Irina - prep according to Excelsior rules for CPNE (and I mean literally to the dot!) and you'll be fine. You knowledge from med school will serve you well - mine did :)
I am also using 18th edition - and I don't think it's a big deal, since they are going to be releasing the changes summary of all changes from 18 to 19. Not that many changes. SO I will continue studying from my printed out 18th edition plus apply changes from 19th.