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AEDs or Automated External Defibrillators are medical devices that can deliver a controlled electric shock to a patient who has gone into cardiac arrest. They can be either semi automatic, whereby the user has to push a button to activate the shock or fully automatic, where the machine generates the shock automatically.

AEDs save lives.

Once someone has gone into a cardiac arrest the AED or defib, as they are often known, is needed to enable the heart to restart.

To understand how they work you first need to understand the functions of the heart.

The Heart

The heart is a 4-chambered pump with its own electrical system which fires off electrical impulses from the primary pacemaker. These signals pass through the top chambers of the heart and make the chambers contract. Further signals then pass through the bottom two chambers and make them contract, thus forcing the blood in the heart, out around the body. The signals are coordinated so that the top two chambers contract first, followed by the bottom two.

The electrical cells in the heart then recharge ready to fire again. In a cardiac arrest, damage has been done to the heart muscle and this may interfere with the natural rhythm and process of conducting the electrical signals across the heart. The heart is unique in that every cell in the heart is capable of generating an impulse and in the event of damage occurring,  random electrical signals are generated. This  means that the heart does not contract as it should, but quivers instead and doesn’t pump the blood out. This quivering is known as Venticular Fibrillation and is a lethal rhythm if not treated.

How the AED works

The AED determines whether the patient’s heart is in a rhythm that is shockable (i.e one where there is still electrical activity occurring, albeit very abnormal activity). If it recognizes a shockable rhythm it will charge up and semi automatic machines will both audibly alarm and flash to indicate you should press the Red Flashing button in order to deliver the shock.  Fully automatic machines will alarm but there is no flashing button to push, the machine generates the shock automatically. In either case a controlled electrical shock is delivered across the heart which overrides the abnormal electrical signals and allows the heart’s own internal electrical system the opportunity to start working effectively again. It is  not a “jump start”, more a type of “re-booting” effect.

Once a shock has been delivered it is imperative that you recommence CPR immediately.  The machine will count down two minutes and will then re-analyse the patient’s heart rhythm to see if the shock has worked.

See flowchart (right) for details of the current UK guidelines on the use of CPR with AED.

Continue with CPR/AED until skilled help arrives or the patient wakes up and is breathing and moving normally.