Does higher RMV cause higher DCS risk?

[rossh]

Hello Ross,

In recent on-line discussions with me you have on many occasions quoted the standard equations used to track gas uptake and elimination from tissues. I presume, therefore, that you know what the symbol "Q" represents because that is your answer right there.

To the OP,

Breathing more than you need to makes very little change to the alveolar inert gas composition, and would not of itself be expected to materially change gas uptake or the risk of DCS.

In contrast, since it is the blood that carries dissolved gas to the tissues, anything that increases tissue blood flow (perfusion) during the process of gas uptake will result in faster uptake of inert gas into tissues. Thus, if the increase in breathing was precipitated by exercise (which increases cardiac output and tissue perfusion) then it will be associated with faster inert gas uptake and (all other factors like ascent rate etc being equal) greater decompression stress.

The same principle applies during decompression - improved tissue perfusion will accelerate inert gas washout whereas merely hyperventilating would not make any difference. That is why gentle exercise is sometimes recommended during decompression. It is also why the prebreathe protocols for de-nitrogenation prior to space walks incorporate exercise. The astronauts wash the inert gas out more quickly.

Hope this helps,

Simon M

You are adopting the "perfusion limited" concept of on /off gas levels. So by using that explanation and interpretation of human tissue gas kinetics, we should see clear trends within in the DCS history and case reports. It would show that dives that involve more exercise, but use the same levels of deco time, would have more prevalent injury rates. Is that the real world case? Probably not. So what does that say about "perfusion limited" on / off gas levels concepts?

Astronauts are not a good example for diving or normal circulation. They float around with no effort or motion of the extremities for hours and days. Their circulation issues are unique to the environment. NASA - Your Body in Space: Use It or Lose It

For divers, a big potion of the problem is warding off the (proven) effects of cold, and our most common method is motion and exercise in water to restore/improve circulation of heat. But that situation cannot be confused / substituted with the perfusion limited on gassing concept.

Above I asked DDM to show some definitive testing or results to back up this concept of "perfusion limited" levels of on/off gas levels in diving. Same goes for you Simon. But none has been provided yet.

.

 

[boulderjohn]

Hello Ross,

In recent on-line discussions with me you have on many occasions quoted the standard equations used to track gas uptake and elimination from tissues. I presume, therefore, that you know what the symbol "Q" represents because that is your answer right there.

To the OP,

Breathing more than you need to makes very little change to the alveolar inert gas composition, and would not of itself be expected to materially change gas uptake or the risk of DCS.

In contrast, since it is the blood that carries dissolved gas to the tissues, anything that increases tissue blood flow (perfusion) during the process of gas uptake will result in faster uptake of inert gas into tissues. Thus, if the increase in breathing was precipitated by exercise (which increases cardiac output and tissue perfusion) then it will be associated with faster inert gas uptake and (all other factors like ascent rate etc being equal) greater decompression stress.

The same principle applies during decompression - improved tissue perfusion will accelerate inert gas washout whereas merely hyperventilating would not make any difference. That is why gentle exercise is sometimes recommended during decompression. It is also why the prebreathe protocols for de-nitrogenation prior to space walks incorporate exercise. The astronauts wash the inert gas out more quickly.

Hope this helps,

Simon M

Simon--would this be be an acceptable summary of your post?

Increased exercise causes increased blood flow, and this will have an impact on both ongassing and offgassing. It is also likely to cause an increase in the rate of breathing. However, it is the increased blood flow that is causing the difference, not the increased breathing rate. An increased breathing rate by itself does not have a significant impact upon ongassing and offgassing.​

 

[Dr Simon Mitchell]

Simon--would this be be an acceptable summary of your post?

Increased exercise causes increased blood flow, and this will have an impact on both ongassing and offgassing. It is also likely to cause an increase in the rate of breathing. However, it is the increased blood flow that is causing the difference, not the increased breathing rate. An increased breathing rate by itself does not have a significant impact upon ongassing and offgassing.​

Exactly - I wish I could have expressed it as clearly!!

