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.
rossh:
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.
rossh:
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.
rossh:
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.
rossh:
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.
rossh:
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…
rossh:
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.