The AED is for use after the pulse stops. It should be used right when it is available at the scene and CPR should immediately be stopped for the use. The AED is self explanatory and has a voice to guide you if you get stuck.
In this instructable I will just cover how to basically use it and some other facts. This instructable should be used in conjunction with the CPR instructable.
An AED is for when a persons heart starts an irregular beating pattern that doesn't let the blood circulate properly. The two ways the heart acts up are by V-tach and V-fib.
V-tach is when the ventricles pump so fast the blood cannot circulate properly.
V-fib is when the ventricles quiver or shake in an unorganized way so the blood cannot circulate properly.
The AED puts a shock through the heart so it stops and restarts so the heart can re-set and pump normally again. It cannot be used for someone who's heart has stopped completely.
Both V-tach and V-fib will show no pulse because the blood cannot circulate properly so you won't be able to tell if it's V-tach V-fib or stopped completely. The AED however can tell so it will be able to do the right amount of shock.