CPR course using automated external defibrillator device - AED

What is an AED? Our Guide to Defibrillators

Learn the basics of what a defib is, how it works, when you should use one, and how to get your hands on one with our simple guide to defibrillators.


What is an Automated External Defibrillator?

We need to start somewhere, so we'll begin with a definition of what an AED, or Automated External Defibrillator is.

Defibrillator Definition

According to the Cambridge Dictionary Online, a defibrillator is:

"a machine that uses an electric current to stop any irregular and dangerous activity of the heart's muscles"

In practice an Automated External Defibrillator, or AED is a portable medical device that can deliver a controlled electric shock to a patient who has gone into cardiac arrest. It can be either semi automatic, whereby the user has to push a button to activate the shock. Or it can be fully automatic, where the machine generates the shock automatically.

An AED consists of two main elements. The first is a 'control box', which in simple terms contains a microprocessor, electrical circuitry and a long-life battery. The second is a pair of defibrillator pads (or electrodes), which attach to the patient. They collect heart rhythm information from the patient and pass it to the control box. Then if required, the control box delivers a shock to the patient's heart.

Example automated external defibrillator showing defib pads

By the way, if you want a whole load more information than we can include here, the Resuscitation Council (UK) and the British Heart Foundation (BHF) have written in-depth guide. Clicking on the link will download their guide to your device.

Want to learn to use a Defibrillator?

Come along to one of our Workplace First Aid courses

How does a Defibrillator work?

Defibrillators are the magic boxes that deliver a controlled electric shock that helps to get a heart that has gone into an irregular lethal rhythm back into a normal one.

So how do they work? To understand that, you first need to know a little about the human heart:

The Human Heart – The Basics:

The heart is a 4-chambered pump with its own electrical system which fires off electrical impulses from the primary pacemaker. 

Diagram showing the human heart and circulation of blood

These signals pass through the top chambers of the heart and make the chambers contract. Then, further signals pass through the bottom two chambers and make them contract, thus forcing the blood in the heart out around the body. The signals are coordinated so that the top two chambers contract first, followed by the bottom two.

The electrical cells in the heart then 'recharge' ready to fire again. 

The chambers on the right pump blood to the lungs, where oxygen is collected. The oxygenated blood then enters the left side of the heart. From there it is pumped around the rest of the body, delivering the oxygen to all of the vital organs.

You can find out more about how the heart works by taking a look at the BHF's how your heart works article.

Ventricular Fibrillation

For the heart to pump effectively, the heart muscles must contract and relax rhythmically, and in the correct sequence. This is the heart beat that you can feel when taking your pulse, and can feel pulsing in your chest.

The heart is unique in that every cell in the heart is capable of generating an impulse. When something goes wrong, for example when damage occurs to the heart as a patient has a heart attack, very often the damage triggers random impulses, causing their heart to go into an abnormal rhythm.

The most common of these is ventricular fibrillation (VF). Instead of the heart muscle pumping correctly, in VF the electrical signals and muscle fibres fire randomly. This can cause the heart to vibrate or quiver. In this rhythm, the heart can't pump blood round the body. If left untreated, the patient will die. This is what is referred to as Cardiac Arrest.

The Defib Difference

Estimates suggest that the ambulance service treats around 28,000 people for 'out of hospital cardiac arrests' (OHCA) in the UK each year. There are an estimated further 30,000 each year who may have suffered OHCAs, and who might have been treatable by the emergency services if they had been called earlier or if bystander CPR had been carried out in time (source: Resuscitation Council (UK)).

Research has shown that if an AED can be attached to a person whose heart has stopped and a shock delivered within the first 3-5 minutes of an arrest, the chances of survival are high, between about 50 % and 70%.

However, the chances of the patient surviving long enough to be discharged from hospital reduce by about 10% for every additional minute of delay in treating them with a defibrillator.

To put that in context, in the UK, fewer than 2% of patients are treated with a defibrillator before the ambulance service arrives.

