Could have been negligence, but in all honesty, the doctor might have very well have been telling the truth, and there was nothing he/she could have done. The body does sometimes reject blood supplies and go into shock and shuts down.
Check the records, talk to an attorney if you want, but sometimes you just have to admit that things don't always go your way. In fact, sometimes it was an honest mistake by the doctor. Really, the guilt of having messed up will probably be enough if that is the case.
Malpractice is delicate and shouldn't be taken lightly or as revenge or just wanting someone to pay for the loss.