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James Owen - for National Geographic News - Published April 8, 2010

Many people who have recovered from life-threatening injuries have said they experienced their lives flashing before their eyes, saw bright lights, left their bodies, or encountered angels or dead loved ones.  In the new study, researchers investigated whether different levels of oxygen and carbon dioxide—the main blood gases—play a role in the mysterious phenomenon. 

The team studied 52 heart attack patients who had been admitted to three major hospitals and were eventually resuscitated. Eleven of the patients reported near-death experiences.  During cardiac arrest and resuscitation, blood gases such as CO2 rise or fall because of the lack of circulation and breathing.

"We found that in those patients who experienced the phenomenon, blood carbon-dioxide levels were significantly higher than in those who did not," said team member Zalika Klemenc-Ketis, of the University of Maribor in Slovenia.

Other factors, such a patient's sex, age, or religious beliefs—or the time it took to revive them—had no bearing on whether the patients reported near-death experiences.

The drugs used during initial treatment—a suggested explanation for near-death experiences after heart attacks—also didn't seem to correlate with the sensations, according to the study authors.

Christopher French, a psychologist at the Anomalistic Psychology Research Unit of the University of London, who was not involved in the new research said that “The hospital study bolsters previous lab work done in the 1950s that found the effects of hypercarbia [abnormally high levels of CO2 in the blood] were very similar to what we would now recognise as NDEs,".

The research also supports the argument that anything that disinhibits the brain—damages the brain's ability to manage impulses—can produce near-death sensations, he said. Physical brain injury, drugs, and delirium have all been associated with a disinhibited state, and CO2 overload is another potential trigger.