An automated external defibrillator (AED) detects and treats cardiac arrest due to the cardiac arrhythmias, ventricular fibrillation (VF) and ventricular tachycardia (VT). Uncorrected, these arrhythmias rapidly lead to irreversible brain damage and death.

In ventricular fibrillation the electrical activity of the heart becomes chaotic preventing the ventricle from effectively pumping blood. In ventricular tachycardia the heart beats too fast to effectively pump blood. Frequently, ventricular tachycardia leads to ventricular fibrillation.

Defibrillation, by applying a shock to the entire heart muscle, uniformly clears the heart's electrical system, hopefully allowing it to resynchronise.

Unlike regular defibrillators, an automated external defibrillator automatically determines if a shock is indicated, and automatically selects and delivers the appropriate energy level. The user cannot override a "no shock" advisory by an AED. It is called external because the operator applies the electrode pads to the patient's chest, unlike internal defibrillators with electrodes surgically placed inside a patient's body.

Once the pads are attached to the patient, the machine diagnoses the heart rhythm and determines if a shock is needed to treat ventricular tachycardia or ventricular fibrillation. If the machine determines that a shock is necessary, it will charge in preparation to deliver the shock. When charged, the machine instructs the user to ensure no one is touching the patient and then to press a button to actually deliver the shock.

After the shock is delivered, the machine monitors the patient's rhythm to determine if further shocks are necessary.

Unlike some more sophisticated defibrillators used by health professionals, if the heart rate is too slow, these devices do not typically pace the heart to make it beat faster.

There are two main types of AEDs on the market today: semi-automatic and fully-automatic. Semi-Automatic AEDs prompt the user to stand clear and then push a shock button to defibrillate. Fully-automatic units sound a 'stand clear' alarm and then deliver the shock automatically without the user having to push the button.

All AEDs approved for use in the US use a synthesized voice to prompt users through each step. Most units today are designed for use by non medical operators. Their ease of use has given rise to the notion of public access defibrillation, which experts agree has the potential to be the single greatest advance in the treatment of out-of-hospital cardiac arrest since the invention of CPR[1] http://circ.ahajournals.org/cgi/content/full/102/suppl_1/I-1?ijkey=0ea84b1fa73ef
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The use of AEDs is taught in basic life support (BLS) classes.