02 Saturation

[Maggie]

If someone was living at altitude for some time and therefore had an increased red cell volume would this need to be taken into account if they then descended to sea level and dived using Nitrox.
Maggie

[danreind]

Hi Maggie,

Practically speaking I suspect it isn't a concern, though I haven't crunched any numbers on this. The simplified version of the nitrox story is that oxygen is transported to tissues in both red blood cells and as dissolved gas. Usually the tissues use up the oxygen in the red blood cells, and then start to eat up most of the dissolved O2 as well. Because the oxygen partial pressure is "burned off", we really only need to worry about nitrogen in our decompression calculations.

But wait a minute, isn't the oxygen converted to carbon dioxide you ask? Well yes it is, but carbon dioxide is SO soluble (loosely speaking) that the metabolism doesn't really make a dent in the CO2 partial pressure. It's like using an eye dropper to fill a bucket. So functionally the oxygen really is "burned off" for decompression purposes.

But if the dissolved oxygen level is high enough (by going too deep on nitrox) then not enough oxygen is burned off and it's partial pressure becomes significant and we have to worry about it for decompression purposes. I believe this effect starts to occur at around 2 atm of oxygen, depending on such variables as blood flow and tissue metabolism. Because nitrox is limited to 1.2-1.6 atm of oxygen we never run into this effect and that's why the equivalent air depth concept works.

In the situation you describe, where more red blood cells are present, then the partial pressure where we have to factor oxygen into decompression calculations would be lower. The tissues get more oxygen from the red blood cells and don't have to dig as far into the dissolved oxygen. The real question is at what point the EAD idea breaks down. My suspicion is there is plenty of room between 2.0 atm (normal limit) and 1.2-1.6 (nitrox limit), and that this isn't something that needs to be worried about. I will hedge my bets though by saying it all depends on how much more hemoglobin high altitude adapted people carry, and this number I don't know.

In addition, even where oxygen contributes to bubble growth, it is also quickly burnt off upon reaching the surface, so these bubbles shrink quite quickly as well.

I hope that helps,

Dan Reinders

[Dr Deco]

Hello Maggie:

Another aspect of this is the increase in blood viscosity resulting from the increased number of red blood cells. I am not aware of any studies on this, but just as a decrease in an individual’s hydration status is considered a risk factor for DCS, so I might believe this would be true for an excess of red cells.

When one comes down to the tissue level, I do not know how this translates quantitatively into a DCS risk increase. Thus, my answer is more on the theoretical side unless someone had a reason for performing a laboratory study.