In Italian, I could say Lo sai cosa è successo? ("Do you know what happened?") for introducing a new topic. I would not generally expect an answer, as it is just a way to let the other person know I am changing topic; in the case I see the other person is not paying attention, maybe because he is thinking to something else, or something else is distracting him, I would not continue until he doesn't reply back with Cosa? ("What?") or a similar phrase. If the person to whom I am speaking would be paying attention, and he notices I am not saying anything, he would ask Cosa?
Would "Do you know what happened?" be understood in the same way from English native speakers? What would happen if I say "Do you know what happened?" and I don't say anything else?
I am speaking about a face-to-face communication.