With simple audio and visual commands, AEDs are designed to be simple to use for the layperson, and the use of AEDs is taught in many first aid, certified first responder, and basic life support (BLS) level cardiopulmonary resuscitation (CPR) classes.

The rhythms that the device will treat are usually limited to: In each of these two types of shockable cardiac arrhythmia, the heart is electrically active, but in a dysfunctional pattern that does not allow it to pump and circulate blood.

In ventricular fibrillation, the electrical activity of the heart becomes chaotic, preventing the ventricle from effectively pumping blood.

To assist this, the vast majority of units have spoken prompts, and some may also have visual displays to instruct the user.

When turned on or opened, the AED will instruct the user to connect the electrodes (pads) to the patient.

The device system is not only safer - charging only when required, but also allows for a faster delivery of the electric current.

Some units also have voice recording abilities[18] to monitor the actions taken by the personnel in order to ascertain if these had any impact on the survival outcome.

All this recorded data can be either downloaded to a computer or printed out so that the providing organisation or responsible body is able to see the effectiveness of both CPR and defibrillation.

This caused increased cardiac injury and in some cases second and third-degree burns around the shock pad sites.

That is possible because all AEDs approved for use in the United States and many other countries use an electronic voice to prompt users through each step.

[22] Observational studies have shown that in out of hospital cardiac arrest, public access defibrillators when used were associated with 40% median survival.

In the United States, Good Samaritan laws provide some protection for the use of AEDs by trained and untrained responders.

In addition to Good Samaritan laws, Ontario, Canada also has the "Chase McEachern Act (Heart Defibrillator Civil Liability), 2007 (Bill 171 – Subsection N)", passed in June, 2007,[27] which protects individuals from liability for damages that may occur from their use of an AED to save someone's life at the immediate scene of an emergency unless damages are caused by gross negligence.

Legislation in Australia varies by state, with separate liability issues relating to providing and using AED equipment.

Each state and territory has enacted "Good Samaritan" laws that offer legal protection to a person who gives assistance in a medical emergency - the standard of care expected corresponds to their training (or lack of training).

Technical malfunctions likely contributed to more than 750 deaths in the 5-year period between 2004 and 2009, in most cases by component failures or design errors.

The agency's strengthened review will focus on the critical requirements needed to ensure the safety and reliability of AEDs and their necessary accessories, including batteries, pad electrodes, adapters and hardware keys for pediatric use.

"[31][32] In the United Kingdom there is concern that poor maintenance may make public defibrillators unreliable.

The Henley Standard reported on 21 July 2017 that more than half the defibrillators in Henley-on-Thames and the surrounding area were at risk of failing, either because of low battery power or because adhesive pads had deteriorated.

[34] The portable version of the external defibrillator was invented in 1957 by Frank Pantridge in Belfast, Northern Ireland, a pioneer in emergency medical treatment.