They call it resuscitation science. It's a new area of research at the University of Pennsylvania, where a Center for Resuscitation Science opened less than a year ago, and where the line between life and death is shifting.
Historically, doctors have defined clinical death as the point at which either the heart irreversibly stops beating or the brain shows no signs of activity according to Dr. Benjamin Abella, the center's clinical research director.
"But researchers … now believe there's a third state of being that hovers somewhere between life and death -- a place where most of the body's cells are still alive, but neither of these two classical signs of life are present," Dr. Abella said.
How can medicine bring patients back from that state without causing irreversible damage to the cells?
"It used to be thought that getting someone back as quickly as possible with CPR, defibrillation and warming … was the best approach," said Dr. Abella. "Where that may in part be true, many initial survivors from cardiac arrest go on to suffer severe debilitating brain injury and sometimes don't live to leave the hospital alive."
Restarting the heart while protecting the brain is where the key to successful resuscitation lies. It is dramatically demonstrated in the National Geographic Channel documentary "I Came Back from the Dead," airing Jan. 29.
Through a cooperative arrangement with the National Geographic Channel, ABC News has looked into two of the cases featured in the documentary, studying how the lessons learned from them have contributed to the knowledge of how people can be brought back from clinical death.
Ward Krenz, now of Sioux Falls, S.D., survived after being submerged for an hour in an icy lake.
"He certainly came back from the appearance of death from all the clinical criteria for death," said cardiovascular surgeon Dr. Dan Waters of Clear Lake, Iowa. "I don't think it's a big stretch to say he came back from the dead."
Today Krenz is active and healthy and works for a railway company in Sioux Falls. In 1993, he was presumed dead when his body was pulled from a frozen lake he fell into after his snowmobile careered onto an open patch of water in Clear Lake.
"That's when you are in panic mode," Krenz said. His companions heard him cry for help but were unable to reach him.
Krenz treaded water. "I believe it was for about five to 10 minutes," he said.
Then Krenz lost consciousness and slipped beneath the surface. Rescuers discovered his body, with only the helmet still floating above the surface, roughly an hour after the accident. His father was told his son had died.
"So on his way to the hospital," said Krenz, "for three hours he was planning my funeral."
"When [Krenz] came to the emergency room he looked like a cadaver," Waters said. "He was stiff, ice-blue, horribly cold to the touch, and he just looked like somebody who had been dead for a long time."
Krenz had flat-lined.
Nevertheless, an estimated two hours after he plunged through the hole in the ice, he was hooked up to a heart-lung machine. Doctors were able to restore a heartbeat, but the prospects, if he survived, were troubling.
"There was at least, I thought, a statistically significant chance we would not get the person he was before back," Waters said. "That he would suffer severe, irreversible brain damage, and still … persist in a vegetative state. So that was my biggest concern."