Decompression and Air: Why we should keep them apart

[And]

Hi

Well we have a deco forum so I may as well use it. It seems all the rage.

Firstly, an intro into what happens when we go underwater. As you know air is made up of 20.9% or .209 Oxygen and 79.1% or .791 nitrogen there or thereabouts. As we go deeper these numbers or partial pressures increase. So at 10 mtrs or 2 bar its .418 and 1.582 (we show these as Po2 and PN2) as it is multiplied by 2. Travelling to 30 mtrs we multiply by 4 bar which is .836 Po2 and 3.16 pN2, you can see how the nitrogen part is growing quite quickly. The oxygen part we don't really worry about as it is a metaboliser (we do but its not relevant today) and our body uses it in other ways, but the inert gas, the nitrogen, is soaked into our body, we could call it saturation.

You will maybe have heard of fast tissues and slow tissues. We use these terms to describe how quickly the tissue in question is saturated with nitrogen. This is determined largely by bloodflow (perfusion) If a tissue has good bloodflow then it is said to be a fast tissue. The blood itself is a fast tissue, along with the major organs and central nervous system. Slow tissues have poor blood flow such as bones, skin and fat. These classifications are quite important in deco it is how they behave which determines how long we decompress and can effect our dive plan.

I mentioned saturation earlier. The nitrogen we breathe underwater is absorbed into our tissues. The faster tissues obviously absorb it faster than the slow ones. At the beginning of the dive all the fast tissues are absorbing that nitrogen like billio, whilst the slower tissues will absorb it slower, obviously. Depending on the depth (which has a bearing on the nitrogen number) and the time some of the slow tissues may not absorb any nitrogen at all, but if they do you have to deco longer. This is because a slow tissue also has poor diffusion, which is a term used to describe the rate at which the nitrogen (or any gas) leaves the tissues as you ascend. If you have stayed at depth long enough some of the slower tissues may have absorbed some nitrogen and now as you ascend you need to let these tissues diffuse the nitrogen to the blood stream and through the lungs. This is why when you play with your favourite deco software sometimes for just a few minutes more bottom time you have to do 2, 3, 4 or more times deco time.

:SIDE NOTE: This is important to understand because it has a bearing on your air supply. If you are not careful and have not stuck to your initial plan you may not have enough air to do the deco safely and may miss stops. If you haven't planned your dive at all and place your faith in the computer you may get a shock if you go into deco and it tells you you need to spend 15 minutes at 6 mtrs or something. So Plan the Dive and Dive the Plan.

I digress. So at this point you now know that the deco performance of your body is largely down to blood flow. A tissue with good perfusion has good diffusion. One point to note is that if we ascend faster than the rate of diffusion then bubbles are created in the tissue, which is DCI. We don't want that so we control our ascent according to the diffusion of the tissues. We can influence the rate of nitrogen perfusion and diffusion with different deco gasses but thats another topic. This is the same for every dive, so EVERY DIVE IS A DECO DIVE, even your 15 mtr red sea bimble. Now to get to the point of my post.

We saw that as we got deeper diving air the partial pressure of nitrogen grows quite significantly. Recent studies have shown that this increased pressure of nitrogen has an effect on our red blood cells. An effect called Red Blood Cell Rigidity, similar in fact to sickle cell. Blood flows through capillaries which are tubes running through our body. As the capillaries reach the tissues the tubes get very thin and the red blood cells need to squeeze through. If they are rigid it is harder for them to travel and this therefore reduces the perfusion of the tissue, which is a bad thing, remember, as it will also effect the diffusion, which we want to be as efficient as possible. As these blood cells are forced through the capillaries they can also cause microcirculatory damage, again hindering the deco process. All this damage then kicks in our immune system, which can cause flu like symptoms after a dive, although this will only be on significantly deep or long dives on air where the damage may be significant.

This is the reason not to dive deep on air, as well as narcosis which we often discuss. The absolute answer is to dive shallower for shorter obviously, but for many of us that is unacceptable and so we have to find a solution. What we do is we reduce the amount of nitrogen we are breathing, either by replacing it with oxygen on shallower dives (nitrox) or with helium on deeper dives (trimix). Smart people will also realise that a nitrox cert should be a mandatory qualification. It is not the increase in oxygen which is the benefit, it is the reduction in nitrogen. So our view is the only use for air is filling your tyres.


Andy

PS Disclaimer: I am no way pretending to be an expert or anything, so feel free to correct any errors.

[chrisch]

As you may have seen on Scubaboard air is not the best gas for tyres.

Second air and diving would have been better since it is only really for filling drysuits...

