Ambulance 2

Using an Automated External Defibrillator (AED) – OSCE Guide

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This automated external defibrillator (AED)Β OSCE guide aims to provide an overview of using an automated external defibrillator (AED). It isΒ intended only for students preparing for their OSCE exams andΒ notΒ for patient care.

Automated external defibrillators

An automated external defibrillator (AED) is a device that is used in cardiac arrest. It delivers a therapeutic dose of electrical energy to the heart through the chest wall.1-4

A cardiac arrest occurs when the heart stops beating effectively.1,3 The patient will lose consciousness and stop breathing. Defibrillation makes up a critical link in the chain of survival for cardiac arrest:

  • Early recognition and call for help
  • EarlyΒ CPR
  • Early defibrillation
  • EarlyΒ advanced cardiac life support

How does an AED work?

An AED works byΒ analysing the heart rhythm and delivering an electrical shock ifΒ the patient is in a shockable rhythm. An AED is an automated device, meaning it can be used by people with little or no medical training.Β 

The two shockable rhythms are:1,3,4

  • Ventricular fibrillation (VT): a chaotic and ineffective heart rhythm that is caused by abnormal electrical activity in the heart
  • Ventricular tachycardiaΒ (VT): a regular and fast rhythm that originates in the lower chambers of the heart

The electrical shock delivered by the AED disrupts the abnormal electrical activity in the heart and restores a normal rhythm. The shock depolarises a large portion of the heart muscle simultaneously. This causes the heart’s electrical activity to reset, allowing the normal electrical conduction pathways to be restored.3

It is important to note that an AED cannot restore a normal heart rhythm in all cases of cardiac arrest. The success of an AED shock depends on several factors, including the underlying cause of the cardiac arrest, the duration of cardiac arrest, and the patient’s overall health. However, the prompt use of an AED can significantly increase the chances of survival for a victim of sudden cardiac arrest.3, 5

The AED will not deliver an electrical shock if the patient is in a non-shockable rhythm (e.g. asystole).

History of AEDs

The history of automated external defibrillators (AEDs) dates to the 1950sΒ when scientists began experimenting with electrical shocks to revive people in cardiac arrest. However, it wasn’t until the late 1970s and early 1980s that the first AEDs were developed.

Physio-Control Corporation developed the Heartstart AEDΒ in the early 1980s. It was a portable device that could be used by non-medical personnel to deliver a shock to a person in cardiac arrest. The Heartstart was a significant advancement in emergency medicine, allowing laypeople to perform defibrillation during a cardiac arrest.

In the 1990s, AEDs began to be installed in public places such as airports, shopping malls, and sports venues. This was due to the growing awareness of the importance of early defibrillation in increasing the chances of survival for people in cardiac arrest.2,3

In the 2000s, AEDs became more widely available and were installed in many public places. In addition, the technology of AEDs has advanced, with newer devices featuring improved design and features such as voice prompts and real-time ECG monitoring.2,3

How to find an AED

In the UK, AEDs are widely available in public places, including shopping centres, airports, sports facilities, and train stations. They can also be found in workplaces, schools, and community centres.1-3

The British Heart Foundation estimates there are approximately 100,000 public AEDs in the UK, with many funded by charities or local communities.1,2

Anyone can use an AED in an emergency, regardless of whether they have received formal training. However, the Resuscitation Council UK recommends that people are trained to use an AED during first aid training, as this can increase their confidence and effectiveness in using the device.3,5

The location of an AED in public places should be indicated by clear signage.3,5 Ambulance services hold up-to-date information on defibrillator locations. There are several apps available which list defibrillator locations (e.g. Defib Finder).3,5

Public access automated external defibrillator (AED)
Figure 1. An example of a public access automated external defibrillator (AED).
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Using an automated external defibrillator

Using an AED is a vital part ofΒ basic life support, and the AED should be combined with high-quality cardiopulmonary resuscitation (CPR).Β 

1. If a bystander or assistant is available, ask them to continue cardiopulmonary resuscitation.

2. Switch on the AED. Once switched on, the AED will start giving instructions (audio-visual) to the operator.

3. Attach the two self-adhesive pads immediately to the patient’s bare chest (as labelled):

  • TheΒ rightΒ of the sternum below the clavicle
  • The mid-axillary line, with its long axis vertical and sufficiently lateral

4. Pause chest compressions whilst the AED analyses the patient’s heart rhythm. Do not touch the patient or electrodes during this time, as this may interrupt the analysis.