“The chance of a patient surviving to be discharged from hospital reduces by around 10% for every minute of delay”

Want to learn to use a Defibrillator?

Come along to one of our Workplace First Aid courses

How AEDs Work and how to use them

Defibs are a critical part of the chain of survival for a casualty who is suffering from Cardiac Arrest.

The CPR Chain of Survival

Chain of Survival, developed by the Resuscitation Council (UK)

Links 1, 2 and 4 are covered in more depth in our guide to CPR, so I won't dwell on them here.

Pad Placement

Once the AED has arrived,  the defibrillator pads need to be placed correctly by the operator / bystander. Correct pad placement is important; there is a diagram on each pad showing where to put them.

The first goes on the right side of the chest (with the casualty on their back), just below the collar bone.

The second goes on the left side of the chest, just below the breast area or below. Once the pads are placed correctly and it's switched on, the defib first analyses readings from the patient's heart.

As we said, they're really, really easy to use. All portable defibs include a speaker which tells the user at every stage exactly what it's doing, and instructs them when they need to intervene, and what they should do. Many models also include a video or text screen which also give the same instructions.

Defibrillator pad placement

“Correct defib pad placement is really important.”

Delivering a Shock

If it recognizes a shockable rhythm it will charge up ready to deliver its shock. Semi automatic defibrillators both audibly alarm and flash to indicate if and when you should press the Red Flashing button in order to deliver the shock. Fully automatic machines will alarm but there is no flashing button to push. Instead, the machine generates the shock automatically.

In either case a controlled electrical shock is delivered across the heart which overrides the abnormal electrical signals and allows the heart’s own internal electrical system the opportunity to start working effectively again. It is not a “jump start”, more a type of “re-booting” effect.

Continuing CPR

In both cases, the defib will then immediately instruct the bystander to commence (or continue) CPR, and will subsequently analyse and shock again, if required.

This CPR and AED process is continued either until skilled help arrives or until the patient wakes up and is breathing and moving normally.

Defibrillators in the Workplace

Should you get a defib for your workplace?

Well, if in doubt the answer is probably 'yes'. And I'm not necessarily talking from a legal point of view, as most organisations are not, at the time of writing, legally required to have one.

But it's worth considering that sudden cardiac arrest is a leading cause of death and that you do have a responsibility for ensuring safety to your staff, visitors and customers. You may consider that the risk of needing one is so low that buying one is unnecessary. But that would be missing the point. After all, you invest in fire extinguishers and in business, car and home insurance and hope you will never need to use any of those either!

Many employers are often put off purchasing an AED on the basis of cost of the device, and the time that it takes to train staff to use one. However, the machines themselves are very reliable and easy to use, and instruct the user how to operate them as they go along. Your first aiders should certainly find them very easy to use, and non-first aiders should have no problem either.

In fact full first aiders should receive hands-on training with them each time they renew or refresh their skills (if not, ask your training provider why not!). And non-first aiders could also receive some basic training on your defib, if you're concerned about them using one.

Purchase Cost vs Cost of Delaying

As far as costs go, portable defibs typically cost between £900 and £1500 + VAT: quite a small outlay for many organisations, particularly when set against the human and business costs of cardiac arrest.

Want to learn to use a Defibrillator?

Come along to one of our Workplace First Aid courses

Make sure your Defibrillator is available

Of course there's no point in having a workplace defib if your staff can't access it, or don't know where it is. We've seen cases of them being locked away in cupboards, with keys held by part-time staff. Or more commonly, locked in the office of someone who works office hours, and so not available to warehouse staff who work shifts.

What about all those staff who don't even know you have one? Or those who've heard a rumour that you have one, but don't know where it is?

As part of our first aid courses, we ask staff to bring out their defib. Sometimes, there are blank looks, with some minutes of hunting around, followed by a sheepish looking staff member bringing out their defib, still in its original box and covered in dust!

