Deco theory question and M-values

[tanzbodeli]

I am currently taking the PADI Divemaster course and some questions have cropped up from the reading material that I'm having difficulty figuring out. My educational background is in molecular biology so I like to understand the physiological reasons behind alot of the things I'm reading/learning, so don't spare any technical details in answering.

My main issue is what the "M-value" for a tissue compartment represents. From the PADI Divemaster manual it states:

"The M-value is the maximum tissue pressure allowed in the compartment when the diver surfaces, so as to prevent exceeding the maximum acceptable gradient"

I've read this chapter over and over, and consulted a few other materials and what I've put together myself (since it's not spelled out anywhere I've yet found) is this:

Tissue compartments exsolve nitrogen at a given rate which is characteristic to the physiology of the tissues involved (e.g. meaning that blood exsolves nitrogen at a higher rate, maybe because it's liquid, as opposed to adipose tissue, lets say). Basically it seems like the M-value of tissue compartments is a measurement of the maximum physiological ability of a tissue compartment to exsolve nitrogen (without bubbles forming). The point being that if a tissue has more nitrogen in it than it can off-gas via physiological means, that gas will come out of solution via bubbles (opening a can of soda), which is a bad thing

Nothing I've read so far has said this directly and if this is WAY off, forgive me, this is only a theory I've cobbled together based on what I've read. But for me to really understand deco theory, I really need to understand what is going on behind the scenes, so any help is greatly appreciated.

note: the term "exsolve" I've seen used in a few articles/books to mean off-gas or come out of solution

Chris

 

[amascuba]

I'm confused by exsolve. Based on dictionary.com exsolve means: Mineralogy. (of two minerals in solid solution) to separate from one another at a critical point in temperature.

I could try to explain it as best as I could, but I don't think I would do it any justice. A good paper to read would be "Understanding M-Values". (Requires Adobe Acrobat Reader) Hope that helps.

 

[Ben_ca]

That's a great paper... I've read it several times and am still digesting it...

Figure 2 on Page 6 does a lot to explain M values for me

 

[Charlie99]

Clearing Up the Confusion About "Deep Stops" article, also by Erik Baker is a good follow-on after reading the Understanding M-Values. The Deep Stops article shows how the M-values are used.

One minor note: The PADI/DSAT model is strictly NDL and only has surfacing M-values, the allowable M-value at the surface. Sometimes signified by M0. Most (all?) models include a slope factor and the allowable M-value increases with increasing depth.

Most of the time there won't be any confusion because the context makes it clear whether the writer or speaker is using "M-value" to mean to M0 or the M-value at depth other than the surface.

Charlie Allen

 

[Gene_Hobbs]

I could try to explain it as best as I could, but I don't think I would do it any justice. A good paper to read would be "Understanding M-Values". (Requires Adobe Acrobat Reader) Hope that helps.

Erik's paper is by far the best resource available.

I have also used this one in the past:
Systematic Guide to Decompression Schedule Calculations.
Braithwaite, WR 1972
RRR ID: 3945, NEDU Report Number: 1972-11

This goes along with:
CALCULATION OF DECOMPRESSION SCHEDULES FOR NITROGEN-OXYGEN AND HELIUM-OXYGEN DIVES.
Workman, RD 1965
RRR ID: 3367, NEDU Report Number: 1965-06

If you need help getting started with the Rubicon Research Repository Please check out the FAQ page.

 

[tanzbodeli]

Ok, I read the Understand M-values article, and I think I'm now more confused than before... :-/ But I'll get there. I'm just having a hard time finding a physiological explanation of what I'm reading (which is completely absent, even in the Baker article). Also, what one of the previous posters mentioned is applicable to me, what I'm studying in the PADI DM Manual is based on surface pressures, M-zero values.

If anyone is familiar with the material taught in the PADI DM course, I'd appreciate them weighing in. Once I get past that, maybe I can move on to the more advanced stuff (like in the Baker article) and tackle that.

thanks,

Chris

 

[H2Andy]

I'm confused by exsolve.

i am pretty sure exsolve is the opposite of disolve.

a gas disolves into solution and then exsolves out of solution.

