Does Exertion Affect N2 Absorption?

[Mr Carcharodon]

@Mr Carcharodon: I would be satisfied if you could tell me what tissue in the human body correlates with the "slow" compartment and how perfusion rate can be accurately measured through that specific compartment.

The idea that gas transport to some tissues (e.g. muscle, skin, nerves) is limited by perfusion goes back at least to Haldane’s time. Also the idea that diffusion limits gas transport to other tissues (e.g. bone and the ear) seems to be nearly as old. Perhaps one of the more accessible discussions of perfusion and diffusion limited transport is provided in Wienke’s text on Basic Decompression. Haldane discussed perfusion rates of various tissues in his 1908 paper in the context of estimating gas loading, and time constants, of those tissues. So given the date of his paper it seems safe to conclude that MRI is not the sole technique available to measure perfusion rates. The diffusion limited tissues are the “slow” tissues. Clearly if you accept that those tissues are diffusion limited you will conclude that their gas loading does not change as a function of perfusion. And equally clearly the rate of gas loading will change for perfusion limited tissues with exertion provided they are not saturated to begin with.


The idea that circulation varies with activity seems almost too obvious to defend, but if you really need backup I would suggest looking at the v-dot studies of the exercise physiologists. The idea that circulation can be locally modulated is a bit less obvious but can be demonstrated by considering what is happening with flushing skin or shock. As far as specific ratios of change of circulation with activity I would have to point you back to Powell and Hennessey again. The 7x ratio is a single value I used to simply express a chart from Hennessey. Perhaps a range would have been better but it still seems like a reasonable value to me.


But this is tangential since the objective is answering the original posters question. Ianr33 is right to think that the lungs will be near ambient pressure. But the rest of the body needs some time to catch up. For recreational dives some of the tissues in the body never catch up entirely. Exertion will increase the rate with which they do. I do not know enough to say how significant this is to DCS, but will point out that the DSAT/PADI approach is to add padding for strenuous dives. Also Buhlmann’s ZH-16A tables are not thought to be conservative enough. And one complaint about those tables is that they were derived from chamber trials with subjects at rest. Subsequent work tightened up the m-values to envelope the gas loading of more typical levels of exertion. What changed were the mid compartments which are the ones that would be expected to represent unsaturated perfusion limited tissues.


So I conclude that exertion does increase gas loading and does increase DCS risk but freely admit that I do not know how much. Strenuous dives are not what the tables are based upon and it seems prudent to do safety stops or min deco to mitigate the risk associated with the extra gas load that exertion causes. In the context of Mr. DeVlieger’s accident we do know with certainty that a safety stop would have helped and would have only taken a few cubic feet of gas.

 

[Puffer Fish]

@Puffer Fish: I don't understand how the equivalent N and He half-time numbers for spinal cord represent a "major violation of several gas permeability laws." Spinal cord tissue is comprised of a highly heterogeneous mix of neurons and various glial cells.

Assuming one does not have charged particles (which should not be the case here), gas permeability is directly related to size. To be crude, little things move faster thru stuff than big things.

On an atomic level, Nitrogen (14) versus Helium (4) is a huge difference.

Note: It is a bit more complex than that, as the number and size of the electron rings comes into play, but consider that it is fairly easy to seperate Nitrogen (14) from Oxygen (16) using a membrane system, because Oxygen is primarily in the form of O2.

Siting the specific gas laws is somewhat silly, because they would require knowledge of the permeability of the material involved.. which we don't have, and we have a trememdously complex mix of materials involved.

"Bone" for example, is not just one homogeneous thing, there are different types, different densities, and the inside is filled a totally different stuff, that is held by several different membranes. Oh, and don't forget cartilage.

What we do know is that Helium moves thru any substance something around 2 - 3 times faster. Short of using very fancy electro-chemical tricks (and our cells have lots of those), this will always be the case.

A local measurement may not show that because one is measuring a sandwich of mateirals. Put the gas on one side of a slow membrane, then put a fast one on the other side, with something in the middle, and the middle will not follow that rule, until the back side reaches saturation...

There is no way to make any material that would have the same permeability for both N and He, so if one measured that, they are not measuring just permeability.

Note: If the size was close, and one could design an exact crystal matrix it would be possible... but any space N can go thru, 2 He could go thru.

Note2: Ionize the gasses and a bets are off.

 

[ianr33]

A
On an atomic level, Nitrogen (14) versus Helium (4) is a huge difference.

Note: It is a bit more complex than that, as the number and size of the electron rings comes into play, but consider that it is fairly easy to seperate Nitrogen (14) from Oxygen (16) using a membrane system, because Oxygen is primarily in the form of O2.

Remember that N2 and O2 exist as molecules rather than individual atoms so the relevant masses are 28 and 32. The mass and size of a nitrogen ATOM is meaningless when discussing gas transport/DCS.

