As research is a key component for your CV, you have to get an early start in residency. The sooner you know you want to do cardiology, the better. You have to start right away in your intern year with research so you can get some abstracts/posters done prior to completing your 1st year. As you know, your applications go out in the middle of your second year, so not that much time to polish your CV and get things done.
What type of research? A double-blinded, randomized anything will be very difficult to do for most people, some might get in on something at big research intuitions, but most will not and should not try to get something like this started.
What I suggest is chart review. This allows some great options to look for trends at your hospital for what you are looking to investigate. Chart review provides you with a fairly larger n (can go back as long as you like), it gives you all the data in hand and if you put together a team, lots of manpower to look at all the data. Just look for simple and clean ideas for your research, only concentrating on 1 variable, this will allow you to come up with many ideas.
Good Luck.