The automated external defibrillator

doi:10.1136/bmj.327.7425.1216

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Roy Liddle,
C Sian Davies,
Michael Colquhoun,
Anthony J Handley

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 …