 

[theduckguru]

I am trying to figure out what mathmatical formulas one could construct where pressure, time and percentage of nitrogen is constant and volumn of gas exchanged is variable and end up with the same result.

 

[Puffer Fish]

Remember that N2 and O2 exist as molecules rather than individual atoms so the relevant masses are 28 and 32. The mass and size of a nitrogen ATOM is meaningless when discussing gas transport/DCS.

Thanks, while I hate typing something wrong... I did, you are correct that N2 is the natural form of nitrogen. I have no excuse for not remebering that at the time.. as it is a kind of famous bond.

Over all size is important though as it is size that effects the speed of gas transport...but not just the atom size.

 

[boulderjohn]

Deleted post--unnecessary

 

[Bubbletrubble]

Assuming one does not have charged particles (which should not be the case here), gas permeability is directly related to size. To be crude, little things move faster thru stuff than big things.

On an atomic level, Nitrogen (14) versus Helium (4) is a huge difference.

Note: It is a bit more complex than that, as the number and size of the electron rings comes into play, but consider that it is fairly easy to seperate Nitrogen (14) from Oxygen (16) using a membrane system, because Oxygen is primarily in the form of O2.

Siting the specific gas laws is somewhat silly, because they would require knowledge of the permeability of the material involved.. which we don't have, and we have a trememdously complex mix of materials involved.

"Bone" for example, is not just one homogeneous thing, there are different types, different densities, and the inside is filled a totally different stuff, that is held by several different membranes. Oh, and don't forget cartilage.

What we do know is that Helium moves thru any substance something around 2 - 3 times faster. Short of using very fancy electro-chemical tricks (and our cells have lots of those), this will always be the case.

A local measurement may not show that because one is measuring a sandwich of mateirals. Put the gas on one side of a slow membrane, then put a fast one on the other side, with something in the middle, and the middle will not follow that rule, until the back side reaches saturation...

There is no way to make any material that would have the same permeability for both N and He, so if one measured that, they are not measuring just permeability.

Note: If the size was close, and one could design an exact crystal matrix it would be possible... but any space N can go thru, 2 He could go thru.

Note2: Ionize the gasses and a bets are off.

@Puffer Fish: I have no idea how one could ever arrive at the measurements cited in DCBC's post...but let's assume that they are correct and focus on the topic that you brought up.

I was once taught that permeability of a cellular membrane to a substance may be influenced by at least 4 different factors: molecular size, charge, solubility in water, and lipid solubility. You brought up size already. It's been pointed out that nitrogen would exist in a diatomic covalently bonded state, whereas helium would be in a monatomic state. Based on molecular size alone, you'd think that nitrogen traverses the cell membrane slower than helium. We'll ignore the charge factor since the molecules have their outer electron shells filled, and it would be highly unlikely for them to exist in an ionized state. At 37°C, nitrogen gas is far more soluble in water than helium is. (In fact, I think one would be hard-pressed to find any single elemental gas that's less soluble in water than helium.) FWIW, we know that the more water soluble a substance is, the less likely it will traverse a lipid bilayer. To complicate things, nitrogen is more lipid soluble than helium. Theoretically, we have factors working for and against nitrogen being more "permeable" than helium. I guess one could argue that it comes down to the magnitude of each of those effects (something I haven't the faintest clue about).

Furthermore, as you pointed out, the tissue types being discussed are very complex. Whether in vivo or in an ex vivo preparation of the tissue in question, the gas would have to traverse non-uniform lipid bilayers, cytosol (intracellular substance), and extracellular matrix.

The reason I was curious about your statement is that you seemed to be applying your knowledge of gas permeability (behavior of gas particles in an idealized system) to a real-world, non-uniform biological substance (spinal cord). In my mind, any number in DCBC's cited reference would be in the realm of possibility.

 

[Bubbletrubble]

I am trying to figure out what mathmatical formulas one could construct where pressure, time and percentage of nitrogen is constant and volumn of gas exchanged is variable and end up with the same result.

@theduckguru: You're forgetting the biological context in which those measurements were collected. Gas laws are a lot of fun to play with. They make the physical world seem ordered and understood. They may be very helpful in creating mathematical models of biological phenomena and generating decompression algorithms. Unfortunately, sometimes we forget that those gas laws hold true only under specific idealized conditions -- conditions which are far removed from that of the microenvironment in/around human tissue.

 

[Puffer Fish]

@Puffer Fish: I have no idea how one could ever arrive at the measurements cited in DCBC's post...but let's assume that they are correct and focus on the topic that you brought up.

I was once taught that permeability of a cellular membrane to a substance may be influenced by at least 4 different factors: molecular size, charge, solubility in water, and lipid solubility. You brought up size already. It's been pointed out that nitrogen would exist in a diatomic covalently bonded state, whereas helium would be in a monatomic state. Based on molecular size alone, you'd think that nitrogen traverses the cell membrane slower than helium. We'll ignore the charge factor since the molecules have their outer electron shells filled, and it would be highly unlikely for them to exist in an ionized state. At 37°C, nitrogen gas is far more soluble in water than helium is. (In fact, I think one would be hard-pressed to find any single elemental gas that's less soluble in water than helium.) FWIW, we know that the more water soluble a substance is, the less likely it will traverse a lipid bilayer. To complicate things, nitrogen is more lipid soluble than helium. Theoretically, we have factors working for and against nitrogen being more "permeable" than helium. I guess one could argue that it comes down to the magnitude of each of those effects (something I haven't the faintest clue about).

Furthermore, as you pointed out, the tissue types being discussed are very complex. Whether in vivo or in an ex vivo preparation of the tissue in question, the gas would have to traverse non-uniform lipid bilayers, cytosol (intracellular substance), and extracellular matrix.

The reason I was curious about your statement is that you seemed to be applying your knowledge of gas permeability (behavior of gas particles in an idealized system) to a real-world, non-uniform biological substance (spinal cord). In my mind, any number in DCBC's cited reference would be in the realm of possibility.

Well written...and an excelllent overview of the issues.

I guess it comes down to whether the limiting factor is membrane trasport (cell walls) or solution limited (don't know of a generic term to discribe all of the various materials that fill the human body).

My assumption is that it would be more membrane limited could be wrong, but Trimix DCS tend to involve the spinal cord much more than air, which is a pretty good indicator that the number is not correct.

Given, as far as I know, that the capillary system in the human body is constructed somewhat the same (watch that be wrong), and all of the gas is delivered thru that system, I could easily see a measure of the spinal cord fluid showing numbers like that. But given the complexity of layers to get there, and the huge distance involved (compared to other areas) from the source, a number like that is possible, but it may not be valid in how much gas is present in the surrounding tissues.

I could also see that being true if the test was spinal fluid in a dish.. but doubt that is really relevent.

 

[Bubbletrubble]

Well written...and an excelllent overview of the issues.

I guess it comes down to whether the limiting factor is membrane trasport (cell walls) or solution limited (don't know of a generic term to discribe all of the various materials that fill the human body).

My assumption is that it would be more membrane limited could be wrong, but Trimix DCS tend to involve the spinal cord much more than air, which is a pretty good indicator that the number is not correct.

Given, as far as I know, that the capillary system in the human body is constructed somewhat the same (watch that be wrong), and all of the gas is delivered thru that system, I could easily see a measure of the spinal cord fluid showing numbers like that. But given the complexity of layers to get there, and the huge distance involved (compared to other areas) from the source, a number like that is possible, but it may not be valid in how much gas is present in the surrounding tissues.

I could also see that being true if the test was spinal fluid in a dish.. but doubt that is really relevent.

Just a few minor points...

• Cell "walls" do not exist in the human system.
• Regarding spinal cord involvement (Type II neurological hit) having a higher incidence in divers breathing trimix vs. divers breathing regular air, the clinical data are what they are. Still, I'll maintain that the measurements cited by DCBC could be correct theoretically depending on how the measurements were made. As you pointed out, a separate issue is whether the measurements are biologically relevant. And that's what we're all really interested in.
• I have issues with saying that certain hard-to-study processes are membrane- or solution-limited. It would take a bunch of work to make those statements. Armed with such data (and using your membrane/solution example), scientists would judiciously use qualifying terminology such as "the data are consistent with a membrane/solution limited process" or "it appears that the process is membrane- or solution-dominated." The qualifying language is important because it leaves some wiggle room down the road should other factors be discovered.
• Gas exchange predominantly occurs in the capillary beds. It is probably incorrect to assume, however, that all capillary beds throughout the body are equally perfused at any given point in time. I can imagine that certain physiological changes, e.g., exercise, might diminish peripheral arterial vasoconstriction to increase flow to/from skeletal muscle. These changes probably involve modulation of the sympathetic autonomic nervous system as well as downstream nitric oxide-mediated mechanisms.
• The cerebrospinal fluid (CSF) can be viewed by neurologists and neuropathologists as the "sewer" of the central nervous system (CNS). The CNS is composed of the brain and spinal cord and it's surrounded by CSF. If cells within the CNS release stuff and/or die, those materials are often released directly into the CSF. A convenient way to get a sample of this system in a living human is to do a spinal tap. In a normal, healthy individual, the CSF system is continuous between the brain and spinal cord. AFAIK, there's no way to isolate the CSF surrounding spinal cord from the CSF surrounding the brain. That's why I'm fairly certain that the measurements conducted on spinal cord tissue did not use CSF. If they had been conducted on CSF, I'd think that the data would have been listed as "CNS tissues" or "spinal cord/brain." If the measurements were conducted on spinal cord tissue in a dish (ex vivo)...once again, we'd all have to question the biological relevance of those numbers. I still haven't a clue how the researchers arrived at those numbers.