 

[scubadada]

Hello Ross,

In recent on-line discussions with me you have on many occasions quoted the standard equations used to track gas uptake and elimination from tissues. I presume, therefore, that you know what the symbol "Q" represents because that is your answer right there.

To the OP,

Breathing more than you need to makes very little change to the alveolar inert gas composition, and would not of itself be expected to materially change gas uptake or the risk of DCS.

In contrast, since it is the blood that carries dissolved gas to the tissues, anything that increases tissue blood flow (perfusion) during the process of gas uptake will result in faster uptake of inert gas into tissues. Thus, if the increase in breathing was precipitated by exercise (which increases cardiac output and tissue perfusion) then it will be associated with faster inert gas uptake and (all other factors like ascent rate etc being equal) greater decompression stress.

The same principle applies during decompression - improved tissue perfusion will accelerate inert gas washout whereas merely hyperventilating would not make any difference. That is why gentle exercise is sometimes recommended during decompression. It is also why the prebreathe protocols for de-nitrogenation prior to space walks incorporate exercise. The astronauts wash the inert gas out more quickly.

Hope this helps,

Simon M

Simon--would this be be an acceptable summary of your post?

Increased exercise causes increased blood flow, and this will have an impact on both ongassing and offgassing. It is also likely to cause an increase in the rate of breathing. However, it is the increased blood flow that is causing the difference, not the increased breathing rate. An increased breathing rate by itself does not have a significant impact upon ongassing and offgassing.​

Exactly

Thanks Simon and John, the original question has been answered quite clearly

 

[Schwob]

Thanks Simon and John, the original question has been answered quite clearly

I concur. Thank you all!

 

[rossh]

Thanks Simon and John, the original question has been answered quite clearly

You are all assuming that on/off gassing is limited by perfusion. But none has provided any evidence, test data, report, field data, case report summary, or anecdotal indications of this assumption. It's been assumed it must be true, based on what ...?

In the results or the case reports, I see no evidence or implied suggestions that different perfusion rates has a dramatic / any effect on the dive outcome .

.

 

[Dr Simon Mitchell]

You are adopting the "perfusion limited" concept of on /off gas levels.

Ross,

This is what you had to say about the very same "perfusion limited concept" on another forum:

Most existing models all follow the same basic gas tracking formula, including VPM-B and ZHL. There is no error, there is no problem to solve, nothing needs to be fiddled with. Of course if you don't like that, then you have to invalidate every model, every plan and every dive computer... because they follow the same basic formula... millions of dives.

But now you are asking questions / making statements which seem to suggest that you don't actually understand the formula, or don't agree with it, or ???.

So by using that explanation and interpretation of human tissue gas kinetics, we should see clear trends within in the DCS history and case reports. It would show that dives that involve more exercise, but use the same levels of deco time, would have more prevalent injury rates. Is that the real world case? Probably not.

We have known since the earliest days of decompression research that exercise at depth increases the risk of DCS. Try:

van der Aue OE, Kellar RJ, Brinton ES. Surface decompression: derivation and testing of decompression tables with safety limits for certin depths and exposures. NEDU Report 5-45. Panama City, Florida, US Navy Experimental Diving Unit: 1945.

That study showed a doubling of the rate of DCS for work dives compared to rest dives. There are others I could cite. If you go to our pathophysiology chapter in Bennett and Elliott they are listed there.

So what does that say about "perfusion limited" on / off gas levels concepts?

Well, you can figure that out for yourself, but can I point out that in posing this question you are directly contradicting your advocacy for the basic gas tracking formula that you explicitly state your own model is based on.

Astronauts are not a good example for diving or normal circulation. They float around with no effort or motion of the extremities for hours and days. Their circulation issues are unique to the environment. NASA - Your Body in Space: Use It or Lose It

OK (not that I agree with any of that), but how about U2 pilots whose exercise enhanced denitrogenation has been studied at 1 ATA in the presence of normal gravity. Reduces DCS by 35 - 40 % according to Andy Pilmanis.

