Algorithms, Conservative Factors, Altitude, Planned Deco - Questions

[dmaziuk]

Valid point. Because RGBM is proprietary, we don't know how different the formulas are. But we do know the Buhlmann formula and developers are just changing two variables within the same formula; GFlow/GFhi.

Not entirely true: -D and -C have different a and b coefficients, and who knows what the "ADT" version has.

ZHL computes M-values as P * a - b on the assumption that the amount of overpressure tissues can tolerate is depth-dependent. GFs modify a and b in a fairly simple way; nothing's stopping you from modifying them with a more complicated formula that factors in e.g. "biometric parameters" or "folded bubbles".

ZHL8 has 8 tissue compartments with 8 half-times whereas ZHL16 has 16. The math is no different, just the number of "slices" is.

Furthemore, Haldane counted all gas as perfusing/diffusing, Buhlmann only counts nitrogen (and helium if present) with no accounting for O2 that is not metabolised. Using 0.79 (when diving air) of total gas, together with chosen a and b coefficients does not get people bent all the way into nitrox. Nothing's stopping you from raising the fraction of inert gas and changing a and b to compensate for that.

One could argue that all Haldanean models are the same formula with a number of changing variables. Main difference with RGBM is that here we know what can be changed.

 

[scubadada]

To expand a bit: Breathing rate does not affect nitrogen loading.

It's easy to mix breathing rate and workload together and have the impression that breathing is a factor. That is because working hard DOES increase the chance of a DCS hit. Working hard would generally correlate with increased breathing. So, it's easy to understand how one might think that increased breathing means increased inert gas loading.

However, (and note that I am NO expert here - I am now venturing off into my own layman's pet theory) the increased chance of a DCS hit after working hard is not because of an increased load of inert gas. It is because the work makes the existing load of inert gas more likely to bubble.

I think it is very much like a soda bottle. The same amount of gas is in the bottle, no matter what you do. But, if you shake it before you open it, it's a LOT more likely to fizz.

Great analogy Stuart, not sure at all if it applies

 

[scubadada]

Remember that tissue compartments to not track to specific tissues but to their half lives of ongassing and offgassing. This may or may not be affected by perfusion, it is taken into account by the model

 

[KenGordon]

Remember that tissue compartments to not track to specific tissues but to their half lives of ongassing and offgassing. This may or may not be affected by perfusion, it is taken into account by the model

Are you saying perfusion doesn’t matter? Or work rate doesn’t matter?

 

[scubadada]

Are you saying perfusion doesn’t matter? Or work rate doesn’t matter?

High work rate at depth is a risk factor for DCS, whether this is related to perfusion of a certain tissue is a different question, for which I do not know the answer, do you?

 

[stuartv]

Great analogy Stuart, not sure at all if it applies

Ha! I'm sticking with it. It makes the point I want it to make. :-D

 

[KenGordon]

The mechanism is assumed to be an increase in blood flow to the tissues, and as a result greater uptake of inert gas into the tissues. Exercise results in increased blood flow to the exercising muscles (exercise hyperaemia) to satisfy the increased metabolic demand, and also increases blood flow to some other tissues. The principal determinate of the rate of tissue uptake and washout of nitrogen and helium (often characterized by half-times) is the blood flow per volume of tissue (perfusion). These tissue inert gas kinetics seem to me modified by diffusion, but the main determinate is perfusion. :

Buhlmann ZHL16 assumes that the perfusion remains the same, that the half time of compartments is constant through the dive, so that gas accumulated in a given compartment is off gassed In the same way it was on gassed.

If the perfusion changes, on the dive, then the half time for a given tissue will change (see David Doolette above) so now the model is using the wrong compartment to track the gas.

 

[KenGordon]

To expand a bit: Breathing rate does not affect nitrogen loading.

It's easy to mix breathing rate and workload together and have the impression that breathing is a factor. That is because working hard DOES increase the chance of a DCS hit. Working hard would generally correlate with increased breathing. So, it's easy to understand how one might think that increased breathing means increased inert gas loading.

However, (and note that I am NO expert here - I am now venturing off into my own layman's pet theory) the increased chance of a DCS hit after working hard is not because of an increased load of inert gas. It is because the work makes the existing load of inert gas more likely to bubble.

I think it is very much like a soda bottle. The same amount of gas is in the bottle, no matter what you do. But, if you shake it before you open it, it's a LOT more likely to fizz.

Please see my quote from David Doolittle from the thread on RMV etc.

There is a different factor about excercise AFTER the dive which is a risk. That is partly (and I am certainly not an expert) about gas crossing from one side of the heart to the other and avoiding the lungs.

This is why you really do want a lift, tea and cake when you are finished with the dive.

 

[CandiveOz]

@CandiveOz, @stuartv is correct here, I believe you have a misconception on this topic

Hi Scubadada, its not a matter of misconception but probably due to poor wording on my part. I thought my post clearly explained the relationship of workload, breathing rate and heart rate. If people misunderstood my comment then it is my fault.

Please let me clarify. I’m not suggesting that increased breathing by itself increases gas loading. You have to understand it in the context to our discussion of “workload.”

As I explained in my post, an increase in workload increases breathing and heart rate naturally. The reason this occurs is because the muscles are demanding more oxygen to perform. So the body works to deliver. Breathing increases to capture more O2, your heart pumps harder to deliver it, your circulation increases, blood vessels dilate, blood pressure increases, temperature increases etc. Everything works in unison and the body’s homeostasis has changed. Your tissues are alive and their properties change in a way that cannot be accounted for in some static mathematical compartment model. With the increase demand, absorption and utilisation of O2 to breakdown glucose to generate ATP; more nitrogen is absorbed in the process. Perhaps John Lippmann explains it better and more succinctly in his book Deeper Diving on page 52.

“Exercise during a dive increases the demands for oxygen. The breathing rate and circulation increase and more nitrogen is delivered to the tissues.”

 

[CandiveOz]

Your nitrogen loading is based on the difference in partial pressure of the gas you're breathing versus the gas in your tissues. Your breathing rate has no effect on how much nitrogen is absorbed.

Stuartv, I agree. You are absolutely correct.

It's easy to mix breathing rate and workload together and have the impression that breathing is a factor.

Breathing, such as deliberate hyperventilation just for fun isn’t correlated to anything. If your body doesn’t need the oxygen, you can breathe as hard as you want; nothing will happen except you may become a bit dizzy (and as a diver, use a lot of gas). But in the context of our discussion of increase workload, it is correlated; absolutely. So breathing rate is a factor and part of the process to deal with the increase workload as per my response to Subadada.

the increased chance of a DCS hit after working hard is not because of an increased load of inert gas. It is because the work makes the existing load of inert gas more likely to bubble.

Unforunately I do not agree with your first comment but I do agree with your second. Alexs Brylske in his book Beating the Bends clarifies on page 17;

"Heavy exercise during the dive increases circulation and accelerates uptake of nitrogen. After you stop exercising, outgassing of nitrogen occurs more slowly...Additionally, just as it does at the surface, exercising while at depth increases the turbulence of blood and creates low-pressure centres that could increase the number of gas seeds."

So in conclusion; both events occur. BTW, I did like your soda analogy.