An automated external defibrillator, or AED, is a portable medical device designed to analyse a person's cardiac rhythm during suspected cardiac arrest and, when appropriate, deliver an electrical shock intended to terminate certain life-threatening abnormal rhythms. AEDs are designed specifically so members of the public can use them with limited or no formal training. They contain adhesive electrode pads that detect the electrical activity of the heart and transmit it to the device.
The AED's software analyses the rhythm and determines whether a defibrillation shock is appropriate. It does not simply deliver a shock every time it is switched on. Current Resuscitation Council UK guidance states that anyone can use an AED and that modern devices are designed to guide the rescuer through the process.
Defibrillation is different from CPR. CPR provides mechanical circulation through chest compressions and, when appropriate, ventilation, whereas defibrillation uses a controlled electrical discharge to interrupt certain chaotic cardiac electrical rhythms. The classic shockable rhythms are ventricular fibrillation and pulseless ventricular tachycardia. In ventricular fibrillation, the heart's electrical activity is chaotic and the ventricles do not contract effectively. A defibrillation shock can temporarily stop the disorganised electrical activity, giving the heart's normal pacemaker and conduction system an opportunity to re-establish an organised rhythm. CPR remains essential before and after the shock because successful defibrillation does not guarantee immediate effective circulation.
An AED normally consists of the defibrillator unit, electrode pads, cables or integrated connectors, a power source and a control system that provides spoken or visual instructions. When the device is turned on, it tells the rescuer what to do, including attaching pads to the person's bare chest.
The pads are usually positioned so that electrical current passes through a substantial portion of the heart, commonly one pad below the right collarbone and the other on the left side of the chest. The exact placement shown on the pads or device should be followed. The chest should be sufficiently exposed and dry to permit good electrical contact, while excessive chest hair may occasionally interfere with adhesion or electrical transmission.
AEDs are called “automated” because the device analyses the rhythm and makes the determination about whether a shock is indicated rather than requiring the lay rescuer to interpret an ECG. Some AEDs are fully automatic and deliver the shock themselves after warning rescuers to stand clear, whereas semi-automatic models instruct the rescuer to press a shock button.
The device will not ordinarily permit a shock to be delivered when its algorithm determines that the rhythm is not one for which defibrillation is appropriate. This design substantially reduces the technical knowledge required of the user and makes public-access defibrillation feasible in places such as railway stations, airports, shopping centres, gyms and workplaces.
An AED is intended for cardiac arrest, not for a conscious person experiencing ordinary chest pain or other symptoms while maintaining normal circulation. The typical situation is an unresponsive person who is not breathing normally. CPR should begin immediately and should not be delayed while someone searches for an AED. If another person is available, that person should retrieve the device while CPR continues. When the AED arrives, it should be connected as soon as possible, but the rescuer should follow its prompts and stop touching the person whenever the device instructs them to do so for rhythm analysis or shock delivery.
AEDs are deliberately designed to be safe for public use. The device analyses the heart rhythm before deciding whether a shock is appropriate, and current guidance states that anyone can use one. The rescuer should nevertheless obey the safety instructions: nobody should touch the person while the AED is analysing the rhythm or delivering a shock. The pads must be attached correctly to bare skin, and water or other conductive contamination should be addressed according to the device instructions. If the person is lying in standing water or the environment itself is dangerous, safety must be considered before treatment.
The effectiveness of defibrillation is strongly time-dependent. In ventricular fibrillation or pulseless ventricular tachycardia, the chance of successful defibrillation generally decreases as time passes, which is why public-access AEDs are placed in locations where they can be reached rapidly. Resuscitation Council UK notes that early defibrillation, together with immediate CPR, is a key component of survival from cardiac arrest. AEDs therefore complement rather than replace CPR: chest compressions maintain some circulation while an AED is located and prepared, and the AED addresses the electrical abnormality when a shockable rhythm is present.
The broader significance of an AED is that it moves a sophisticated element of resuscitation from specialist hospital equipment into the community. Modern public-access defibrillation programmes are built around rapid recognition of cardiac arrest, immediate emergency activation, bystander CPR and early AED deployment. UK guidance recommends that public AEDs be clearly signposted, readily accessible and available continuously, with their locations registered so ambulance services can direct callers to them. The device does not diagnose the entire medical condition, replace ambulance care or guarantee survival, but it can provide the critical intervention of defibrillation within minutes when the cardiac rhythm is one that can respond to an electrical shock.
