Does higher RMV cause higher DCS risk?

[doctormike]

You know what they say. If something happens once, it's "in my experience". If it happens twice, it's "in a recent series". If it happens three times its "case after case after case".

 

[Duke Dive Medicine]

Hopefully my input here wont end up at the same point we got to in another thread in this neigbourhood, but here is a real world 'result' relating to said question, that is I am simply reporting a fact, not imputing a so-called 'hypothetical scenario'. Make of it it you will but it is a fact. What it means though is not mine to deduce.

On Kevin Gurr's HMHS Britannic 98 Expedition there was a Greek diver who had a very high sac rate and consistantly arrived at his first deco stop with his doubles sucked dry to the extent of being concave (well, maybe not concave, but you now what I mean i.e. e m p t y). He also did a pretty good job of draining his deco gas too. Needless to say KG chewed him out several times for this practice.

However, when all bottom divers were dopplered post dive after every dive by a qualified registered medical practioner experienced with dopplers and diving and hyperbaric medicine, it was the Greek gas guzzling guy that cosistantly had the lowest doppler score. Make of that what you will, coincidence, or.............................?

I think this is great. Like Mike said, one case doesn't prove an entire concept but this one does illustrates the large individual difference in propensity to bubble.

Best regards,
DDM

 

[boulderjohn]

John,

Though Neal may not have mentioned it, perfusion is an integral part of that mechanism. Exercise increases heart rate, blood pressure, and tissue perfusion, which will increase nitrogen uptake.

Best regards,
DDM

Correct, but that is not what he said. He said merely breathing more gas is what does it. Is that what he means?

 

[Duke Dive Medicine]

Correct, but that is not what he said. He said merely breathing more gas is what does it. Is that what he means?

John,

That's not how I heard him. Also, if you look at his graphic, exercise is the component that he's stressing.

Best regards,
DDM

 

[Alastor]

All those things led us to a couple of thoughts:
1) the actual model is just ideal: if exercise bring in more inert gas, the model should account for that in some way - simple pressure gradients and tissue parameters are not enough. Maybe the algorithms will not change dramatically if they'll change at all, but we need a scientific evidence and reason of the difference of this inert intake.
2) Everyone has its own off-gassing "ability" - it will be phisiological for the majority of the phenomenom, but, if evidence of the previos point will come out, the diver behaviour could take part of the process too.

 

[doctormike]

All those things led us to a couple of thoughts:
1) the actual model is just ideal: if exercise bring in more inert gas, the model should account for that in some way

You are assuming that moving more inert gas in and out of the lungs causes more inert gas to be dissolved in the blood and tissues. Those are two different things.

2) Everyone has its own off-gassing "ability" - it will be phisiological for the majority of the phenomenom, but, if evidence of the previos point will come out, the diver behaviour could take part of the process too.

The most important concept in medicine is the fact that there is a bell curve for everything. Yes, people have different RMVs but they also have differences in lung function, in left to right shunting, and in susceptibility to tissue injury. One person might bubble more than another but be at less risk for DCS if their tissue inflammatory response to bubbles is less pronounced.

Individual variance is something that is very difficult to correctly model, because we don't know what variables to include.

 

[Alastor]

Sorry, maybe I was misleading. I prefer the actual theory: volume of gas in and out of our lungs do not affect sensibly the gas dissolved in the blood and tissues more that the effect from partial pressures gradient and time.
I did that thoughts because someone with a bit of authoritativeness is saying that intake is potentially affected from the stress in not negligible way.

 

[admikar]

Since this is hypothetical, I'll take a try. In OP, everything is the same, heart rate, blood flow, perfusion level....
Like @fmerkel said, there is finite amount of gas that can be transferred through alveoli, for given level of activity. Therefore, if you breathe more, you're basically flushing gas that is not saturated/desaturated as much as possible. You can intake as much gas as you want, but there will not be more intake of inert gases (at constant depth).
Look at it like this: you have a large container with small exit pipe on the bottom. If that container is full of water, it will exit at some predefined volume through that pipe. You can put in more water than goes out, but container will overflow, you will not get more water through that pipe.

