Near-death experiences explained by carbon dioxide? [or: How journalism works]

Has a simple rational explanation for near-death experiences finally been found?

First there came a Google news announcement:

CO2 may explain ‘near-death experience’: study (AFP)

Then other news outlets repeated the story, essentially word-for-word but with increasingly emphatic headlines.

‘Near-death experience’ explained by carbon dioxide (Times of India)

Is that paradise beckoning, or just CO2 in your blood? (New Scientist)

Life flashing before your eyes? It’s just raised CO2 levels (Independent)

Near death experiences ‘trick of the mind caused by high levels of CO2 in the blood’ (Telegraph)

The phrase ‘trick of the mind’ seemed to catch on as it was then used by many others repeating the same story. My personal favourite, though was probably

Near-death experiences may be gas-fired (The Australian)

It was fascinating to watch this story spread around the globe, even returning to the Times of India which repeated its own story with a brand new headline:

Revealed: Truth behind near death experience

Given all this coverage, we could be forgiven for believing that an actual explanation has indeed been found. As a scientist, however, I decided to do something which none of the above writers had apparently bothered to do … and read the actual study. It turns out that the reality is nowhere close to what the headline-writers imagine.

20.04.10: The LiveScience website has belatedly jumped on the bandwagon with a bizarre twist in its headline: ‘Near death experiences linked to oxygen deprivation‘. The study found no such link. Ironically, this take on the story comes from LiveScience‘s ‘Bad Science’ columnist.

The case for carbon dioxide: Slovenian study

Sceptical doctors and scientists have long sought in vain for a rational (non-mystical) explanation for near-death experiences (NDEs). A standard medical-scientific explanation for NDEs would be that they are meaningless hallucinations triggered by the physical condition of the dying brain, then misconstrued by lay folk (non-doctors, non-scientists) as having religious significance.

To date no evidence for such an explanation has ever been found. For while the NDEs themselves are impressively consistent, the physical, psychological and cultural circumstances of those experiencing them vary greatly and show no consistent pattern.

This week, however, Critical Care, a peer-reviewed open-access journal, published a report by a team of researchers in Slovenia which seemed to hold the answer. Their paper is entitled

The effect of carbon dioxide on near-death experiences in out-of-hospital cardiac arrest survivors: a prospective observational study.

Near-death experiences (NDEs) are reported by 11-23% of cardiac arrest survivors whose hearts have stopped for significant intervals. We only know of the existence of such experiences because, increasingly, modern resuscitation techniques are able to bring some heart attack victims back to life.

Zalika Klemenc-Ketiš, MD, and her colleagues at the University of Maribor, contacted 52 hospital patients in Slovenia and asked them to complete a questionnaire about their experiences. All of them were cardiac arrest survivors who had been successfully resuscitated outside of hospital and then admitted to an intensive care unit during the period from January 2008 to June 2009.

The 16-item questionnaire asked the patients about any mental, emotional, paranormal, and transcendental experiences which they could remember. A total score of 7 or more qualified as a near-death experience. Those patients who scored 7 or more were assigned to the NDE group, others were assigned to the non-NDE group.

After questionnaires were completed, the NDE group consisted of 11 patients (21%) — 10 male, 1 female — with an average NDE score of 11.5. The non-NDE group consisted of 41 patients (79%) with an average NDE score of 0.9.

The patients also provided information on their demographics, education level, religious belief, fear of death, and any previous near-death experiences. In line with earlier studies, the researchers found no correlation between near-death experiences and any of the patient characteristics.

Interestingly, of the NDE group, 3 were Catholics and 1 was Muslim, but the remaining 7 were all atheists. So much for religious wishful-thinking.

The researchers compared all of the above with clinical and lab data on the patients which had been obtained within five minutes of their emergency admission to hospital. These included blood serum levels of oxygen, carbon dioxide (CO2), sodium and potassium.

After analysing the data, there was found to be a significant correlation between NDEs and two measures of CO2: the serum partial pressure of carbon dioxide (pCO2) and the partial pressure of end-tidal carbon dioxide (petCO2). In other words, those patients in the NDE group tended to have, on average, significantly higher carbon dioxide levels in the blood than the other patients. “We can conclude,” says the report, “that CO2 might be one of the major factors for provoking NDEs.”

Graph showing serum CO2 pressure levels for the NDE and non-NDE groups.