5. If a shock is required (i.e., the patient is in a shockable rhythm), follow the AED instructions and deliver aΒ shock.

  • Stand clear: check that no one is in contact with the patient before delivering a shock; otherwise, the electrical current may travel to others.Β 
  • Oxygen away: remove free-flowing oxygen (e.g. a mask or nasal cannula) to at least 1 meter away. Ventilation with an endotracheal tube or supraglottic airway device does not increase the oxygen concentration around the area of defibrillation. Therefore, it is acceptable to leave the ventilation bag connected.

6. If no shock is advised, leave the self-adhesive pads in place and follow AED instructions.Β 

7. Re-start chest compressions.

8. Follow instructions from the AED: the AED will time CPR, prompt the user to stop for a rhythm check every 2 minutes and deliver additional shocks if necessary.

Special circumstances

There are some special circumstances to consider when using an AED to ensure that the AED is used effectively.3,4,6

Obstructive clothing

Remove clothing and any other objects obstructing the patient’s chest to allow the correct placement of the electrode pads.

Age and size of the patient

The AED electrode pad size needs to be selected based on the age and size of the patient to ensure that the device is used effectively and safely.

Some AEDs require specific (smaller) paediatric pads for use on a child, depending on their body weight. The manufacturer’s guidance should be followed, and where necessary, both adult and paediatric pads should be kept with the AED.

Adult pads can be used on a child if no paediatric pads are available.

Wet conditions

If the patient is lying in water or is wet, move them to a dry area before using the AED. The moisture can interfere with the device’s ability to deliver a shock and pose a risk of electrocution to the operator.

Wet skin

Dry the patient’s chest as much as possible before applying the electrodes/pads. You may need a dry cloth or towel to dry the area before applying the electrodes.

Hairy skin

To use an AED on hairy skin, you should shave the patient’s chest hair to expose the skin. This will help ensure that the electrodes make good contact with the skin and that the AED can accurately analyse the patient’s heart rhythm.

Metal objects, jewellery, buttons

Remove any metal objects in contact with the patient’s chest. This includes jewellery, buttons, and other metal objects that may interfere with the AED’s ability to deliver a shock.

Pacemakers or implantable cardioverter defibrillators (ICDs)

If the patient has a pacemaker or ICD implanted, avoid placing the AED electrode pads directly over the device, as this can interfere with the electrical signals of the device. Instead, place the pads at least one inch away from the device.


  1. British Heart Foundation. Cardiac Arrest. 2023. Available from: [LINK]
  2. British Heart Foundation. How and when to use a defibrillator. 2023. Available from: [LINK]
  3. Resuscitation Council UK, & British Heart Foundation. A guide to automated external defibrillators (AEDs). 2019. Available from: [LINK]
  4. Resuscitation Council UK. Adult Basic Life Support Guidelines. 2021. Available from: [LINK]
  5. BΓ¦kgaard, Josefine S.; Viereck, SΓΈren; MΓΈller, Thea Palsgaard; ErsbΓΈl (2017). “The Effects of Public Access Defibrillation on Survival After Out-of-Hospital Cardiac Arrest: A Systematic Review of Observational Studies”. Circulation. 136 (10): 954–965.
  6. Sudden Cardiac Arrest UK. Frequently Asked Questions. Available from: [LINK]

Image references

  • Figure 1. Leonard Bentley. Public access AED. License: [CC BY-SA]


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