 

[Schwob]

Since this is hypothetical, I'll take a try. In OP, everything is the same, heart rate, blood flow, perfusion level....
Like @fmerkel said, there is finite amount of gas that can be transferred through alveoli, for given level of activity. Therefore, if you breathe more, you're basically flushing gas that is not saturated/desaturated as much as possible. You can intake as much gas as you want, but there will not be more intake of inert gases (at constant depth).
Look at it like this: you have a large container with small exit pipe on the bottom. If that container is full of water, it will exit at some predefined volume through that pipe. You can put in more water than goes out, but container will overflow, you will not get more water through that pipe.

I like your analogy, .... if it holds any water ...
(pun intended)
Thats sort of how I pictured it, simplistically...

Thanks to all. Interesting!

And yes, my example is an intentionally oversimplified... step #1 to lead to understanding...

Of course, in the real world there usually is a real reason that causes the higher RMV. Also interesting, but another question:

Like in my case, the example sort of pictures me towards the beginning of the dive and me in the last 1/3rd of the dive. I seem to have some issue with something, maybe some sort of covert anxiety.
(and of course if that sort of thing still falls into the "all else being the same" bracket or what does anxiety (that you don't even seem to notice... if that's ehat it is) actually do to you... that's another interesting question...)

It always seems to take me 1/2 the dive plus to settle around an RMV of 0.5 ... If I don't watch it, I may start the dive as bad as 1.0 ish and then it slooowly (asymptotically) it settles by mid to two thirds through the dive.
If I try hard to rest at the surface prior to descent and to zen out... breath calm, not worry about the buddy... etc... I may start the dive at 0.75 and then it slooowly will settle at around 0.5 (or even a tad below when I do good, 0.6 when not so much) by mid to 2/3rds into the dive. Haven't figured it out yet. Watching it. I seem to not hyperventilate (short breath), just breath a lot, but deep, with "pressure"... Sometimes I catch myself feeling that chest pressure when inhaling so deep, no more goes in.... and it seems to (if I don't watch it) cause me to breathe out way faster than I need to, ... so I can get more in again... or maybe so I don't screw my buoancy... not sure...(it's different from concious deeper breath to rise over something, it's really, deep breath that just happen...)

When I watch it, I do better, but still, at the beginning of the dive and part ways into it I "waste" a lot of air... always...
But at least, so it seems, I may not do much additional harm other than shortening my dive ... and worse, my buddies ... but I am not really increasing my risk for DCS (maybe... 'cause the real world reason behind it all still evades me).

But, if I could find that (mental) button to do better there, I sure would tweak it... working on it...

 

[rossh]

John,

Though Neal may not have mentioned it, perfusion is an integral part of that mechanism. Exercise increases heart rate, blood pressure, and tissue perfusion, which will increase nitrogen uptake.

Best regards,
DDM

Hi,

Can you point to any testing that answers that assumption, or some data that answers the general question being raised here - does a higher workload require a higher deco time?

That would be a fundamental component of all decompression model calibrations. It should have been easlily tested and shown. But models do not come with a limit based on workload. So does this workload / deco time change really exist, in any minor or major way?

It would be such a trivial thing to test for - one group is worked hard while another sits still, and then compare the results.

How does the real life experience stack up? Do we see any real difference in the case reports? Do scooter divers get a free pass for deco time and injury, while the self propelled diver has a higher rate of injury? Similar question - do the newbie / nervous divers get bent more than their experienced confident buddies? Do we see more injuries with divers swimming against the current?

These are good questions for the PDE data we need answered.


We know through existing science reports, that the effects of cold has a dramatic effect on the deco result. Neal Pollock has a presentation on this. But this must not be confused with the exercise level question being asked here. However, cold soak and heat loss is intrinsic in most dives to some amount, so it may be difficult to separate the effects of cold from exercise.

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