Peter Houghton, I learned from his obituary this week, lived 7 1/2 years with an artificial heart, and is thus the longest surviving patient with this device. Love the story of how a thief tried to steal his bag with the battery pack, but got scared off by the alarm (built in to avoid accidental unplugging, not theft!).
They don't mention what he died of, though. Nor which factors limit the survival times.