So that’s the finding. And if this finding were to prove valid and reliable, it could be the first time that any physical factor has been definitely associated with NDEs. But that’s actually a very big IF. Because as yet, we have no idea how reliable this finding is.

For one thing, the sample size of this study was pretty small. There were only 11 in the NDE group. So the finding will need to be repeated at least once, preferably using much larger samples, before it can be considered to be reliable. A single result from a small sample can never count as proof of anything.

For another thing, the results contradict those of earlier NDE studies.

In an email to MedPage Today, Bruce Greyson, MD, a psychiatry professor and NDE researcher based at the University of Virginia (and also the creator of the original 16-item questionnaire adopted for this study), referred to previous studies looking at blood CO2 levels in relation to NDEs. In contrast to the Slovenian findings, one earlier study found an association between NDEs and below-average carbon dioxide levels, whereas another failed to detect any significant difference in patients with versus without near-death experiences.

All of which simply highlights the lack of any reliable association in the evidence to date. If CO2 were the cause of NDEs, why did it not show up in all the previous studies?

Greyson has also criticised the team’s conclusion that CO2 plays a role in “provoking” near-death experiences, pointing out that correlation does not prove causality. (Shame on them for such a schoolboy error.) According to Greyson, high blood CO2 levels are associated with better cardiac output and coronary artery perfusion pressure, which, in turn, would help stave off the amnesia that often occurs in cardiac arrest. “Thus,” says Greyson, “the association between NDEs and higher carbon dioxide levels may indicate simply that patients who are able to recall more of their cardiac arrest also report more NDEs.”

In other words, a high level of carbon dioxide could simply enhance the memory of having had an NDE, rather than “provoke” an NDE in the first place.

And even if it were eventually established that there is a consistent association between raised CO2 and NDEs — this is still far from providing an explanation. The researchers themselves admit that the hallucinatory experiences that are typically induced by physiological factors such as CO2 tend to be fragmented, random and confusing — unlike NDEs which are clear, highly structured, deeply meaningful and easily recalled.

And finally, I would also make the following observation. The NDEs of these patients all occurred before hospital admission — either before or after an ambulance had been called. The blood CO2 measures, in contrast, were taken some time later, during or shortly after hospital admission — by which time the patient had already been resuscitated and their NDE was, presumably, over.

So what is the blood CO2 measure in this study actually telling us? That there could be a pattern of raised CO2 some time after having had an NDE.

On the strength of this study we could just as legitimately (and just as unreliably) conclude that having an NDE causes a subsequent increase in the level of CO2 in the blood, rather than the other way around!

Bad news for patients

It is possible that CO2 plays a role in NDEs, or in the memory of NDEs, or in the recovery from NDEs, or that it plays no role at all if this result was simply a glitch from a small sample. It is far too early to say on the basis of such flimsy evidence.

But, rather typically, a number of headline-writers have made the leap from tentative correlation to proven causation, announcing that NDEs have now been explained.

The experiences which near-death survivors report — which include floating out of their bodies, moving toward a bright mystical light, meeting the spirits of deceased friends and relatives, and feeling extreme joy and peace beyond anything known on Earth — are often described as the most joyful and most meaningful experiences of their entire lives. For many, they are conclusive proof of the existence of eternal life and a spiritual basis to reality.

The bad thing about all this misreporting is that for the next few months or even years, anyone who has a near death experience will be told by their family, friends and medical staff alike, “Oh, it was nothing but a trick of the mind caused by carbon dioxide. It’s been proven, you know. It was all over the news.”



Klemenc-Ketis, Z., Kersnik, J. & Grmec, S. (2010). The effect of carbon dioxide on near-death experiences in out-of-hospital cardiac arrest survivors: a prospective observational study. Critical Care, 14(R56). doi:10.1186/cc89doi:10.1186/cc8952. Open access provisional PDF copy.

See also

Physician says near-death experiences are evidence for life after death.Personality & Spirituality

No medical explanation for near death experiencesNew Scientist

For more of my posts about near-death experiences, here’s an index:

2 thoughts on “Near-death experiences explained by carbon dioxide? [or: How journalism works]

  1. I just found your incredible blog.

    Thanks for this post. Since first reading about the supposed causality between NDEs and CO2, I’d been thinking that no such causality can be established merely because of an apparent association.

    The fact that so many people are quick to claim causality shows that we are not only eager to explain the inexplicable but that we are willing to do so without critical analysis of the “explanation”.

    This is a topic for which few of us can hold the tension of the opposites.

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