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Acclimatization and DCS
By Dr. Sawatzky
Does repetitive diving increase your risk of DCS? Do multiple days of consecutive diving increase your risk of DCS, or reduce the risk as you 'acclimatize'? The answers to these questions are not simple. We do not completely understand what is going on in these situations, but an increasing body of information is available. There are really two separate topics here. The first is repetitive diving, on the same day or over multiple days. The second topic is multi-day diving that is NOT repetitive diving.
We've all been taught that repetitive diving increases our risk of DCS (at least we should have been!). A repetitive dive is a dive where you start the dive with some residual nitrogen (and/or helium) still left in your body from a previous dive. Dive tables typically assume that if it has been more than 12 or 18 hours since your last dive, all of the inert gas you absorbed during the last dive has been eliminated. We know that uptake and elimination of inert gas is approximately exponential and therefore complete elimination of the excess inert gas absorbed during a dive will take a VERY long time. However, 12 to 18 hours after a typical recreational dive the amount of retained gas is so small that it usually has no practical significance. For example, 24 hours after a shallow, no decompression dive, the amount of retained gas might be equal to the amount of inert gas you will absorb in 15 seconds during your next dive to 60 fsw.
Let's look at this in a bit more detail. Decompression equations assume that the various tissues of the body take up and eliminate inert gas at various rates. Given that inert gas is transported between the lungs and the tissues in the blood, it makes sense that tissue with a very high blood flow (working muscle) would take up and eliminate inert gas far faster than tissue with a very poor blood supply (bone).
Every tissue can tolerate a certain amount of excess inert gas and it seems that faster tissues can tolerate more excess inert gas than slower tissues. After a single, recreational, no decompression dive, most of the excess inert gas will be in the fast tissues. These tissues will lose the excess inert gas quickly and they will be pretty much back to normal several hours after a dive. Slower tissues will absorb relatively little inert gas during this kind of dive.
If we do a dive deeper than recreational limits, or a dive where we stay down longer than standard recreational limits, more inert gas will be absorbed by slower tissues and it will take much longer to eliminate this gas after the dive. If we do a second dive before we have completely eliminated the excess inert gas, the amount of inert gas in the various types of tissues becomes very hard to estimate.
By now you are most likely thinking, Òit is easy, my computer does the math and calculates it?Ó Your computer does the math, and most computers will generate a pretty picture to show you how much inert gas is in each 'tissue compartment', BUT what your computer is doing is solving a mathematical equation. What we really need to know is how much inert gas is retained in the various tissues of our bodies. YOUR COMPUTER HAS ABSOLUTELY NO IDEA HOW MUCH INERT GAS IS IN YOUR BODY!!! The capitalization is because I cannot emphasize this point enough. The problem is made worse because many decompression equations 'pretend' that they somehow represent what is happening in the body. In reality, all are simple math and have very little to do with physiology.
Life is even more complex than you might imagine. I gave the example of working muscle as a 'fast' tissue. But, blood flow to a muscle can be as much as 100 times greater at maximum work compared to blood flow at rest. What is the blood flow to a muscle during a dive? Well, it depends on how hard the muscle is working, and it can change dramatically during the dive (hard working dive followed by cold, long decompression stops). Which compartment in the decompression equation represents this muscle? The answer of course is, none.
But the complexity is even worse! We tend to be warm at the beginning of the dive (bottom phase) and cold at the end (decompression). These temperature variations cause huge changes in the distribution of blood flow in our bodies during the dive. Again, no decompression equation can know about these changes. Some dive computers measure water temperature and adjust the decompression based on this. However, it is not the water temperature that matters, it is the tissue temperatures of the diver. Depending on how you are dressed, how hard you are working, etc. you can be very hot or very cold in any temperature of water.
