AEDs are designed to treat two shockable rhythms: ventricular fibrillation (VF) and ventricular tachycardia (VT). (As a side note, you can have a pulse with VT.)
CPR is designed to provide effective circulation, whether the heart has stopped entirely (asystole), or is not providing effective circulation. You can even have in-betweens, like PEA (pulseless electrical activity) where the heart is providing the electrical signals that should contract the cardiac muscle, but it's just ... not happening, or not happening strongly enough to provide cardiac output.
CPR is really: the heart is not doing what it should, or it's not doing enough of it, or not doing it effectively enough.
In the case of an entirely stopped heart, CPR can provide enough sustenance to get a shockable rhythm that can then be defibrillated.
AEDs are also different from what EMS will use (at the ALS/paramedic level)—we can defibrillate or shock rhythms other than VF/VT, but the rules are different, as are things like pacing, etc.
As a VERY rough guide, a lot of arrests are a devolution, from NSR (normal sinus), i.e. all good, which can become VT, and some time in VT (typically minutes) before degenerating into VF, which is even less effective at cardiac output, and then subsequently into asystole or PEA.
AEDs are designed to treat two shockable rhythms: ventricular fibrillation (VF) and ventricular tachycardia (VT). (As a side note, you can have a pulse with VT.)
CPR is designed to provide effective circulation, whether the heart has stopped entirely (asystole), or is not providing effective circulation. You can even have in-betweens, like PEA (pulseless electrical activity) where the heart is providing the electrical signals that should contract the cardiac muscle, but it's just ... not happening, or not happening strongly enough to provide cardiac output.
CPR is really: the heart is not doing what it should, or it's not doing enough of it, or not doing it effectively enough.
In the case of an entirely stopped heart, CPR can provide enough sustenance to get a shockable rhythm that can then be defibrillated.
AEDs are also different from what EMS will use (at the ALS/paramedic level)—we can defibrillate or shock rhythms other than VF/VT, but the rules are different, as are things like pacing, etc.
As a VERY rough guide, a lot of arrests are a devolution, from NSR (normal sinus), i.e. all good, which can become VT, and some time in VT (typically minutes) before degenerating into VF, which is even less effective at cardiac output, and then subsequently into asystole or PEA.