Near-death experiences and CO2

Behind the Headlines

Thursday April 8 2010

Is the light at the end of the tunnel just caused by gas?

“Near death? It’s a gas”, exclaimed a headline in the Daily Express. The newspaper said a study has shown that near-death experiences, “such as seeing life flash before one’s eyes” or “intense feelings of joy and peace”, may be linked to levels of carbon dioxide (CO2). According to the Daily Mail, the researchers believe that CO2 may alter the chemical balance of the brain and trick it into seeing lights, tunnels or dead people.

This news story is based on a small observational study of the near-death experiences of people who had survived a heart attack. The researchers found that these appeared to be linked to the concentration of CO2 in the patients’ exhaled air and the levels of CO2 and potassium in their blood.

This main weakness of this study is that its conclusions are based on the experiences of 11 people. As such, the results should be interpreted with caution. The implications of these findings are not clear and the results will need to be repeated in larger, more rigorous studies. For now, near-death experiences remain an unexplained phenomenon.


Where did the story come from?

The study was carried out by Dr Zalika Klemenc-Ketis and colleagues from the University of Maribor in Slovenia. It is unclear who funded the research. The study was published online in the peer-reviewed medical journal Critical Care.


What kind of research was this?

The authors say that near-death experiences are reported by up to 23% of people who survive cardiac arrest, but there is little explanation for the mechanisms behind them. In this study, they investigated the effects of various body chemicals and gases on the occurrence of near-death experiences.

The authors say that their study was a prospective observational study. They describe the experiences of 52 people who had a heart attack outside of a hospital setting and who were admitted to the intensive care units in one of three main hospitals between January 2008 and the end of June 2009. The researchers relied on data reported by patients about their experiences of a heart attack and on medical records kept during the resuscitation and early admission period. It is not possible to know at what time in relation to the near-death experience itself the measurements were taken (the study cannot prove causation).


What did the research involve?

To be eligible for inclusion in the study, participants had to be over 18 years old, defined as clinically dead when they were admitted to hospital (breathing and effective cardiac output had stopped) and have had low brain activity. They were approached during their hospital stay and asked to fill in a 16-item questionnaire about their near-death experience. The questionnaire appears to have been designed specifically for this study and investigated different aspects of the person’s near-death experience, including cognitive, emotional, paranormal and transcendental (unnatural) aspects. It assigned an overall score of the experience from 0 to 32. The researchers defined a near-death experience as having a score of seven or above.

Various other variables were also assessed, including age, sex, education, religion, previous near-death experiences and how fearful the participant had been of dying before and after the heart attack. Other relevant details were obtained from the patients’ files. These included time until resuscitation, time until return of circulation, what drugs were received, petCO2 (measure of CO2 in exhaled air) and the amount of CO2, O2 and sodium and potassium in blood samples taken in the first five minutes of admission.

The researchers used simple statistics to compare the various measures between the people classed as having had a near-death experience (a score over seven) and those who did not. They then did some regression modelling, adding in some of the other variables they measured, such as age and religion, to see whether these explained some of their findings.


What were the basic results?

Eleven of the 52 patients had a near-death experience. Patients with higher levels of CO2 in their exhaled air and in their blood had more near-death experiences. The levels of potassium and CO2 in blood were also linked with the score on the near-death experience scale.

There was no link between the occurrence of near-death experiences and gender, age, education, religion, fear of death, time to resuscitation, drugs used during resuscitation and the level of sodium in the blood. However, patients who had previous near-death experiences were more likely to have had them on this occasion.

The researchers then used regression modelling on their results. This involved putting all the significant variables into one model so that they could tell which of them independently (i.e. after adjusting for the others) predicted the number of near-death experiences or the score on the scale. The model showed that a higher level of petCO2 was an independent risk factor for the number of near-death experiences and for the score on the near-death experience scale. Levels of blood potassium and previous near-death experiences were also independently linked with the score on the scale.


How did the researchers interpret the results?

The researchers concluded that higher levels of CO2 in exhaled air and higher CO2 levels in arterial blood “proved to be important in the provoking of near-death experiences”. They say that higher levels of potassium in the blood may also be important.



This small observational study found an association between blood potassium and levels of CO2 (in exhaled air and blood) and the occurrence of near-death experiences. The researchers themselves highlight some weaknesses of their study and say that the results should be interpreted with care and that further research is needed.

The greatest limitation here is the sample size, as only 11 people in the total sample of 52 had a near-death experience. Any conclusions based on analyses of sample sizes this small should be interpreted with caution. Another important limitation is the fact that the study cannot establish the temporal relationship between raised CO2 and near-death experiences, so cannot provide conclusive evidence that it “causes” near-death experiences.

Analysis by Bazian

Edited by NHS Choices


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