Effect of slow compartments size in relation to NDL and DECO

[tbone1004]

@stuartv
I think @boulderjohn hit it on the head for @narke

Here's the TLDR
Narke's logic implies that diver A has a 5 gallon bucket, and diver B has a 55 gallon drum.
Both take say 10 minutes to fill up with water, but both only have a 1" hole to drain water from, so it takes Diver B a lot longer for his 55 gallon drum to empty vs. Diver A who only has to move 5 gallons through the hole.

Simplified reality, making a lot of assumptions.
Diver A still has his 5 gallon bucket, and it has say 1 hole drilled in it that allows 1gallon per minute of water to flow into or out of it depending on depth, etc.
Diver B still has his 55 gallon drum, but it has 11 holes drilled into it that allow 11 gallons per minute of water to flow into or out of it.
Both take 5 minutes to fill, and 5 minutes to drain.
This is the half-time principal greatly simplified

Assuming that both divers are healthy and are just of different sizes *say me at 275lbs, and one of my buddies at 110lbs*, it doesn't matter.
Where it gets complicated is if there are health concerns attributed with poor circulation in one of the divers that can cause gas flow to be reduced. This is relatively poorly understood, so if you have known bad circulation, it behooves you to adjust your conservatism accordingly. I have bad knees from several injuries, so I am not comfortable running a gf-hi of 85 in a technical setting, and not comfortable diving anything over a gf-hi of 85 in a recreational setting because I don't want to run the risk of bubbles showing up in my knee *which they have before*.

 

[boulderjohn]

Just to point something out....

• More than 100 years ago, John Haldane's extensive research on decompressing goats led to the first dive table and the idea of theoretical tissues, halftimes, etc.
• Dive organizations, including especially the U.S. navy, continued to research and tweak those early tables.
• From 1959 through his death in the 1980s, Swiss scientist Albert Buhlmann studied decompression extensively, creating the decompression algorithm that is still widely used today, with some adjustments based on other research.
• Bruce Weinke's research led him to create the Reduced Gradient Bubble Model (RGBM), still used by a lot of computers today (although often criticized).
• In the early 1980s, scientists in the Diving Science and Technology group (DSAT) performed extensive research on divers and decompression, using Doppler bubble imaging, research that led to the PADI Recreational Dive Planner.
• Over 20 years of research by DCIEM in Canada led to the creation of their tables.
• The United States' Naval Experimental Diving Unit (NEDU) has been studying diving and decompression for decades.

These scientists and others have created the methods we use for dive planning and decompression, whether we use tables or computers for that purpose. In all cases, it does not matter what you look like. A 95 pound female marathon runner will use the same table or the same computer as a 285 pound overweight male couch potato. Now, that couch potato is rightly advised to be more conservative than the marathon runner, but they still use the same algorithm. None of those scientists over more than a century of data crunching saw a need to build that difference in physique into their algorithms.

Think that through for a minute.

 

[stuartv]

@stuartv
I think @boulderjohn hit it on the head for @narke

Here's the TLDR
Narke's logic implies that diver A has a 5 gallon bucket, and diver B has a 55 gallon drum.
Both take say 10 minutes to fill up with water, but both only have a 1" hole to drain water from, so it takes Diver B a lot longer for his 55 gallon drum to empty vs. Diver A who only has to move 5 gallons through the hole.

I understand the theory.

And in your example, the simpler way to explain it is that the buckets/drums fill up and drain through the same holes. So, if they fill up at the same rate (i.e. they are the same "theoretical compartment"), then they will drain at the same rate.

An analogy that implies they fill one way (through an open top), but drain a different way (through holes in the bottom/side) is just making it complicated and does not match up with the theoretical model that is being analogized.

Two different sized buckets, each with a hole in the bottom. You set them down in water and they start to fill as they sink. If they fill at the same rate (so they are both full - aka saturated - at the same time), then when you lift them up out of the water and they start to drain, they will both empty at the same time, too.

If they drain at a different rate, they would have also filled at a different rate and, by definition, they would not be the same "theoretical compartment".

Recall that the OP was completely predicated on talking about the same compartment in two different divers. The key point is that if the compartments in the two divers don't on- and off-gas at the same rate, then they are not the same "theoretical compartment". Saying that it is (just for example) fatty tissue on the liver in both divers does not mean it's the same theoretical compartment in both divers. If the fatty tissue on the liver in Diver A off-gasses at a different rate than fatty tissue on the liver in Diver B, then even though they are the same physical type of tissue they are not the same theoretical compartment. With respect to the model, the actual physical composition of the compartment doesn't matter. The compartment is defined solely by its on- and off-gassing rate.

 

[Dan_P]

You do realize, don't you, that ratio deco was just studied in comparison with other algorithms, and it did not do so well?

Another algorithm, not other algorithms

It was compared to a 30/80 in a trial setup that locked the parametres (in fairness, because of the scientific model) disallowing the use of adjustment in the water, and to a particular depth/time that only just favored deeper stops in the RD framework.

You know that means that a diver actually using RD - without being locked by a scientific trial - would approach the ascend with less emphasis on deep stops, on that dive.

And the result?
No cases of dcs in either case and no statistically significant difference in bubble formation. Increased inflammation count on the RD side, but nothing conclusive as the report clearly says in the conclusion section.

