Complex sentence: 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.
Split into simpler sentences: When carbogen is inhaled, the increase of carbon dioxide in the lungs causes a perception, both psychological and physiological, of suffocation. This is because the brain perceives an increase in carbon dioxide levels in the lungs as a decrease in oxygen levels.