The thing that stuck out for me the most on my last first aid course was that you don't perform CPR expecting someone to revive right then and there, you are merely adding oxygen into their lungs and manually pumping blood to spread that oxygen around to prevent brain death until they can be taken to a hospital and revived. The story they told us was of a firefighter who didn't have a proper seal on his mask and collapsed while fighting a fire. His friends carried him out and performed CPR on him for about 45 mins while they drove him to a hospital, when they got there the doctors didn't want to try anything as they were sure they he was dead if CPR didn't wake him for 45 mins. His friends insisted that the doctors try while never stopping the CPR until the doctors made the attempt, which worked.