It is a very interesting read, though.

 

[stuartv]

And the result?
No cases of dcs in either case and no statistically significant difference in bubble formation. Increased inflammation count on the RD side, but nothing conclusive as the report says in the conclusion section.

It is a very interesting read, though.

Link?

 

[boulderjohn]

UTD Decompression profile study results published

 

[Dan_P]

Link to the actual report:
A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. - PubMed - NCBI

(note, membership requirement for the full report)

On a side note, this scientific project was largely supported by UTD who, in lieu with it's findings, raffined further the RD blueprint.

 

[Dr Simon Mitchell]

Hello,

This is an interesting discussion.

I think that the genesis of the OP's original question arises from an assumption that as a person accumulates more fat, this is simply added to a tissue in which the blood supply does not change. This is not correct. As we become fatter, the addition of fat cells has to be supported by the laying down of new connective tissue and microcirculation. Thus, a fatter person does not change the kinetics of the fat compart per se, but rather adds more tissue with the same kinetics.

I should observe at this point that that literature is somewhat divided on whether or not being fat actually increases the risk of the DCS. The truth is that we probably don't have definitive studies of high enough quality to be sure of the answer, but there are certainly theoretical problems associated with being fat.

In calculating the time constant for a notional tissue (and ultimately its half life) one needs to know the perfusion. In that regard, as Ross and others have pointed out, it is not so much the tissue volume per se that is important; perfusion is the blood flow per unit tissue. Fat is actually better perfused than most people think. Its perfusion is similar to resting skeletal muscle, but in saying that, it is potentially quite variable which is where problems can start (see below). What makes fat a `slow` tissue is that the blood-tissue partition coefficient for nitrogen is low. This is the ratio of solubilities of nitrogen in the blood and the tissue. If you think about it, if nitrogen is poorly soluble in blood compared to the tissue it is carrying nitrogen to, then trying to fill the tissue with nitrogen is going to take a long time because it is like a small truck carrying dirt to fill a huge hole.

What this comes down to is that if you are fat you have a larger amount of a slowish tissue whose perfusion can be somewhat variable. If we get the decompression of this tissue wrong (which could happen if the model assumes it is not the controlling tissue when it actually is), then it can start to bubble more than expected, and if you have more of it, then as leadduck has implied, this will mean more bubbles. Bubbles forming outside the blood vessels in fat are unlikely to harm you, but if the bubbles are forming in the fat tissue capillaries, then this will mean more venous gas emboli, and a greater probability that these could cross a PFO or a pulmonary shunt and cause problems that way.

For completeness, being obese is also a risk factor for coronary artery disease (which contributes to a significant proportion of diving deaths) and your functional capacity / ability to exercise your way out of trouble may be reduced.

And the result? No cases of dcs in either case and no statistically significant difference in bubble formation. Increased inflammation count on the RD side, but nothing conclusive as the report clearly says in the conclusion section.

Dan, this is a separate topic, but a couple of points. The lack of cases of DCS is irrelevant. DCS was not an outcome measure and the study was underpowered to seek differences in DCS rates. There were more bubbles on the RD side but you are right - the difference was not statistically significant. However, two things you did not mention. First, the RD decompression was significantly longer than the GF one, which was a design flaw in the study because it constitutes an extremely strong bias in favour of the RD profile. Despite this the primary outcome measure was worse in the RD profile. Second, the Swedish navy study has shown that 30:80 also probably over-emphasises deep stops and so it does not constitute the ideal comparator for RD. Put another way, if RD had been compared to another algorithm with even less deep stops the differences may have been greater.

Simon M

 

[Duke Dive Medicine]

Just as I was gathering references, the source of the references jumped in. Great post Simon.

Best regards,
DDM

 

[Dan_P]

Dan, this is a separate topic, but a couple of points. The lack of cases of DCS is irrelevant. DCS was not an outcome measure and the study was underpowered to seek differences in DCS rates. There were more bubble on the RD side but you are right - the difference was not statistically significant. however, two things you did not mention. First, the RD decompression was significantly longer than the GF one, which was a design flaw in the study because it constitutes an extremely strong bias in favour of the RD profile. Despite this the primary outcome measure was worse in the RD profile. Second, the Swedish navy study has shown that 30:80 also probably over-emphasises deep stops and so it probably does not constitute the ideal comparator for RD. Put another way, if RD had been compared to another algorithm with even less deep stops the differences may have been greater.

Hi Simon,
You're right, it's a separate issue, I simply felt it was called for to make some of the above points in reply to a previous post on the matter.

A final note on my part (I promise);
I agree with your point that there was a design flaw in the trial.
However, I should at the same time point out that the specific time/depth in the trial design exactly put the RD side into a "bracket" with more emphasis on deep stops. In fact, if accounting for the actual average depth of the dive, the emphasis on deep stops (as well as total decompression time), would be less on the RD side.

If we compare the RD decompression framework to standard gasses, it would equate to using a gas that is exactly within desired density level for a given depth, instead of a gas significantly below desired density level on a dive 1m deeper - and then compare the effect of the overall standard gas table to an optimal blend approach on that basis.

That said, I do not argue that deep stops have been overemphasised, and am thrilled to see scientific advances in the area of decompression physiology.