Scuba Geek

Scuba Diving Air and its Effects on Decompression December 25, 2005

Posted by Andy Carroll in : Decompression , trackback

Firstly, an intro into what happens when we go scuba diving 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. Scuba diving 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 scuba diving deco as it is how they behave which determines how long we decompress and can effect our scubadive plan.

I mentioned saturation earlier. The nitrogen we breathe whilst scuba diving underwater is absorbed into our tissues. The faster tissues obviously absorb it faster than the slow ones. At the beginning of the scuba 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 scuba diving 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 scuba dive plan you may not have enough air to do the deco safely and may miss stops. If you haven’t planned your scubadive 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 scuba dive, so EVERY SCUBADIVE 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 scubadive, 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 scubadive deep on air, as well as narcosis which we often discuss. The absolute answer is to scuba 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 scubadives (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. You can download the Enhanced Aggregability of Human Red Blood Cells by Diving paper here