Recent discoveries are changing the way doctors are looking at resuscitation:

If the patient doesn’t receive cardiopulmonary resuscitation within that time, and if his heart can’t be restarted soon thereafter, he is unlikely to recover. That dogma went unquestioned until researchers actually looked at oxygen-starved heart cells under a microscope. What they saw amazed them, according to Dr. Lance Becker, an authority on emergency medicine at the University of Pennsylvania. “After one hour,” he says, “we couldn’t see evidence the cells had died. We thought we’d done something wrong.” In fact, cells cut off from their blood supply died only hours later.

But if the cells are still alive, why can’t doctors revive someone who has been dead for an hour? Because once the cells have been without oxygen for more than five minutes, they die when their oxygen supply is resumed.