In real life, there's something called "anesthetic awareness" which apparently is a condition in which you are awake during surgery, but paralyzed. You can feel and hear everything, but you can't scream or move. This is the basis for this unrealistic movie that wastes a perfectly good plot by stretching it too far.
Short Attention Span Summary (SASS)
1. Clay Beresford (Hayden Christensen) is 22, rich, successful, and scared of his mommy
2. He's got a heart condition, which I'm guessing is made worse by the fact that he's secretly engaged to the beautiful Sam Lockwood (Jessica Alba)
3. There's more chemistry between Clay and his mother, than between Christensen and Alba
4. His name finally comes up on the heart waitlist, and he chooses his friend Dr. Harper (Terrence Howard) to perform the operation.
5. The Doc advises him to put his affairs in order, which he does
6. Mommy isn't pleased
7. Christensen displays less emotion than Darth Vader, even when he's not in surgery.
8. No prizes for guessing that a goof up leads to him experiencing anesthetic awareness
9. Movie moves from "science" to "ridiculous"
10. Plot with many twists unfolds, and these are quite good if you can ignore the ridiculous parts of the movie.
In summary, the ending of this movie has a few good moments, but these are largely eclipsed by an unbelievable plot, and some lackluster acting.