The automated external defibrillator

Introduction

The automated external defibrillator (AED) is generally considered to be the most important development in defibrillator technology in recent years. Its development came about through the recognition that, in adults, the commonest primary arrhythmia at the onset of cardiac arrest is ventricular fibrillation or pulseless ventricular tachycardia. Survival is crucially dependent on minimising the delay before providing a countershock.

Use of a manual defibrillator requires considerable training, particularly in the skills of interpreting electrocardiograms, and this greatly restricts the availability of prompt electrical treatment. Conventional emergency medical systems often cannot respond rapidly enough to provide defibrillation within the accepted time frame of eight minutes or less. This led to an investigation into ways of automating defibrillation so that defibrillators might be used by more people and, therefore, be more widely deployed in the community.

Modern automated external defibrillator

Modern automated external defibrillator

An AED automates many of the stages in performing defibrillation. The operator has only to recognise that cardiac arrest may have occurred and attach two adhesive electrodes to the patient's chest. These electrodes serve a dual function: recording the electrocardiogram and giving a shock if indicated. The process of interpreting the electrocardiogram is automatic, and if the sophisticated electronic algorithm in the device detects ventricular fibrillation (or certain types of ventricular tachycardia) the machine charges itself automatically to a predetermined level. Some models also display the heart rhythm on a monitor screen.

When fully charged, the device indicates to the operator that a shock should be given. Full instructions are provided by voice prompts and written instructions on a screen. Some models feature a simple 1-2-3 numerical scheme to indicate the next procedure required, and most illuminate the control that administers the shock. After the shock has been delivered, the defibrillator will analyse the electrocardiogram again; if ventricular fibrillation persists the process is repeated up to a maximum of three times in any one cycle.

Positioning of electrodes for automated external defibrillator

AEDs are programmed to deliver shocks in groups of three in accordance with current guidelines. If the third shock is unsuccessful, the machine will indicate that cardiopulmonary resuscitation should be performed for a period (usually one minute), after which the device will instruct rescuers to stand clear while it reanalyses the rhythm. If the arrhythmia persists, the machine will charge itself and indicate that a further shock is required.

Ventricular fibrillation

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Advantages of automated defibrillators

The simplicity of operation of the AED has greatly reduced training requirements and extended the range of people who are able to provide defibrillation. They can be used by all grades of ambulance staff (not just specially trained paramedics), and in the United Kingdom every emergency ambulance is now equipped with a defibrillator. Many other healthcare professionals are able to use AEDs, and in most acute hospital wards and many other departments defibrillation can be done by the staff present (usually nurses), well before the cardiac arrest team arrives.

It is almost impossible to deliver an inappropriate shock with an AED because the machine will allow the operator to activate the control only if an appropriate arrhythmia is detected. The operator, however, is still responsible for delivering the shock and for ensuring that everyone is clear of the patient and safe before the charge is delivered.

Public access defibrillation

Defibrillation by first aiders

Conditions for defibrillation are often optimal for only as little as 90 seconds after the onset of arrhythmia so any delays can be critical. Public access defibrillation has been introduced to minimise the delay before delivery of a countershock outside hospital, where members of the public usually witness the arrest. Under the scheme, lay people (often staff working in public places) are trained to use automated defibrillators. These individuals operate within a system that is under medical control but respond independently, usually on their own initiative, when someone collapses.

Early schemes to provide defibrillators in public places have reported favourable results. In the first year after their introduction at O'Hare airport, Chicago, several airline passengers who had a cardiac arrest were successfully resuscitated after defibrillation by staff at the airport. In Las Vegas, security staff at casinos resuscitated 105 patients in ventricular fibrillation, 56 (53%) of whom survived to be discharged from hospital. The closed circuit television surveillance at the casinos enabled rapid identification of potential patients, and 74% of those defibrillated within three minutes of collapsing survived.

Continuous electrocardiogram showing successful treatment of ventricular fibrillation by a countershock (given at the arrow)

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Other locations where trained lay people undertake defibrillation are in aircraft and ships, when a conventional response from the emergency services is impossible. In one report, the cabin crew of American Airlines successfully defibrillated all patients with ventricular fibrillation, and 40% survived to leave hospital.

