I can't speak for experienced CVSICU nurses as I'm new to this specialty myself, but I can give some insight from the "newbie" perspective.
Questions. Ask them. A lot. The amount of knowledge these nurses possess is staggering. Be a sponge! You are in a unique position to learn without all the stress of extreme expectation.
If you don't know the answer, say you don't know. Trust me, your preceptor knows the answer and can smell BS from a mile away.
Study, Study, Study. When you think you're done studying... study some more. i make it a point to make a list throughout the day of things that are mentioned that I want to know more about. Then I look it up as soon as I get home.
If you have down time (which is rare, but may happen from time to time), and your preceptor doesn't mind (which they shouldn't), seek out interesting cases, new admits, transfers... anything that you haven't been exposed to or want to know more about.
You'll know almost right away if this is the specialty for you (or so I've heard; I couldn't imagine not absolutely loving it!). I could go on and on about how amazing this unit is, but in all honesty its not for everyone. Its fast paced, crazy intense, and you're expected to be on top of your game no matter what rolls out of the OR. You MUST have thick skin. You WILL feel dumb at times. You WILL screw up. Learn from it and move on. The gym, my family, and chocolate cake get me past the tough days.
All that said, I wouldn't trade it for the world!