M value

[The Iceni]

Hi Dr Deco,

Could you explain, in simple terms, the concept of "M" values and how they are derived?

I gather it is related to the degree a tissue can retain gas in supersaturated solution, after the external pressure is reduced, before it comes out of solution within the tissues, or "boils".

Thanks in advance.

 

[Alban]

Hello Dr T.

Not diving on this fine weekend ?

I thought I would sneak in while Dr D. is out. I am not at home so I have no references only a bad memory.


M nought values are the maximum surfacing supersaturation values for a theoretical tissue compartment. Faster compartment can withstand a greater change in ambient pressure I think the No 1 compartment on the Bullmann tables has a M nought value of 30 Mts. in theory at least this compartment could be saturated at 30 Mts. and be brought to the surface where as compartment 16 would have a very low M nought value maybe only 6 Mts.

M delta values are maximum supersaturation values .On deeper decompression dives they set the ceiling for the dive or they set the decompression stop depth., as you said this compartment the leading tissue would be at its maximum supersaturation value the table would then call for a stop to allow this compartment to release some tissue tension until all compartments are able to ascend without exceeding the critical values.

I think J workmann originally stated that compartments could tolerate an overpressure of 2: 1 this was latter proved to be slightly incorrect as the 1 was infact 0:79 N2, Haldane improved on this and linked the compartment half times to M values by equations using a set of figures known as A and B values.

I am sure Dr Deco will correct me and explain it more fully.

Alban

 

[Dr Deco]

Dear Readers:

M-values

They are variously referred to as ”matrix values” or “maximum values.” The method was instigated by Dr Robert Workman of the US Navy to assist in the calculation of decompression tables.

The initial concept of JS Haldane was to allow the pressure to be reduced by ½ (2:1 ratio) in all of the “tissue compartments.” [This was actually 1.58:1 if allowance was made for the fact that much of the air was actually oxygen and not pure nitrogen.] This ratio for pressure reduction was modified when it was noted from experiments that “fast” compartments could tolerate a much higher supersaturation than “slow” compartments.

Start with NDLs

If one starts with the no-decompression limits determined experimentally for a group of test subjects (age dependent, lean/fat dependent, physical fitness dependent, work load dependent, hydration dependent, etc., etc), it can be found that deep dives are short and are “limited” by small halftimes (5-10 minutes). Intermediate depths (60 fsw) are limited by medium halftimes, and shallow dives (35-40 fsw) are long half time dependent. The pressure in the limiting tissue is the “surfacing pressure” and its ratio to absolute pressure is the “limiting ratio.” This value (in fsw) is the pressure in a given half time compartment for ascent to the surface and is the Mo-value.

Deeper depths

As one moves from the surface to the ten-foot depth, the allowed tissue gas ratio becomes a bit smaller. The ATTACHMENT (from my Decompression Physiology class) shows the allow tissue pressure (M-value) for ascents to the ten-foot stop and deeper stops. There are increments in the Mo value that allows the calculation of tables down to hundreds of feet.

Problems

These M-values do not always work as the tables go deeper and decompression becomes extended. This deco is conducted in chambers for commercial and military divers. I believe that the failure in deco is the result of tissue nucleation as the divers walk about in the chambers over the hours to days of decompression.

Safe decompression

It is clear for recreational SCUBA that you can add to the safety of decompression by

• simply reducing bottom time,
• slowing ascent, and waiting a bit at the surface before boarding the boat, and
• reducing strenuous physical activity on the surface.

You can always reduce your strenuous surface activities, however. This increases safety and costs noting in terms of new equipment.

One advantage of tables is that it is easy to conservatively read and apply the tables. Computers always give numbers that are the “table limits.”

Dr Deco :doctor:

 

[rcohn]

Paul,

Take a look at Understanding M-values By Erik C. Baker, P.E.: ftp://ftp.decompression.org/pub/Baker/Understanding M-values.pdf

Ralph

 

[Dr Deco]

Dear Readers:

Table Testing

(1.) I have looked at the piece by Eric Baker and it is very informative. The tables and graphs are very helpful.

(2.) I have been involved in decompression tests for three decades. I do wish to emphasize once again that surfacing limits (Mo values) and decompression tables are determined in individuals at rest following the hyperbaric exposure. It is assumed that subjects will move about in a “normal” fashion. Test subjects do NOT

• run up and down stairs or ladders;
• simulate climbing onto the boat with full dive gear; or
• swim or play volleyball

Clearly, if one does this, you are imposing a different set of conditions and increasing the chances of DCS.:boom:

It is my finding at NASA that some altitude depressurization exposures carry a 10-fold (not 10%) increase in DCS with activity in the legs. What this would be for divers has not been checked.

