Bright lights, tunnels to the unknown and mysterious voices give near-death experiences an mystical air. The explanation may be something rather more mundane: plain old carbon dioxide. People who have near-death experiences during heart attacks tend to have higher levels of the molecule coursing through their veins.

“We cannot say that this is the only cause, but it partially explains the physical mechanism of near-death experiences,” says Zalika Klemenc-Ketiš at the University of Maribor in Slovenia. “I think higher levels of carbon dioxide cause hallucinations, and this can be part of near-death experiences.”

Hers isn’t the first paper to offer a physiological explanation for near-death experiences, which are reported by 10 to 20 per cent of people who survive a heart attack. Others have proposed lack of oxygen or hallucinogenic drugs as a trigger. A 2001 study in the Lancet, however,found none of these explanations adequate.

Klemenc-Ketiš’s team tracked down 52 people who had lived through cardiac arrests and persuaded them to allow blood taken right after the heart attack to be analysed. Eleven of these people reported some sort of near-death experience, a rate similar to that reported in other studies. These people tended to have significantly higher levels of dissolved CO 2 in their blood, as well as potassium.


Heightened perception

Other studies have linked high levels of CO 2 to visual hallucinations, Klemenc-Ketiš notes. People at high altitudes, where blood CO 2 spikes, sometimes have hallucinations such as bright lights that are similar to near-death experiences.

Other factors also play a role in near-death experiences, however, she says. Not everyone with high CO 2 levels had a near-death experience, and previous studies have found that demographic and psychological factors are also involved. For instance, younger people are more likely to report near-death experiences, as are the religiously devout, those afraid of dying and people who have already reported a near-death experience.

Journal reference: Critical Care, in press