The phenomenon known as "near-death experience" could be caused by raised levels of carbon dioxide, researchers suggest.
People who say they have experienced the phenomenon describe sensations such as their life flashing before their eyes, feelings of peace and joy, and supernatural encounters.
Scientists in Slovenia studied 52 patients who had suffered cardiac arrest, using questionnaires to determine if they had had near-death experiences (NDEs). They also looked at data including carbon dioxide levels.
According to the study, published in Critical Care, 11 patients reported NDEs, with more incidences among those with a higher concentration of carbon dioxide in the breath and arteries.
It concluded: "As much as one fifth of out-of-hospital cardiac arrest patients report NDEs during cardiac arrest. Higher initial petCO2 [carbon dioxide in air exhaled from the lungs] and higher arterial blood pCO2 [the pressure of dissolved carbon dioxide in the blood] proved to be important in the provoking of NDEs.
"Higher serum levels of K [potassium] might also be important. Since these associations have not been reported before, our study adds new and important information to the field of NDEs phenomena."
Zalika Klemenc-Ketis, who worked with a team of researchers from the University of Maribor, said: "Several theories explaining the mechanisms of NDEs exist. We found that in those patients who experienced the phenomenon, blood carbon dioxide levels were significantly higher than in those who did not.
"The association with carbon dioxide has never been reported before, and deserves further study," she added.