I wouldn't stress out too much about it at this point. I think once you start working with vents and get to apply the book knowledge, it will click for you.
I read a thousand times how when someone goes into a-fib, they lose 20% of cardiac output w/ loss of atrial kick, but until I actually witnessed this in the OR, with CO done before and immediately after pt going into a-fib, it was book knowledge, which of course is very important, but when you get to see it feel it, touch it, experience it, it is much more meaningful.
So, the first time you make vent changes and see a pt improve clinically and abg-wise, it will make a lasting impression. Well, it did for me. It also depends on what kind of a learner you are too.
Don't worry. No one will expect you to walk in the first day knowing all of this. Just the fact that you are worrying and want to do your best is a great sign!! You will be a great Critical Care nurse!!