Not believable in the slightest!
Like most movies this one had a whole TON of medical facts wrong. Biggest one was of course the whole 'awake during heart transplant surgery' thing.
In real life there is no way they would have gone that far with the operation without noticing that the patient was in severe pain. He would get a huge adrenalin rush. His heart rate would have gone up, as would his blood pressure and most likely they'd monitor brain activity with an EEG or other type of monitoring and there is just no way they would miss that he is wide awake. He'd most likely also be sweating heavily.
I know they wanted to make a movie where the main character was awake and in excruciating pain during surgery, I mean let's be honest, most of us watched this movie because that's what we heard it was going to be about. While I liked how the truth unfolded through his eyes while he was on the operation table... I kinda did feel that maybe this movie would be better if they hadn't included that part and just had us experience the surgery through a third person view, because the gross medical inaccuracy kinda makes the movie a bit ridiculous.
Don't get me wrong, I thought it was a decent movie despite it... it just seemed so silly to me. Especially since they made sure to get so many other medical facts right.
But then again, I guess if they didn't have that rather horrifying aspect of patient being awake during surgery, it would have been overlooked by most as just a run of the mill thriller about some guy who needs heart surgery.
I'd also like to add that why I never ever saw the twist coming... I still kinda felt it was a bit too far-fetched to believe his doctor and girlfriend had worked on this evil scheme for over a year involving getting him to fall in love with and marry her so he could kill him during surgery and then they could run off with his millions.
It was a bit over-the-top.
And what happens when he meets his mother in limbo? At first I thought maybe it was just his own hallucination/dreaming and finally facing up to the truth he had repressed for so many years, namely that his father was an abusive coke addict. But then how could he know his mother was already dead and donating his heart to him? How could his mom explain to him how she had figured out what the doctor and his wife had been plotting? I mean, she told him about stuff that had happened in the waiting room while he was in surgery. So they MUST have really met there in limbo, or whatever it was.
So suddenly we have a supernatural thing going on.
Weird.