Deco theory question and M-values

[rawls]

UGH...M-Values...I feel that they actually do have an indirect physiological aspect to them in that they are determined based on testing of live subjects. Dopplers are utilized for detection of bubbles and an acceptable gradient...m-value...is determined. I am finishing up my DM cert and the math involved in table design is way beyond me. But I look at it as... the greater the pressure is exerted in your tissues, the greater the gradient (difference between pressure inside the tissues and outside). As your tissues become more saturated secondary to pressure at depth, they exert more pressure compared to blood and alveolar pressure. On ascent, the tissues become supersaturated with gas and thus begin to come out of solution. This happens at different rates depending on the compartment. And there is always one compartment that will control the dive (tell you when your dive is over). The m-value is telling you what rate of dissolution will be allowed in the controlling compartment when you surface (considering NDL dives), i.e., allowable gradient. It is setting an acceptable rate for offgassing, at the point where you reach the surface and begin the offgassing process, which will minimize the risk of pathalogical bubble formation. In that determinations of m-values are based on testing of live subjects, there is a quasi physiological aspect to the calculation of these values.

Someone correct me if I'm off base here. Think of it as candy on a conveyer belt being emptied into a box. If the rate of the conveyer belt is set too fast the candy would fill the box to quickly and the candy would then spill over onto the floor. The m-value is the acceptable rate by which the conveyor belt is set to avoid overflowing the box.

 

[Creekyphil]

I dont understand the need to compartmentalize tissues in the creation of tables. After finishing the John Lippman book The Essentials of Deeper Sport Diving, I took my first crack at making my own tables last night, with a slightly different approach. Instead of following the Haldanian approach of seperating the tissues into compartments, I started off with a bell curve from 0 to 635(the slowest tissue in the Buehlmann system), and used this as a representation of the body tissues. I skewed the curve slightly to the fast tissue , but with a high standard deviation extened toward the high tissues. This was based on the idea that blood, muscle and fat represent most of the tissue that I was modeling. The numbers didnt come out great, and I definately wouldnt dive this table (yet), but thats because my curve is off and doesnt really represent body tissue well. I'm having a tough time finding references that suggest how much of the body each tissue compartment represents(approx) in other models. The only signifigant problems I see in making this a workable model would be in differentiating between which part or the curve would bubble first and thus create the limiting NDL. Any input would be great.

 

[rawls]

I took my first crack at making my own tables last night, with a slightly different approach.

You're gonna die

 

[donacheson]

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"
....

From my viewpoint, if one plots tissue absolute nitrogen partial pressure on the ordinate (vertical axis) vs. ambient nitrogen partial pressuere as Baker did in his Figure 1 in "Understanding M-Values", then before ascent one is always below the 1:1 diagonal line - tissue partial pressure never exceeds ambient. As long as one stays below this line, tissue partial pressure increases or, if already saturated, remains constant. However, when one wants to ascend, one MUST go above the 1:1 line in order to decompress. The maximum allowable amount one can safely (according to the model) go above that line is given by the M-value for that model.

 

[Dr Deco]

Hello readers:

I am currently having a problem with a cervical disk. Have trouble typing; please bear with me. Dr D.

M-values

As “rauls” points out, there must be physiological basis to M-values since they derive from human testing. Actually, the no-D limits [NDLs] will result in a safe ascent from a single dive at a single bottom depth. Applying half times to the NDLs, as done by JS Haldane, will allow a link of NDLs to “tissues.”

The NDLs are physiologically linked to the concept of limited, stable saturation. This is not correct and micronuclei exist. Their concentration is not known.

Halftimes make a convenient method of creating tables but are probably incorrect in their present incarnation. Exceeding gas loads in a 10, 50, or 250-minute tissue can all result in a pain in, e.g., the elbow. As conceived, it cannot all be in the same place.

The M-value method simply adds fixed amounts of partial pressure that is “allowed” to halftime as one descends. It is empirical, but it will result in useful decompression tables for deep diving. It does not allow for the advantages of deep stops.

Dr Deco :doctor: