Does higher RMV cause higher DCS risk?

[admikar]

Back to our diving world. Where are the injured divers that show some connection to excess activity in the water? When was the last time we read something like ".... I swam real fast on the bottom, and now I have a DCS..." It doesn't happen.

Or how about ".... the divers who choose to swim against the current on this dive site, have more DCS injury ....". Never heard that one either.

Maybe, just maybe, because all models are made with some safety clearance?

If on/off gassing was limited by perfusion, then those statements would be true... but they are not.

Topic of this thread was does and how much RMV influence on/off gassing. If blood flow is twice as fast, twice as much of inert gas will be transported (simplistic I know). So, in this example, perfusion is limiting factor.
For some reason, you just choose to ignore OP, and go on tangent here....

 

[Dr Simon Mitchell]

Ross,

This particular forum is one where divers come to ask questions about decompression physiology from experts, and they expect factual answers. I have provided exactly that.

I am incredulous that you would come on here and claim that exercise and tissue perfusion has no relevant effect on inert gas uptake and decompression risk when no one in diving science agrees with you, and virtually everything written on the topic says the opposite to what you claim.

Existing model gas kinetics theory and formula, follow the concept that the individual tissue is the limiting component of the uptake / off gas rate, as represented by a tissue half time value set. Its the balance of partial pressures against the tissues density and its ability to absorb excess inert gas, represented by a half time value.

Yes, gas tracking formulae use half times, but you omitted to mention tissue perfusion as a crucial influence on half time. All other things being equal the greater the perfusion the shorter the half time. If you increase perfusion and gas is carried to the tissue more quickly, it will accumulate gas more quickly (just as Admikar says above). So, while I agree with your comment about half times, it seems that you don’t fully understand what influences them, and this is causing you to draw incorrect conclusions.

That is the basic theory you will find in all current model gas tracking, in use in (VPM-B, ZHL, VVAL and more). These parallel tissue models do not support a "perfusion limit" in their calculations.

This is simply wrong. The majority of models use gas tracking calculations for a series of parallel perfusion limited compartments. Here is a quote from an account of inert gas exchange in hyperbaric conditions [1] published in an authoritative peer reviewed physiology reference work:

The most commonly used tissue model is the single, well-mixed compartment in which perfusion is considered the rate-limiting process, tissue and venous partial pressures are in equilibrium, and arterial-tissue inert gas partial pressure difference declines monoexponentially. Since the tissue sites relevant to DCS are unknown, it is common to model uptake and washout of inert gases in a collection of such compartments in parallel (i.e., no gas flow between compartments), each with a different time constant spanning some range thought to encompass all relevant tissues.

There are examples of approaches that are different, but most of those used by recreational divers (including ZHL) conform to this description.

Your sample science reports are not supportive to the perfusion limit concept. The Nedu one from 1945, is using very old (fast) tables sets, and was exploring the introduction of surface decompression (SUR-D) procedures. The test involved aggressive deco, and all SUR-D is an abuse of the deco ceiling anyway.

Who cares if the decompression was aggressive? Indeed, if you are trying to compare the effect of strategies like rest versus exercise at the bottom on DCS risk, you NEED aggressive decompression strategies that result in an high incidence of the outcome of interest so that you can see a difference if there is one. In many respects this was the PERFECT setting for this evaluation. In this case exercise at the bottom produced more DCS than rest at the bottom. It is totally relevant and exactly the evidence you were asking for.

The U2 pilot sits in a pressure suit and is strapped in tight to an ejection seat. He likely has issues with circulation being limited and cutoff, and maybe his suit pressure was too low for the flight duration anyway. So where is the connection to increased / limited perfusion and increased off gassing, in someone at rest flying an aircraft, versus a diver? - none.

You were disputing that exercise and perfusion could alter tissue inert gas kinetics, and I have provided you with incontrovertible published proof from exercise enhancement of denitrogenation in both astronauts and U2 pilots that exercise does exactly this.

Back to our diving world. Where are the injured divers that show some connection to excess activity in the water? When was the last time we read something like ".... I swam real fast on the bottom, and now I have a DCS..." It doesn't happen.

Or how about ".... the divers who choose to swim against the current on this dive site, have more DCS injury ....". Never heard that one either.

Ross, those of us who actually treat sick divers hear stories like that all the time and it is widely recognised by diving physicians that a history of hard work during a dive is a factor that can be added to the diagnostic mix when trying to decide whether a diver has DCS or not. I will certainly be teaching this to the attendees at the NOAA diving medicine course in Seattle on Saturday. Other experts who will be there like Jim Holm and Neal Pollock will not disagree with me.

Speaking of Neal (who you generally quote positively), this is what he wrote on the subject in a recent article on the DAN site.

Juggling Physical Exercise and Diving — DAN | Divers Alert Network — Medical Dive Article

Physical activity during the dive also has a direct impact on decompression safety. (4 references cited) Exercise during the compression and bottom phase increases inert gas uptake, effectively increasing the subsequent decompression obligation of any exposure. (my emphasis)

Which puts this comment of yours into perspective…

What ever amount of on/off gassing changes occur from exercise on the bottom, it does not seem to accumulate to anything that makes a difference. Models are correct to ignore this effect, as its just not big enough to make a difference.

I think the truth is that models have little choice but to ignore this effect because they cannot predict how hard a diver will exercise on any given dive. However, divers themselves should be properly educated on the matter (which you are not helping here) so that they can pad in some compensation for hard working dives, and maybe computers that attempt to monitor physical activity by some means will one day be sophisticated enough to help.

Simon M

Reference 1: Doolette DJ, Mitchell SJ. Hyperbaric conditions. Comprehensive Physiol. 2011;1:163-201.

