The AED is designed to only shock if ventrical fibrillation is present. It will not shock any other heart rhythm. Nevertheless, AEDs are not perfect and in 1% of cases it may shock a rhythm other than VF. That’s why it should only be attached to a person who is unconscious and has collapsed. If the AED determines the heart is not in VF, it will not deliver a shock and will advise you to begin CPR.