Pilmanis A. Altitude DCS risk mitigation - U2 practice. In: Bennett PB, Michaelson R, Butler F (eds). Best practice guidelines for prevention and effective treatment of decompression illness proceedings: Part 1. Durham NC; Undersea and Hyperbaric Medical Society. pp87-122. 2010.

Above I asked DDM to show some definitive testing or results to back up this concept of "perfusion limited" levels of on/off gas levels in diving. Same goes for you Simon. But none has been provided yet.

Well it has now.

Simon M

 

[Dr Simon Mitchell]

You are all assuming that on/off gassing is limited by perfusion. But none has provided any evidence, test data, report, field data, case report summary, or anecdotal indications of this assumption. It's been assumed it must be true, based on what ...?

In the results or the case reports, I see no evidence or implied suggestions that different perfusion rates has a dramatic / any effect on the dive outcome .

Ross,

I just wanted to capture this. I look forward to our next discussion of the "standard gas tracking formula" if we ever have to go through the pain of another debate about decompression.

Simon M

 

[Kevrumbo]

Trying to learn something. Indulge me:
In this example:

If all else is the same, same sex, same age, same health, same percentage of body fat, same height, same weight, same ethnicity, same level of hydration, same fitness level, same lung volume, same muscle mass, same distribution thereoff, same amount of sleep, same amount of average alcohol consumption, etc.,
exact same serious of dives under exact same conditions, ..., etc...
...
But person 1 has an RMV of 0.5 cf/min and person 2 has a 50% higher RMV at 0.75 cf/min...

Does person 2 actually, somehow metabplize more gas?
If yes, how?
Or is it all just extra gas blown through the system, not metabolized and wasted?
In either case, does person 2 have a higher risk of DCS?
If yes or no, why?

Would / could one diver potentially more CO2 in his system?
Which one?
Why?

Which is the bigger risk here, DCS or CO2 hit? Why?

. . .
The issue of CO2 retention is complex, and hypoventilation can cause this. However, remember that hypoventilation is a relative term. A diver with a lower RMV has that for a reason. Your RMV comes from your body's own physiological parameters, including lung volume, CO2 production, CO2 transport efficiency, etc... So it doesn't stand to reason that a person with a baseline RMV of 0.5 CFM is blowing off CO2 half as efficiently as a person with an RMV of 1.0.

Another thing to consider is that even for a given diver who is changing their ventilation (due to workload, anxiety, etc...), tachypnea (faster breathing) isn't necessarily the same thing as hyperventilation. Fast shallow breathing means that a greater proportion of your total gas movement is "dead space ventilation", meaning that you are moving air in and out of your trachea and large airways but that ventilation doesn't contribute to CO2 blowoff because that exchange takes place in the alveoli (the little air spaces at the end of the airways that are surrounded with blood vessels separated from the gas space by a thin membrane). So a diver breathing quickly and moving more air in and out of his airways may actually be expelling CO2 even less efficiently than someone with slow deep respiration.

The DCS issue settled above, a greater more patently immediate risk in this instance at depth is metabolic CO2 retention. As @doctormike points out, RMV of itself isn't a predictor of efficient CO2 elimination. There are a lot more dynamic immersion physical effects and individual physiological reactions-interactions-factors to consider along with RMV.

For example, look at this simple graphic demonstration of work-of-breathing, exercise, and ambient pressure differential just by trying to talk and tread water at the surface at the same time:
Imagine the gross effects at depth exercising and breathing Nitrox 21 (Air) through a regulator at ten times the ambient pressure in the video above. . .

Good additional explanation here:
Advanced Knowledge Series: Basic Carbon Dioxide Physiology
Advanced Knowledge Series: Carbon Dioxide Retention
Advanced Knowledge Series: The Gas Density Conundrum

 

[Schwob]

...
Good additional explanation here:
Advanced Knowledge Series: Basic Carbon Dioxide Physiology
Advanced Knowledge Series: Carbon Dioxide Retention
Advanced Knowledge Series: The Gas Density Conundrum

Thanks & thanks for links.