Chris

[Tim Ingmire]

Hi Andy, I'd be interested in the study references for the observations around RBC rigidity. As a man that has studied more haemotology than is really healthy I'd love to understand the mechanism for this.

[And]

Hi

No worries. I haven't read it, only articles referring to it. I am waiting for a full copy.

Taylor WF, Chen S, Barshtein
G, Hyde DE, Yedgar S. "Enhanced aggregability of human red
blood cells by diving." Undersea Hyper Med 1998;
25(3)167-170. This study concluded that increased ambient
pressure during diving increases the aggregability of red
blood cells, which could block microcirculation.

Interestingly, although the scientists performing the study
did not make the connection, the study’s results also
suggest that increased nitrogen pressure is the culprit for
the red cell aggregation, and that replacing nitrogen with
helium stops the progression of the aggregation.

The study found a "dramatic" increase of aggregability of
red blood cells from the surface down to 66 fsw. From 66
fsw to 300 fsw there was a "relatively small additional
effect". What appears significant here is that the
experimenters maintained a constant pressure of nitrogen
from 66 fsw on down by starting to add helium at this depth.
But, surprisingly, they did not correlate the change in red
blood cell aggregability with the fact that the pressure of
nitrogen was no longer being increased:

”If the increase in RBC aggregation is in part caused by an
increase in partial pressure of inert gas, one would expect
a proportional increase of RBC aggregation at 300 fsw. That
was not observed. Together with the in vitro experiment,
these results suggest that the elevation of RBC
aggregability is due to hydrostatic rather than to partial
gas pressure, but this remains to be confirmed.”

[wreckweasel]

Originally Posted by And

Hi

No worries. I haven't read it, only articles referring to it. I am waiting for a full copy.

Andy,

Ask me nicely... veeeeery nicely

I tried to attach it here, but this site won't let me. The interesting bit is in the conclusion with says "these results suggest that the elevation in RBC aggregability is due to hydrostatic rather than to gas pressure, but this remains to be confirmed"

basically, they found that clumping would occur dramatically in the 66ft compression range and significantly less so in the 300ft range. They also played around with gas fractional composition, which was found to have minimal impact.

Theres a whole lot of interesting references.... Im working on collating this stuff on a website, I'll try and get it up soon. (ooer!)

Having said that, I tend to agree... air is baaaad, its only saving grace is price/simplicity. I havent dived air in years.

[And]

Hi

Cool. Is there anything in there which is contrary to what I posted mate?

Pretty please

Andy

[Tim Ingmire]

Without seeing the study, was it in vivo or vitro? If there is an effect on RBC cell membrane surface tension then this might suggest that other cells would be similarly affected? What is the mechanism by which increased hydrostatic or N partial pressure exerts such an effect?

Interesting stuff and probably poses more questions than it answers.

[And]

In vitro Tim

Here is an intro to the study

Enhanced aggregability of human red blood cells by diving.

Taylor WF, Chen S, Barshtein G, Hyde DE, Yedgar S.

Diving and Environmental Physiology Department, Naval Research Medical Institute, Bethesda, Maryland 20889-5607, USA.

In vitro studies have shown that mild pressure increases red blood cell (RBC) aggregation. Enhanced RBC aggregation in pathologic states can drive the circulation into stasis. This investigation examined the effects of pressure on RBC aggregation in human volunteers. The hypothesis tested is that RBC aggregation is increased during hyperbaric exposure. Eleven subjects participated in dives to 300 feet of seawater (fsw) in a man-rated chamber complex. Blood samples were taken at the surface, at 66 fsw, and at 300 fsw. Data were analyzed with a repeated measures one-way analysis of variance for a complete randomized design. Statistical significance was achieved at P < 0.05. Data are expressed as mean +/- SEM. The median aggregate size (number of RBC/aggregate) of RBCs was significantly increased at depth. At a shear stress of 0.1 dyne/cm2, median aggregate size was 12.0 +/- 2.1, 33.0 +/- 7.3, and 48.8 +/- 10.8 at the surface, at 66 fsw, and at 300 fsw, respectively. These results show that mild pressure increases RBC aggregation in the human circulation.

Andy

[wreckweasel]

Both.. as far as I can see theres references to both.

I've sent a copy to andy, will also put a copy on our website tonight/tomoz and post a link

[Dr Stevil]

The reference is certainly interesting and I for one would like to see more research properly cited, but an "n" of eleven is too small to perform reliable statisical analyses, and p=0.05 while significant, doesn't exactly rock the boat, especially with in vitro work. There are numerous examples of in vitro results which are not seen with in vivo research, plus there are many factors, diet especially, which can affect blood flow and cellular effects.