NEW YORK — The line between life and death is not as clear as once thought, now that developments in the science of resuscitation have made it possible to revive people even hours after their heart has stopped beating and they are declared dead, medical experts say.

"Historically, when a person's heart stopped and they stopped breathing, for all intents and purposes, they were dead," said Dr. Sam Parnia, an assistant professor of critical care medicine at State University of New York at Stony Brook. "There was nothing you could do to change that," Parnia told an audience at the New York Academy of Sciences last week.

However, in the process of unraveling mysteries of death at the cellular level, scientists have learned that death does not occur in a single moment, but instead is a process. It is actually after a person has died -- by our current definition of death -- that the cells of the body start their own process of dying. [After Death: 8 Burial Alternatives That Are Going Mainstream]

This process "could take hours of time, and we could potentially reverse that," Parnia said.

The death process

It was once thought that after the heart stops pumping blood throughout the body, a person has only few minutes before suffering permanent brain damage caused by lack of oxygen and nutrients getting to the brain cells. This notion, scientists now say, is outdated.

When the heart stops beating, the process of death is only beginning, said Dr. Stephan Mayer, a professor of neurology at Columbia University and a panelist at the discussion.

Brain damage from lack of oxygen to the brain comes in stages. Within seconds, brain activity is affected, but it isn't until several minutes later that sugar-deprived cells start going through the steps of programmed cell death.

"When somebody's been without oxygen, we know there’s a whole bunch of signals that are now starting to tell cells that it's time to die. So we have an opportunity to modify that programing just a little bit, to say 'wait put the brakes on,'" said panelist Dr. Lance Becker, a professor of emergency medicine at the University of Pennsylvania.

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Some insights for how to halt the dying process come from case reports of people who were brought back to life with little or no brain damage after hours of a silenced brain and heart.

The key to these successful cases, in addition to good critical care, is hypothermia, experts say. Hypothermia is a state in which the body's core temperature is brought a few degrees lower than its normal temperature of 98.6 degrees Fahrenheit (37 degrees Celsius).

How long can one remain pulseless?

Studies have found that hypothermia seems to protect the brain by decreasing its need for oxygen and aborting activated cell death pathways. Still, there are limits -- although body-cooling techniques have improved recovery in many patients after cardiac arrest, there will be a moment when the damage is too much and it's too late to come back, the experts said.

Moreover, scientists have learned that successful recovery depends on how the patient is treated after the heart is restarted and how the body is warmed after hypothermia.

"What we are learning is counterintuitive, because what we were all taught, if somebody's oxygen is low, I should give them oxygen, if their blood pressure is down, I should crank their blood pressure up," Becker said. [The Science of Death: 10 Tales from the Crypt & Beyond]

In reality, however, if a patient responds to initial care and his heart is restarted, a sudden rush of blood and too much oxygen to the brain could actually worsen the neurological damage. Instead, moderating the amount of oxygen delivered to the brain may be crucial in resuscitation.

A state-of-the-art resuscitation

The idea of cooling the body after cardiac arrest has been around for a few decades, but scientists were not certain if it truly was beneficial to patients.

In recent years, however, studies have provided evidence that hypothermia improves patient's survival and recovery, and professional societies such as the American Heart Association recommend considering hypothermia after patient's blood circulation is restored.

Nevertheless, not all hospitals have implemented hypothermia as part of their critical care protocol.

"What is sad is that this knowledge out there, the system is available but is not implemented," Parnia said. Less than 10 percent of people in the United States who might benefit from cooling therapy actually receive it, he said.

In an ideal world, resuscitation protocols would use machines instead of people to deliver chest compressions as long as needed, and to ensure right amounts of oxygen and blood are getting to the brain, Parnia said. Cooling and reducing oxygen after the heart is restarted are among factors that should increase people chances of coming back without brain damage, he said.

New ethical questions with a new concept of death

The conventional wisdom in medical practice is to not revive a patient who has suffered extensive brain damage and would only survive in an unending coma. Attempting to bring back a patient hours after cardiac arrest may even pose higher risks of brain injury, raising an ethical question for those who support a more comprehensive resuscitation protocol.

However, Mayer argued that our knowledge of brain damage and dying is incomplete, and it's not always clear how much injury one has endured, and whether it's reversible.

"What we've come to learn is that those notions of irreversibility of brain damage are dead wrong," Mayer said. "If you make those judgments too soon without going fully all the way, you may be actually writing people off."

Becker said while extending life artificially may not be appropriate in every case, doctors should apply all available methods if they decide to resuscitate.

"If we are going to do anything, I don't know why we do less than everything we can to save a person. So the question is, why would you want to save a person half-way?" Becker said.

Email Bahar Gholipour or follow her @alterwired. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.

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