My parents are GPs. They somewhat recently had an old man have a heartattack in their waiting room, and basically had to give him CPR + rescue breaths (via one of those squeeze bottle ventilators) for about 10 minutes before the ambulance arrived.

The hell of it was, it did work. The guy kept waking up from it while they were doing it, being very grumpy that people were pressing on his chest, and the moment they stopped he'd go unconscious again. Continued through the normal CPR and the ambulance's CPR apparently.

Unfortunately in the end he didn't make it (heart just would not restart fully).