An aggravated assault may become a murder when the victims dies. But exactly when that is may be debatable. If the doctors were wrong and turned off the respirator, was the victim killed by the perp or the doctors?
Amy Dockser Marcus has this article in the WSJ on the current controversy over "brain death."
I think this is just a subset of the doctor negligence defense when a victim dies of arguable malpractice when criminally-inflicted wounds are treated.
Generally, there is no intervening cause, and the criminal act remains the probable cause.