Carbogen
Carbogen, also called Meduna's Mixture after its inventor Ladislas J. Meduna, is a life-supporting mixture of carbon dioxide and oxygen gas.  Meduna's original formula was 30% CO2 and 70% oxygen, but carbogen can refer to any mixture of these two gases, from 1.5% to 50% CO2.

# Mechanism
When carbogen is inhaled, the increase of carbon dioxide in the lungs causes a perception, both psychological and physiological, of suffocation because the brain perceives an increase in carbon dioxide levels in the lungs as a decrease in oxygen levels. In nature, this is generally true, and has evolved as an evolutionary mechanism to prevent all mammals from drowning by failing to surface in water. Therefore, inhalation of carbogen causes the body to react as if it were not receiving sufficient oxygen. Breathing quickens and deepens, heart rate increases, and cells release alkaline buffering agents to remove carbonic acid from the bloodstream. Not surprisingly, the sensation of inhaling carbogen is highly unpleasant and is likely to cause panic, near-death experiences, and other discomfort.

# Psychotherapy
Carbogen was once used in psychology and psychedelic psychotherapy to determine how a patient would react to an altered state of consciousness or to a sensation of loss of control. Individuals who reacted especially negatively to carbogen were generally not administered other psychotheraputic drugs for fear of similar reactions. Meduna administered carbogen to his patients to induce abreaction, which, with proper preparation and administration, he found could help clients become free of their neuroses.  Carbogen users are said to have discovered unconscious contents of their mind, with the experience clearing away repressed material and freeing the subject for a smoother, more profound psychedelic experience.
One subject reported: 
"After the second breath came an onrush of color, first a predominant sheet of beautiful rosy-red, following which came successive sheets of brilliant color and design, some geometric, some fanciful and graceful .... Then the colors separated; my soul drawing apart from the physical being, was drawn upward seemingly to leave the earth and to go upward where it reached a greater Spirit with Whom there was a communion, producing a remarkable, new relaxation and deep security."
Carbogen is rarely used in therapy anymore, largely due to the decline in psychotherapeutics.

# Modern Uses
Carbogen is used in biology research to study in vivo oxygen and carbon dioxide flows.

# Notes
-  Prisman E, Slessarev M, Azami T, Nayot D, Milosevic M, and Fisher J. (2007). Modified oxygen mask to induce target levels of hyperoxia and hypercarbia during radiotherapy: a more effective alternative to carbogen.  International Journal of Radiation Biology.  Jul;83(7):457-62.
-  Jump up to: 3.0 3.1 3.2 Walsh, Roger N. (2005). Higher Wisdom: Eminent Elders Explore the Continuing Impact of Psychedelics. State University of New York Press. pp. 5758, 98. ISBN 0791465179. Unknown parameter |coauthor= ignored (help).mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
-  Markoff, John (2005). What the Dormouse Said. Penguin. pp. 25, 62. ISBN 0-670-03382-0.
-  Meduna, Ladislas J. (1950). Carbon Dioxide Therapy. Charles Thomas.
-  Arnold JF, Kotas M, Fidler F, Pracht ED, Flentje M, Jakob PM. (2007). Quantitative regional oxygen transfer imaging of the human lung. Journal of Magnetic Resonance Imaging. Aug 8;26(3):637-645
de:Carbogen
et:Meditsiiniline karbogeen