 

[rossh]

For some reason, you just choose to ignore OP, and go on tangent here....

I have not ignored anybody. The assumption has been put forward that our tissues gas uptake is being limited by perfusion alone, and I challenge DDM to verify that.

Maybe, just maybe, because all models are made with some safety clearance?


Topic of this thread was does and how much RMV influence on/off gassing. If blood flow is twice as fast, twice as much of inert gas will be transported (simplistic I know). So, in this example, perfusion is limiting factor.

Yes, models and planning does come with an extra margin for error and variances. However, if exercise level (and therefore increased perfusion) was a real issue, then it would be the leading cause of injury, and figure heavily in the case reports.

Where is the identifiable trend line in case reports, with a common exercise component? It does not seem to exist. Tech and Rec divers are a diverse and varied lot, with millions of random exposures. If exercise was a prevalent feature of extra on gassing and subsequent injury, then it would show up.

Our experience shows that on/off gassing rates / quantities, seemingly are not controlled by perfusion limited. Instead we appear to be limited by tissue absorption rates, exactly as the models are designed and operate.

.

 

[Storker]

I was under the impression that it's well established that both exercise and thermal stress during bottom time, and during decompression, has a profound impact on the DCS risk. Would it be sensible to ascribe at least a part of this phenomenon to differences in tissue perfusion? After all, both exercise and thermal stress affect tissue perfusion quite strongly.

 

[Capt Jim Wyatt]

I was under the impression that it's well established that both exercise and thermal stress during bottom time, and during decompression, has a profound impact on the DCS risk.

Me too.

 

[rossh]

I was under the impression that it's well established that both exercise and thermal stress during bottom time, and during decompression, has a profound impact on the DCS risk. Would it be sensible to ascribe at least a part of this phenomenon to differences in tissue perfusion? After all, both exercise and thermal stress affect tissue perfusion quite strongly.

The direct effects of thermal stress, in isolation, are tested and proven, and was found to be quite a dramatic effect. The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness.

The direct effects of extra exercise, as an individual component of decompression stress, and its changes to gas load, do not appear to be well tested or verified, or calibrated to any known value.

You can imagine that every model calibration might be affected by exercise levels, and each model maker would be interested in such variances. But models do not come with any limits or caveats along those lines. The model maker seems to have skipped this part.

Such an important component - exercise in a dive and how it affects gas uptake, and its an unknown quantity.

All the things that Simon posted above... there is nothing definitive there. In Neal Pollock's references, still nothing definitive.


There is a great deal of assumption that "in dive" exercise has a dramatic effect on gas uptake and therefore required deco, but the case reports and history simply do not indicate this as a cause. If "perfusion limited" on/off gassing was a realistic and determining value, then we would see some indication of this in the DCS reports of individuals. But its not there. So what does that say about the "perfusion limited" concept?

Of course the whole cold/exercise subject gets convoluted, because increasing thermal stress acts by restricting perfusion. But that does not automatically imply that normal perfusion was the limiting on gas factor beforehand.


Models and their internal gas tracking, are not designed as perfusion limited. The basic sequence of mono-exponential gas kinetic formula, uses the partial pressure of gas versus the tissue absorption rate. This rate is represented with a half time value.

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[Duke Dive Medicine]

I have not ignored anybody. The assumption has been put forward that our tissues gas uptake is being limited by perfusion alone, and I challenge DDM to verify that.

I never said that, nor do I think I implied it. What I said was that exercise on the bottom increases tissue inert gas uptake and hence the probability of DCS. I provided one reference written by some of the best diving physiology minds in the business, and Simon has kindly provided several others. Please correct me if I'm misinterpreting, but you seem to be assuming that the only thing that exercise does is increase tissue perfusion.

Best regards,
DDM

 

[Duke Dive Medicine]

But wouldn't a longer deco help to reduce that probability of DCS?

The way I understand it, and I am by no means an expert, is that deco algorithms are designed with a safety margin in them to account for individual physiology and the possibility of a higher workload on the dive. A larger workload during a dive affects where you fall in the safety margin, in other words moves you closer to the DCS/ no DCS line, which is different for each person and for each day. A longer deco would move the diver more into the safe zone, and seems like a good precaution to take if you feel like the workload may have been greater than what the designers of the algorithm considered typical for an average dive.

Do you think that at some time the deco algorithms will evolve to the point that a dive computer would learn your baseline heart rate or SAC and alter your deco based on that data? for example: if your HR is x% of your baseline then your deco time in increased by y%.

Maybe there are too many factors that affect your DCS risk. Is this idea like trying to measure with a micrometer when you are cutting with a chainsaw?

Yes, a longer deco after heavy work on the bottom would help reduce the probability of DCS. Commercial dive companies and the US Navy will bump up their deco when the diver has worked hard. It's subjective and a bit of a blunt instrument as you pointed out, but it's the best we have for now. At some point we may be in a place where we can exactly predict how much a diver's individual probability of DCS increases with a measured increase in workload but we're nowhere near there yet. There are too many variables, as the ongoing discussion has elucidated.

Best regards,
DDM

 

[Storker]

exercise on the bottom increases tissue inert gas uptake and hence the probability of DCS

That means I've understood it correctly (to the best of my abilities, at least). Thank you.

Now, since we know that both physical activity and cold stress affect perfusion, would it be a reasonable hypothesis to assume that perfusion at least plays a role in both situations?

 

[Duke Dive Medicine]

That means I've understood it correctly (to the best of my abilities, at least). Thank you.

Now, since we know that both physical activity and cold stress affect perfusion, would it be a reasonable hypothesis to assume that perfusion at least plays a role in both situations?

It would be reasonable to assume that, yes. Measuring it and quantifying its effects is something else.

Best regards,
DDM