(i think)

 

[scarefaceDM]

Having followed the DM course and read over and over again the M values, off gassing saturation etc...it breaks down to this. Stop thinking like a scientist and break it down to a simplistic nature. Your looking at it scienifically. I had the same issues and my instructor having a PHD is Physics and Chemistry made us look at it in two phases. One being RAW science the other was to break it down to be able to explain it to a non diver.

We used the analogy of 6 sponges. Rate of saturation per sponge being of different sizes. And rate of desaturation per sponge with different pressures. The result was an understanding of saturation and off gassing. Not being knowedgable in this field of science, this is how it worked for many in our DM course. Plus as we made our presentation to NON divers, they had to understand the basics of saturation and off gassing.

We supplied moist towellets to the folks as we sprayed pop to demonstrate off gassing.

Stephen

 

[limeyx]

Ok, I read the Understand M-values article, and I think I'm now more confused than before... :-/ But I'll get there. I'm just having a hard time finding a physiological explanation of what I'm reading (which is completely absent, even in the Baker article). Also, what one of the previous posters mentioned is applicable to me, what I'm studying in the PADI DM Manual is based on surface pressures, M-zero values.

If anyone is familiar with the material taught in the PADI DM course, I'd appreciate them weighing in. Once I get past that, maybe I can move on to the more advanced stuff (like in the Baker article) and tackle that.

thanks,

Chris

I'm not certain there really is much of a physiological explanation of it.
The "tissue compartments" are a theoretical device (different theories use different number of them, and I think different "M"-values)

I think it is more of a concept that has been used to allow people to generate certain tables that seem to "work" for a certain proportion of people in given circumstances.

(where "work" means you dont have bad enough DCS to be able to distinguish from just being tired from diving all day -- getting up early, exertion, dehydration from breathing dry air etc)

dont look at it like a panacea.

 

[Dr Deco]

Hello tanszbodeli:

The historical explanation of allowable supersaturation goes like this. JS Haldane believed that substances (solids or liquids) could remain in solution in a supersaturated condition. This might be correct if micronuclei were not normally present in liquids. The allowed supersaturation was what made possible an ascent to the surface without bubbles forming or DCS being instigated. This is somewhat correct but not the whole story. No-decompression limits derive from this stable supersaturation effect.

Haldane was aware that blood flow was not the same to all tissues of the body. Also, there were gas solubility difference being large between “aqueous” tissues (e.g., muscle) and “lipid” (e.g., fat). Brain tissue [a “fast” tissue] was more richly supplied with blood than muscle tissue [medium]and this in turn received more than adipose (fat) tissue [a “slow” tissue]. Different tissues were believed to be able to sustain differing degrees of supersaturation. No good explanation was provided for this effect.

One “loads” these tissue at differing rates and they “control” the ascent depending on how long one has been submerged. One could remain at shallow depths for a long duration, but the slow tissue was loaded, and it could not sustain a high supersaturation. Thus, NDLs for shallow bottom depths are not very large.

Half Time Tissues

The problem with all of this is that, in time, no one believed that there were different tissue types responsible for DCS. The term halftime “compartment” was then developed. It was believed to be actually only a mathematical construct. It did have a link to regional blood flow and regional gas solubility in some unclear fashion.

In truth, it is difficult to describe the physiological processes associated with DCS in terms of half time compartments. It is only a method to calculate dive tables.

M-values

Progressing to deeper depths, the NDLs are modified to allow the calculation of decompression tables. M-values allow one to add small amounts of dissolved gas – expressed as depth – to the NDLs. All of this is based upon the concept of different “tissues” and stable supersaturation, ideas that are not actually correct. One must know how supersaturation limits change with depth and with each compartment type. That is the job of the table designer, and it was proprietary for deep, commercial tables.

Dr Deco :doctor: