs (Automated External Defibrillators) as opposed to . AEDs are found across the country in key, publicly accessible locations such as phone boxes, post offices and local landmarks, as well as in workplaces, schools and shopping centres.

How Defibrillators (AEDs) Work

An AED, sometimes called a defibrillator or a PAD (Public Access Defibrillator), works by detecting ventricular fibrillation during Sudden Cardiac Arrest (SCA), and automatically applies a shock of up to 3000 volts to the heart of a patient at the correct moment. An AED will only work if it can detect a shockable heart rhythm, most commonly ventricular fibrillation (where the heart muscles flutter).

When the AED can detect a shockable heart rhythm, the AED will send a brief, but powerful, electrical shock through the heart. This depolarises the cardiac muscle, allowing the body’s pacemaker to beat at a normal rhythm. Multiple shocks are often necessary, but an AED will be able to detect this, and apply the correct shocks at the correct time. An AED will not send an electric shock if it cannot detect treatable ventricular fibrillation.

When is a defibrillator used?

A defibrillator is used when someone is suffering from Sudden Cardiac Arrest (SCA). SCA will result in someone collapsing and being unresponsive. During SCA, a person’s heart will get stuck in an irregular pattern and will be unable to correct itself, resulting in a type of arrythmia called ventricular fibrillation.

What happens during Sudden Cardiac Arrest (SCA)?

SCA starts with ventricular fibrillation, which is where the heart ventricles flutter rather than maintain the normal muscular motions. This fluttering, or fibrillation, is what causes someone to go into cardiac arrest.  

As the ventricles that pump blood around the body are no longer working as needed, blood cannot be moved around the body, causing the victim to collapse, resulting in death in minutes if CPR is not performed immediately. A defibrillator can be used to shock the heart back into a normal rhythm again.

Using a Defibrillator (AED)

A defibrillator (AED) is not a standalone device, it is used in conjunction with CPR to provide lifesaving assistance. This process is called the chain of survival.

When someone goes into sudden cardiac arrest, the first thing to do is to call for the emergency services, stating that the casualty is in cardiac arrest. Then another person should begin performing CPR. As CPR is so important to the chain of survival, understanding the basics of CPR can help you to potentially save a life.

During this process, another person nearby should go and get the nearest defibrillator (AED) if one is available. The emergency services handler should instruct you where to find the nearest defibrillator and provide the unlock code to the cabinet if necessary.

Then, while CPR continues, the person with the defibrillator will need to remove any clothing from the patient’s upper half, including underwear, and apply the electrode patches as instructed.

The AED will then instruct on the CPR, if it has this capability, before instructing the CPR performer to step back as the AED delivers a shock. If the AED does not have the CPR assistance feature, then whoever is carrying out CPR will need to listen for when the AED is about to deliver a shock.

Once a shock has been delivered, the AED will be able to instruct the user on whether to continue with CPR, or whether another shock is necessary.

Defibrillator (AED) Components

An AED comes with multiple components, all of which work together to deliver lifesaving treatment to someone in need.

  • The defibrillator: This is the central core of the AED and provides instruction to the user, containing visual aids and an intelligent computer that can detect ventricular fibrillation and deliver a shock at the correct moment.
  • Pads (Pad Pak): Defibrillator pads (Pad Pak is the HeartSine brand name) are coated with a conductive gel that, when adhered to shaven skin, conduct electricity through a person’s body.
  • Razor: A razor is always included with an AED, as for a shock to be delivered effectively, the pads must be touching skin, rather than hair.
  • Scissors: Scissors are provided to easily cut through clothing, as any clothing on the top half must be removed.

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