In the United Kingdom, the remoteness of rural communities often prevents the ambulance service from responding quickly enough to a cardiac arrest or to the early stages of acute myocardial infarction. Increasingly, trained lay people (termed first responders) living locally and equipped with an AED are dispatched by ambulance control at the same time as the ambulance. They are able to reach the patient and provide initial treatment, including defibrillation if necessary, before the ambulance arrives. Other strategies used to decrease response times include equipping the police and fire services with AEDs.

Automated external defibrillator at a railway station

The provision of defibrillators in large shopping complexes, airports, railway stations, and leisure facilities was introduced as government policy in England in 1999 through the Defibrillators in Public Places initiative. The British Heart Foundation has supported the concept of public access defibrillation enthusiastically. It has provided many defibrillators for use by trained lay responders working in organised schemes under the supervision of the ambulance service.

As well as being used to treat patients who have collapsed, AEDs can be applied as a precautionary measure in people thought to be at risk of cardiac arrest—for example, in patients with chest pain. If cardiac arrest should subsequently occur, the rhythm will be analysed at the earliest opportunity, enabling defibrillation with the minimum delay.

Using an automated defibrillator

The first step is to ensure the safety of approach to the victim. If two rescuers are present one should go for help and to collect the AED while the other assesses the patient. If the patient has arrested it may be necessary for an assistant to perform basic life support while the equipment is prepared. If possible, screens should be used to provide some dignity for the patient if members of the public are present. People accompanying the casualty may also require support.

Algorithm for using automated external defibrillator (AED)

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Once the defibrillator is ready to use, the following sequence should be followed:

Switch on the machine and apply the electrodes—One electrode should be placed at the upper right sternal border directly below the right clavicle. The other should be placed lateral to the left nipple with the top margin of the pad about 7 cm below the axilla. The correct position is usually indicated on the electrode packet or shown in a diagram on the machine itself. It may be necessary to dry the chest if the patient has been sweating noticeably or shave hair from the chest in the area where the pads are applied. A sharp razor should be carried with the machine for this purpose.

Follow the voice prompts and visual directions—The electrocardiographic analysis is usually automatic, but some machines require activation by pressing an “analyse” button.

Administer shock if indicated—Ensure that no one is in contact with the patient and shout “stand clear.” Press the shock button once it is illuminated and the machine indicates it is ready to deliver the shock.

Repeat as directed for up to three shocks—Do not check for a pulse or other signs of a circulation between the three shocks because this will interfere with the machine's analysis of the patient's electrocardiographic trace. Most machines have motion sensors that can detect any interference by a rescuer and will advise no contact between shocks.

Give cardiopulmonary resuscitation as required—If no pulse or other sign of a circulation is found after three shocks, perform cardiopulmonary resuscitation for one minute. This will be timed by the machine, after which it will prompt the operator to reanalyse the rhythm. Alternatively, this procedure may start automatically, depending on the machine's individual features or settings. Shocks should be repeated as indicated by the machine.

If circulation returns, check for breathing—Continue to support the patient by rescue breathing if required. Check the patient every minute to ensure that signs of a circulation are still present. If the patient shows signs of recovery, place in the recovery position.

Liaise with emergency services when they arrive and provide full details of the actions undertaken.

Report the incident to the medical supervisor in charge of the defibrillation scheme so that data may be extracted from the machine. Ensure all supplies are replenished ready for the next use.

Roy Liddle is resusitation training officer at Wythenshaw Hospital, Manchester; Sian Davies is programme manager for the National Defibrillator Programme, London

The ABC of Resuscitation was edited by Michael Colquhoun, senior lecturer in prehospital care, Wales Heart Research Institute, University of Wales College of Medicine, Cardiff (colquhoun{at}bishopsfrome.u-net.comAnthony J Handley, senior research fellow, Prehospital Emergency Research Unit, University of Wales College of Medicine, and Tom Evans, consultant cardiologist, Royal Free Hospital, London

The diagram of the algorithm for the use of AEDs is adapted from Resuscitation Guidelines 2000, London: Resuscitation Council (UK), 2000.