From a Newsweek article:
Consider someone who has just died of a heart attack. His organs are intact, he hasn’t lost blood. All that’s happened is his heart has stopped beating—the definition of “clinical death”—and his brain has shut down to conserve oxygen. But what has actually died?
It seems that, starved of oxygen, the cells of the body don’t actually die for an hour, not the 5 minutes that was thought. It’s getting a supply of oxygen back that kills them. And the reason could be even more fascinating: the cells kill themselves, because the body’s anti-cancer mechanisms get confused. Fascinating stuff.