By this time, in a real emergency, it would already be too late to use it!

“Make sure everyone knows where your defib is. If they have to go looking, it will already be too late.”

9 Rules of Successful Deployment

So, when you buy a defib for work, follow these rules:

  1. Don't delay - unbox it as soon as it arrives.
  2. Read the instructions to make sure you know how to use it.
  3. Decide where it should live - perhaps get a wall-mounting, and place it in an obvious and accessible area.
  4. Put up signs to highlight that it's there.
  5. Send out a notice, email or whatever (probably more than one) to let EVERYONE know that it's there.
  6. Add to the note an outline of how to use it, how important it is, and how crucial it is that someone fetches it quickly in an emergency.
  7. Put a note in the diary to check it periodically (and to make sure you have in-date pads, charged batteries etc).
  8. Decide if you need another one on another floor, in another building or on another site.
  9. Add a note (perhaps annually) about your defib to your internal newsletter, to make new staff aware, and to remind existing staff.
Automated External Defibrillator Sign

Defibrillator Training

Historically only those who have been trained to use one could do so. However since the latest guidelines on resuscitation have been published there is a greater emphasis on anyone being able to use one if it is available, although obviously having some training helps.

Is formal training necessary?

Out-of-hospital defibrillators are specifically designed to be used by untrained staff as well as trained first aiders. They provide help to the user, including clear imagery (both on the device and the pads), verbal instructions and semi or automatic operation.

They also won't shock unless a shock is required, so you can't cause an injury by using one.

So in theory, you don't need to be trained before you use a defribrillator. And if you find yourself needing to use one to help save someone's life, you should do so, rather than holding back from lack of training.

But if you are unsure about having untrained staff using a defib, you'll be happy that almost all first aid courses include instruction in defibs.

Types of Training

All of our practical first aid courses, from the most basic to the most advanced, include instruction, demonstration and hands-on practise in the use of defibs. So at the end of any of those courses, everyone should be confident to use one.

If you're looking for formal workplace first aiders, you should be looking at the Emergency First Aid or First Aid at Work courses. During both courses you'll be assessed in your use of an AED, as well as being taught a whole range of other first aid skills

If you already have enough workplace first aiders, a basic first aid or AED course may be suitable for you. This teaches fewer skills, but can be used to inform those who don't want to be first aiders, but would still benefit from learning these key skills.

In either case, your staff will learn to use a defib in context of a simulated medical emergency, designed to help them to react swiftly and correctly in the event of a real emergency situation.

Want to learn to use a Defibrillator?

Come along to one of our Workplace First Aid courses

Maintaining and Servicing your AED

You'll be pleased to hear that defibrillators are designed to be very low maintenance devices. However, there are a few regular checks you should carry out. Your first port of call for this information is the manual which came with your defib, so please READ THAT FIRST!

Please note that each make of defibrillator is slightly difference, so all of the below information is generalised.

There are a few consumables which need replacing periodically:

Replacing Batteries

Like any other device, defib batteries have a limited useful life. This is usually between 3 and 5 years. Check in the manual, and make sure you record when the battery is due for replacement.

SCHEDULE the purchase of the replacement, well in advance of its expiry date. It's no good recording that it needs replacing, if no-one gets a reminder when it's due.

If the defib is used, the batteries may need replacing earlier.

Replacing Pads

Defib pads also need changing, usually every 2 to 3 years, even if they've not been used. They're also single use, so if they have been used, replace them immediately. Better still, always keep a spare set (but bear in mind they'll go out of date too).

Depending on the model of your AED, you may require junior pads as well as adult ones. If so, keep a spare set of those too, if you're likely to need to care for children.

Other Maintenance

Many devices have in-built self-test modes which will activate when the defib is switched on. If yours is one of those, it's worth scheduling regular 'switch-ons' so that the machine can check battery levels and other operations.

