Notes from Pim van Lommel,Consciousness Beyond Life: The Science of the Near-Death Experience(New York: HarperCollins, 2010), 119-121:

Oxygen deficiency is accompanied by an increase of carbon dioxide, and this increase has been suggested as a possible cause for near-death experiences. Patients breathing in unusual quantities of carbon dioxide have been known to experience a sense of separation from the body, and there have been occasional reports of a bright light, a tunnel, a sense of peace, and/or memory flashes. It should immediately be pointed out, though, that these memory images or flashes are quite rare, are extremely fragmented, and never involved either a life review or an encounter with deceased persons. Moreover, the sometimes dramatic life changes that have been extensively documented in connection with NDEs have not been reported in cases of carbon dioxide overload.

After a relatively technical discussion of medical resuscitations and the difficulties in measuring levels of oxygen and carbon dioxide during a frantic operating room emergency, Dr. van Lommel offers a simple summation:

The conclusion that a high concentration ofCO2 could be the cause of an NDE seems to be highly questionable, and at least very premature. (118)

But there are plenty of other hypotheses on offer. How about psychedelics such as LSD, DMT, psilocybin, and mescaline? Perhaps surprisingly, Dr. van Lommel is somewhat more friendly to this suggestion than he was to oxygen deprivation or even to carbon dioxide overload. The latter three of these substances can be found fairly abundantly in nature. Psilocybin and mescaline, particularly, occur in plants native to Latin America and in (magic) mushrooms and have been used in potions, powders, and inhalants for centuries to induce mind-expanding experiences. All of them are closely related to the neurotransmitter serotonin, which is easily found in the human body, and their chemical structure is related to tryptamine.

During times of major physical or psychological stress, the body activates large amounts of DMT, notably via the pineal gland. This is probably also true during the dying process, when the cells of the pineal gland are dying and, it is thought, releasing DMT.

The experience induced by psychoactive substances is often surprisingly similar to a near-death experience, especially in the case of DMT although, depending on the dosage, confusing or frightening perceptions may also occur. These substance-induced experiences include the following elements: a sense of detachment from the body, out-of-body experiences, lucid and accelerated thought, an encounter with a being of light, a sense of unconditional love, being in an unearthly environment, access to a profound wisdom, and wordless communication with immaterial beings. Sometimes the characteristic post-NDE transformation, including the loss of the fear of death, is also reported after administration of DMT or LSD.

It is a new and surprising hypothesis that DMT, which occurs naturally in the body, could play an important role in the experience of an enhanced consciousness during near-death experiences. Perhaps DMT, its release triggered or stimulated by events in our consciousness, lifts our bodys natural inhibitions against experiencing an enhanced consciousness, as if it is able to block or disrupt the interface between consciousness and our body (and brain). Mention should be made here of the fact that zinc is essential for the synthesis of serotonin and related substances such as DMT. At a more advanced age, the body has lower levels of this metal, and, as mentioned earlier, NDE reports are less common at an older age. (120-121)

I would point out, though, that attempts to reduce NDEs merely to subjective brain events caused by oxygen deficiency or DMT an option that Dr. van Lommel himself clearly does not embrace fail to account for what seem to be verifiable out-of-body experiences in which the experiencers witness events and observe people from a vantage point distinct from the location of their bodies.

See more here:

Are NDEs caused by carbon dioxide overload? And what about psychedelics? - Patheos