But there is still more complexity! When you generate inert gas bubbles in the venous ends of the capillaries, and those bubbles get carried back to the lungs where they are trapped and the inert gas eliminated, the inert gas is carried from the tissues to the lungs much faster than when no bubbles are present. This means that the amount of inert gas in the tissue will be LESS than calculated by the dive computer. But, if the bubbles stay in the tissues, inert gas will move from the tissues into the bubbles. This will cause the partial pressure of inert gas in the tissues to drop. The amount of inert gas that is carried in solution by the blood from the tissues to the lungs is a function of the partial pressure of inert gas in the tissues. Therefore, tissue bubbles will dramatically slow down the elimination of inert gas from the tissues. In this situation there will be MORE inert gas in the tissues than calculated by the decompression equation.
Recent decompression models try to incorporate the effect of bubbles on decompression (bubble models). Do they calculate the effects of circulating bubbles or stationary bubbles? Another problem is that the same diver doing the same dive might have no bubbles one day, and many bubbles (of either type) another day. In addition, there is a huge difference between divers as to whether they will bubble or not after a given dive profile. By now I hope you fully appreciate that your dive computer has absolutely no idea what is happening in your body.
Another complication is that nitrogen is 4 to 5 times more soluble in fat than in water. This means that the partial pressure of nitrogen in fatty tissues will rise far slower than in watery tissues with the same blood flow. It also means that when the tissues are saturated, fatty tissues will contain 4 to 5 times more nitrogen than watery tissues. The solubility of nitrogen and helium is similar in water but helium is far less soluble in fat than nitrogen.
The reason most decompressions equations work fairly well is because the variables in the equations have been adjusted to give decompression profiles which work most of the time for most divers. Therefore, the decompression equation is only as good as the diving data that has been used to adjust the equation. This is a VERY important point. Bubble models are not necessarily any better, or worse than any other decompression equation. There is not necessarily any difference between 4, 8, 12 or 16 compartment models. More is not necessarily better. It doesn't really matter if the model is parallel or series.
Formal, scientific testing of dive tables and decompression equations has been fairly extensive but it has almost exclusively involved a single dive per day, and usually no diving for at least one day before the experimental dive. There were only a few hundred repetitive dives done to test the DCIEM air model and these dives were limited to two dives in one day (many thousand single dives were done). Other tables had even fewer or no repetitive dives to test them. Therefore, even the fairly well tested decompression tables and models are based on very small data sets of repetitive dives. Many decompression models have NEVER HAD FORMAL TESTING, let alone testing for repetitive diving. They have simply been 'adjusted' to give decompression profiles that 'seem reasonable' to the designer, based on other models and their own personal bias. They are then released to be 'tested' on the diving public. Unfortunately, there is almost no feedback to the equation designer on whether the divers develop DCS or not. The only feedback they get is in the form of law suites when divers developed serious DCS with permanent neurological damage on their tables or using their computers!
In spite of the above, most current tables and dive computers give decompression profiles that work most of the time, for most divers, for single recreational dives. They also work fairly well for two and sometimes even three dives in one day. However, as mentioned above, there has been VERY little formal testing done for doing more that one dive a day. The equations are ASSUMED to work for several dives in one day and several days of consecutive diving, but they have not been well tested for this kind of diving.
When you adjust an equation to fit the data, the more data you have, the better the equation can be adjusted to fit the data (in simple English, lots of good data means the table will be 'safe'). When you have little data, or even worse, when you 'extrapolate' your equation outside your data, the reliability of the equation is unknown (in simple English, the table might be safe, or it might be totally unsafe, you have no way to know). Therefore, when we do more than two dives in one day, we are doing a type of diving that has not been well tested, and our risk of DCS might be much higher.
It is very important to remember that just because we slept, and the date changed on the calendar, we are not necessarily starting a new dive series. If we did a late evening or night dive, the first dive the next day will be a repetitive dive. If we did deep/long dives, the first dive the next day could be a repetitive dive. This could be true even if your table or computer calculates you have eliminated your excess inert gas.
I am out of room in this column but by now it should be fairly clear that we can reasonably expect an increasing risk of DCS with repetitive diving. We will continue this discussion and move on to multi-day diving in the next column.
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