They may also have battery condition lights, which will indicate how much charge remains in your unit. Replace your battery on if your unit shows low condition or if the battery is out of date (whichever is the earlier). 

Servicing Companies

There are companies who charge to service AEDS. They can either be related to the manufacturer, the vendor or be completely independent of either. It's up to you whether you purchase this 'insurance service' or not. Again, we'd suggest taking advice from your owner's manual.

Defibrillators in the Community

Over the last few years we've been delighted to see the number of Public Access Defibrillators (PADs)  on the increase. They are appearing outside many more shops, libraries and community centres as well as in schools.

How do I recognise one?

Often, but not exclusively, recognised by the bright yellow cabinets they are housed in, they are a welcome sight. How many people though are aware that they are for the use of the general public and that anyone can access them?

The cabinets are typically locked (to combat vandalism), and have instructions on the front telling the user which number to call to unlock the cabinet and access the defib. They're then used in the same way as the machine in your workplace.

Wall-mounted Public Access Defibrillator

Where's your nearest one?

Of course trained or not, a defib's only any good if you know where it is. So ask yourself the question... "where's my nearest one?"

And don't just think of the one you have at work. Where's the nearest one to your home, your child's school, the shopping centre that you visit regularly? Knowing this, and how to use one will be doing yourself, your family and your community a valuable service.

Many Ambulance Services hold a list of accessible defibrillators, and you can also take a look at the National Defibrillator Database which maintains a map of public access defibs which is searchable by town, city or postcode. In addition, the BHF manage the 'The Circuit', aiming to register the location of all PADs in the UK.

There are also several PAD finder apps available on mobile app stores, but as yet we've not reviewed the accuracy of them.

Funding a PAD for your Community

It is possible to apply to the BHF for funding to get a PAD in your community. Not all organisations can apply (private businesses, Dental & GP Practices, Schools etc are all excluded). However take a look at their page, and you'll see whether your community qualifies, and if so, what you'd have to do.

Registering a Device with the Ambulance Service

As we know, rapid access to defibrillation dramatically increases the chances of surviving a cardiac arrest. So it's really important that public access devices are registered with the local ambulance service to enable emergency call handlers to direct rescuers to the nearest AED.

You also have the option to register your office device too, and by doing so, you'll be contributing to the ever-expanding pool of defibs available for the public to use.

Want to learn to use a Defibrillator?

Come along to one of our Workplace First Aid courses

Buying A Defibrillator

There are plenty of options if you're buying a defibrillator for your workplace. Apart from minor differences, each model has the same function. It's often best to find out which model your local ambulance service is currently recommending, but you can base your decision on price or add-in functions, or on cost of consumables.

Please note that none of these are a specific recommendation, and we neither sell nor take commission from any AED suppliers or manufacturers.

The main brands, along with links to the manufacturers are:

Other Similar Devices

In addition to AEDs, there are also several similar medical devices which you may have heard about, which have similar functions, but are distinctly different from the AEDs that we use as first aiders. To help you understand the differences, here are some quick details on those other machines.

Manual defibrillator

Manual defibrillators are typically the larger, more advanced devices used by professional medical responders. They often include screens and printers which provide heart traces, and have full paddles rather than pads.

Implantable cardioverter defibrillator

An ICD is a medical device that's planted in the chest, under the skin, to monitor your heart rhythm. It can deliver electric shocks directly to the heart if it detects the appropriate conditions.


A pacemaker is a device similar to an ICD. However a pacemaker sends out a regular electrical signal if it detects a fast or irregular heartbeat, and therefore does more of a heart regulation than heart shocking function.

Have Your Say

As first aid trainers, we think it's great (and overdue) that defibs are included as a compulsory part of all workplace first aid courses. We've been teaching them that way for years, and think it's a big step forward.

The big increase in community defibrillators is also really positive. But what do you think? Are you trained in using an AED? If you're not, would the presence of a defib put you off from trying to resuscitate someone, or would you be happy following the instructions given by the device?

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