An Automated or Automatic External Defibrillator (AED) is a small, portable easy to operate lifesaving medical device designed to be used by the first responder on scene to a medical emergency. A first responder may be a lay person with minimal AED training.

Time to defibrillation, the most critical factor in sudden cardiac arrest (SCA) survival, can be reduced if an AED is "on-site" and can be brought to the victim quickly. This is one of the reasons that survival rates improve in communities with an active AED program, remember, every minute that passes before defibrillation reduces survival rates by 7-10 percent.

The 'Automatic' part of the title refers to the unit's ability to autonomously analyse the patient's heart rhythm and to assist this, the AED has clear spoken prompts, to instruct the user.

An AED is called ‘External’ because the operator applies the electrode pads to the bare chest of the victim.

‘Defibrillation’ is the definitive treatment for the life-threatening cardiac arrhythmias. Defibrillation consists of delivering a therapeutic dose of electrical current to the affected heart. This depolarizes a critical mass of the heart muscle, terminates the arrhythmia, and allows normal rhythm to be reestablished by the body's natural pacemaker in the heart.

How does an AED work?

When turned on, or opened, the AED will instruct the user to connect the electrodes (pads) to the patients bare chest. Once the pads are attached to the patient, everyone should avoid touching the victim so as to avoid false readings by the unit. The pads allow the AED to examine the electrical output from the heart and determines if the patient is in a viable, shockable rhythm. If the device determines that a shock is viable, it will use the battery to charge it's internal capacitor in preparation to deliver the shock. This system is very safe because it will only charge when it determines a shockable rhythm is present.

When charged, the device instructs the user to ensure no one is touching the victim and then to press a button to deliver the shock, or in the case of a fully automatic AED the unit will advise the user that it will deliver the shock without further intervention. After the shock is delivered, the device will instruct the user to commence CPR (Cardio Pulmonary Resuscitation) for one minute at which time it will analyse the victims heart rhythm once again advising a further shock, further CPR or care of the patient depending on the outcome of the previous shock.