Buhlmann questions

[dmaziuk]

Wow, for someone who usually argues that the NDL line is very gray and very fuzzy and very dependent on the individual, you certainly have suddenly changed your position so that you are willing to argue the semantics of safe vs safer. At least you can't be accused of consistency.

You go over M-value, you could get bent. If you stay under the M-value you probably won't get bent. It does not follow that if you stay further away from M-value, you'll get "more not bent". Is that so hard?

 

[tursiops]

You go over M-value, you could get bent. If you stay under the M-value you probably won't get bent. It does not follow that if you stay further away from M-value, you'll get "more not bent". Is that so hard?

You are playing with words. I won't play.

 

[EFX]

So I may be totally wrong but if you suddenly found that the model had issues with some depth.

Would you:

1. examine the underlying assumptions and check if something was missed in the original study
2. Slap a quick fix to make it more conservative by drawing a line in the middle of two other lines

I am not saying #2 is stupid, as it is better to be alive than dead, but by putting quick fixes you may sometimes be at risk of missing something more fundamental ?

I would definitely do #1. No. 2, being a quick "fix" runs the risk of solving one problem but introducing other unintended consequences. Buhlmann found issues with the original A tables of parameters. The middle compartments were not conservative enough for dives planned with tables so a set of slightly changed parameters called B was created. Then the parameters were made more conservative again for use with PDC's. This set was called C. I don't know how Buhlmann arrived at his conclusions for successive conservatism but I doubt it was a quick "fix".

For example, my understanding is that people use different GFs but I have not met many people who can explain to me why they use this set of values and not something else.

I don't know of any divers personally but after reading the reasons why divers here on SB change their GF's based on the dive profile and personal factors is based on an educated guess and not on hard scientific facts. We don't know exactly how the bodies response to decompression stress is affected by hydration, temperature, age, fitness level, past injuries, and other factors. There are too many factors. The assumptions for the model and physiology need to be challenged and hopefully in the future there will be tests with measured results that can help us affirm or deny the validity of those assumptions. For example, on the model side:

The Buhlmann model which I'll call ZHL is based on two mechanisms for inert gas uptake and removal in the tissues: diffusion and perfusion. Diffusion is the movement of gases across a barrier and perfusion is the transport of gases to/from the tissues primarily by the blood. The assumption in ZHL is that the TC's (tissue compartments) are in parallel with other TC's with the blood being the source of the gas. The blood can circulate through the entire body in 30 seconds which is a short time compared to the shortest half-time of 5 minutes for TC1. In addition, it's assumed that the blood is a sufficiently large reservoir to supply every tissue with its gas based on the HT's. There is no provision in the model for a series connection of tissues. That is, across tissue to tissue barriers.

1. ZHL is called a gas-diffusion model in that it ultimately limits the rate of inert gas coming out of the tissues. Other models such as VPM and RBGM incorporate bubble mechanics in the model to limit not only bubble quantity but also bubble size. What is the effect of bubble size on decompression stress? Do we need it at all or is gas-diffusion all we need?

2. Is the parallel model correct or is there some leeching of gas between tissues where maybe a combination of the two, parallel and series, needs to be considered.

3. ZHL employs the Schreiner (or its equivalent Buhlmann) equation to calculate the inert gas pressure in each TC. The equation uses the natural exponent e which produces an exponential rise or decay of pressure over time. Some models use a linear equation. Is using e more accurate or some combination of exponentiation and linearity and what combination of the two do we use for ongassing and offgassing?

4. What about the slope of the m-value lines. They define a linear line of m-values based on depth and TC pressures and are constant for each TC. Should the slopes be changed based on other parameters? Or should they be curves?

5. ZHL provides surfacing limits to TC inert gas content which are called m-values. In light of more aggressive dive profiles should these values be decreased? For NDL diving should they be increased? Or, are they fine?

6. What about GF's (gradient factors)? They introduce deep stops and provide conservatism on dive profiles. But, how exactly do they correlate with decompression stress? With the increase of GF's, is the resulting effect on decompression stress linear or is there a tipping point?

On the physiology side we can ask these questions:

1. We know gas bubbles beyond a certain limit create DCS. Not only can bubbles block blood flow, they can impinge on nerves and cause paralysis but they can also cause an inflammatory response which causes its own secondary problems. The question is: at what level either separate or in combination do these proponents cause the onset of symptoms.

2. Due to the inflammatory response Is there a hormone or other chemical we can measure in the body that correlates with bubble quantity and size and can that be used to verify the model or as a real-time means to reduce decompression stress of dive profiles (this assumes a sensor to measure the chemical component can be inserted in the body and monitored by a PDC)?

3. We need to look at the personal factors mentioned above and see how they correlate with a tendency to increase/decrease DCS risk.

Anyway, it's just me spilling my thoughts. Carry on.