Quote: (Originally Posted by
dteubner)
I would like someone to explain (in simple words) why they think there is a difference in off gassing rates at different depths with a FIO2 of 1.0.
I guess I should have a go, given I made the origional statement.
Decompression rate is driven by the degree of overpressurisation - increase the overpressurisation, and you increase the rate of inert gas removal (both as dissolved gas, and as bubbles if you are in to that sort of thing) - up to a point anyway. (where bubbles start forming in tissue and disrupting the process - apparently that hurts).
As a huge oversimplification, consider just the case of a dissolved inert gas moving across a semi permiable membrane from an imaginary tissue, into blood. Now we are breathing O2 at 10m, so there is no inert gas coming in from the lungs, however there is still inert gas dissolved in the blood due to the ongoing decompression process. In this simplified model we will consider the pressure of dissolved gas in the blood to be relative to absolute pressure, ie the lungs are perfectly efficient inert gas exchangers, the only reason there is dissolved gas in the blood is due to the time taken for it to circulate to the lungs to offgas.
Lets say that the (dissolved gas)pressure in the tissue is X, and the pressure in blood is Y. Overpressure gradient driving rate of decompresion is X:Y.
Now lets go instantly from 10m depth to the surface (this is just for easy maths), assuming we were breathing O2 at 10m and will continue breathing O2 at the surface.
In my simple model, the inert gas pressure in the tissues has not changed, it is still X. The pressure of dissolved gas in the blood (as it tracks absolute pressure) is now HALF Y.
Therefore the overpressure gradient is now X : Y/2 (ie it has doubled).
As rate of gas exchange is driven by this pressure ratio, it will increase.
Hence we will offgass faster at the surface than at 10m, breathing O2 at both points. Same theory for 6m v 4m, just not as big an effect.
Obviously a gross oversimplification, but I think the idea is sound. Fockie is supposed to be dropping in for a beer this afternoon, I'll see if I can convince him
Mike