Are there authorities who hold that a stopped heart constitutes halachic death, but yet permit heart transplants? Stopping and taking out the recipient's heart would seem to be technically considered as causing halachic death (i.e. murder). The fact that it ultimately leads to healing or prolonging the life of the recipient (at time T2) should seemingly have no bearing on the prohibition of murder (which occurs at time T1 < T2).
[While there's a precept of violating Shabbos for someone so that he can keep more of them in the future, an analogous "kill someone so that he can live longer" principle doesn't seem to exist.]
So according to these authorities (if there are any), what is their halachic reasoning to permit stopping a recipient's heart?