[Mal Bridgeman]

Originally Posted by principia

Can some explain the thinking or science behind the fast descents and C02 that people keep mentioning?

I tend to descend pretty quick and the last dive I did out of Brid I got really bad narcosis but put that down to the previous 40 dives being in warm clear water.

J.

I shall attempt a layman's description ... but if Mr Hobbs is reading I am sure he can do a much better job.

I think it's to do with increasing partial pressures of the gases acting upon tissues (synapses) in the brain and causing them to constrict and process info more slowly. The amount of constriction seems to vary with the speed at which you increase the partial pressure so the faster you descend the quicker the pp changes and the more befuddled you get. Adding Helium to the breathing mix, which is a very "thin" gas, has the effect of "relaxing "those tissues and reducing the narcotic effect.

CO2 is a much more dense gas than nitrogen so when the ppCO2 starts to become significant then it's impact on the tissues is much greater. Rather than causing a befuddled forgetful effect of nitrogen, though, it causes the fear dread and panic feelings described.

HTH
Mal