I admit, I'm just an idea guy and not an implementer. I know there have been many attempts at doing the human body, but I think just the heart by itself is an interesting subject and would make a good map.
Here's one example of how it could be done, just to start the subject out. I'm sure someone with more imagination and gameplay knowledge than me could ... er... flesh it out a bit and make it more interesting.
Latest version, 6/8/2010:
In the top diagram below, this map won't work because the red and blue sections do not connect at all. In the bottom diagram, the lungs are (clumsily) added in, which makes the map more playable but takes away somewhat from the "heart attack" theme.