Dr Deco :doctor:

 

[The Iceni]

Alban once bubbled...
Hello Dr T.

Not diving on this fine weekend ?

No, but two geat dives today (Monday) out of Lyme Regis, Dorset, after far too long a journey yesterday.

Almost intact wreck Moidart at 35 M, sunk in 1918 by a U boat, vis abot 8 metres. Then 28 M dive on yet another U boat victim. This time 1943, vis about 12 metres. ( I'll have to check on the name but there was not much of it left intact but two superb boilers about 30 feet in diameter and so much life! I saw at least three conger eels. The largest was at easily 12 feet long.)

No lobster for tea though!

(Unlike most I must be on call every day, and locums come expensive and in addition they are few and far between.

An adage of my own making, " If I have the money to dive I don't have the time. If I do have the time to dive I won't have the money.)

 

[The Iceni]

Dr Deco once bubbled...
Dear Readers:

M-values

They are variously referred to as ”matrix values” or “maximum values.”

Very, informative.

One day I hope to attend your lecture series (maybe I will get some help here next year!) but I am gratified I am not alone in my interest. I note the very useful attachment has been downladed 15 times already!

Kind Regards,

 

[Alban]

Ah Dorset is my favourite place an excellent dive the Moidart , the viz this year after a slow start has been very good I honestly think there is more sea life this year than ever before , perhaps its the cleaner waters.

I read an article which I found interesting
Perhaps Dr Deco would know a little on this ?

Http://home1.gte.net/leetpley/bubble_activ._and_growth.html

There is a question about two thirds down
Q. So why don't cetaceans get the bends??
Have you heard of these protective mechanisms ??

Can we learn from Mother Nature ?

Thanks again Allban

 

[Dr Deco]

Dear Alban:

Cetaceans and Bubble Growth

The question has been in people’s minds for quite a while, viz, why do whales not get “the bends?” There are several possible reasons

• the tissue gas loads are too small because they can only distribute the air (nitrogen) in their lungs during the breath-hold dive;
• they do not have many tissue micronuclei;
• they have few pain receptors in their ligamental tissue and therefore feel nothing painful (just as we do not feel the gas bubbles in our muscle tissues.

If it is assumed that the creatures can get DCS, then the questions arises, can it be exacerbated by powerful sound waves?:sigh:

Sonar and Gas Bubble Activation

The question of forced growth of microbubbles (micronuclei) by means of sound waves is what EN Harvey originally studied in the 1920s in water. It is this area of research that eventually lead him into the area of decompression biophysics and tissue nuclei.

What we are reading about in these papers concerning cetaceans is the injury to their hearing organs resulting from the vibration of the gas present in these structures. Additionally it is proposed that much of the injury can arise from direct damage to tissues from growth of gas bubbles present in them.

Where I would disagree with these papers is in the concept of the impermeable shell around the bubbles. The sonar wave is believed to activate nuclei by breaking this barrier and allowing nitrogen to diffuse in and the bubble to grow. I do not know of any evidence that bubbles in your body contain this type of a barrier. (Echocontrast agents do possess this type of diffusion boundary, however.]

Rectified Diffusion

The ability of a gas bubble to grow by pulsations when it is in a supersaturated medium is referred to as rectified diffusion. As I understand this, more of the effect is the result of mixing of the fluid boundary and renewing it by stirring rather than a change in the area of the gas-liquid boundary. In diving, this stirring is the result of muscle movement (I believe) and might contribute a little to bubble growth. More is gained, though, by activity opening up capillaries and allowing dissolved nitrogen to escape from tissues into the blood stream.

No Bends

There is possibly a reason why DCS is absent, but I doubt if the reason lies in some material in the blood of dolphins, etc. They would simply dive deeper or longer, although the possibility exits that they are at their limit because of oxygen reserves in a breathhold dive. Thus they are protected to the limit of their diving capabilities.:singfish:

Dr Deco :doctor:

 

[Alban]

Thanks Doc

A most imformative answer , one thing that has puzzled me though is how the whales blood gets supersaturated from a breath hold dive ?

Also would the whale slow down it's heart rate reducing prefusion to the tissues ?

I did think I found the answer to all deco problems !
if only life was that easy

Thanks again Dr.D