How it works
When the heart is beating correctly, an electrical and chemical impulse is sent from the top of the heart to the bottom. The impulse travels the same path every time.
Cardiac arrest, or cardiopulmonary arrest, occurs when the heart stops beating for any reason. One reason can be the failure of the electrical system in the heart to conduct impulses along the correct path. Random impulses firing all over the heart will cause it to shiver uselessly, not producing a pulse.
When the AED shocks the heart it causes the heart to stop momentarily. While the heart pauses, the small piece of tissue responsible for the next impulse (the sinus node or sinoatrial node) should fire off the next heart beat.
One thing to remember about AED's: they only work on two specific scenarios. Only those two issues will benefit from an AED, and that's what the machine is set to recognize. It will not save every patient in cardiac arrest. Indeed, AED's often don't help the patients they shock.
Make sure during the AED portion of your CPR class that the instructor explains the process well enough. There is an assumption - often by instructors themselves - that AED's are foolproof. Nothing could be further from the truth.
Using an AED correctly requires the user to listen carefully to the instructions coming from the machine. Pay particular attention to this part of the class; AED's save lives when they are used appropriately.