Does higher RMV cause higher DCS risk?

[Duke Dive Medicine]

Hi Ross,

Vann et al (2011) quote one of the NMRI data sets:

Temple DJ, Ball R, Weathersby PK, Parker EC, Survanshi SS. The dive profiles and manifestations of decompression sickness cases after air and nitrogen-oxygen dives. Volume I: Data set summaries, manifestation descriptions, and key files. Bethesda, MD: Department of the Navy, 1999 NMRC 99-02(Vol.I)

I don't think this says that heavy work requires longer deco time, just that it results in a higher probability of DCS. I'll leave integrating that into a deco algorithm to the experts; that's above my level of knowledge and training.

Best regards,
DDM

 

[Jack Hammer]

I don't think this says that heavy work requires longer deco time, just that it results in a higher probability of DCS.

I've heard similar things said before and find it interesting. To your knowledge has any research focused on this?

 

[Sol518]

I don't think this says that heavy work requires longer deco time, just that it results in a higher probability of DCS.

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?

 

[Sol518]

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...

Can we make the assumptions that two people could be in every way identical, except for their RMV? It seems like there would have to be something that affects the respiratory drive. Anxiety, or efficiency underwater, or some difference in metabolic rate or blood chemistry which could affect both the RMV and DCS risk.

 

[Schwob]

Can we make the assumptions that two people could be in every way identical, except for their RMV? It seems like there would have to be something that affects the respiratory drive. Anxiety, or efficiency underwater, or some difference in metabolic rate or blood chemistry which could affect both the RMV and DCS risk.

Granted.
But in a thought example constructed for the explicit purpose of learning / understanding something, we can allow ourselves to leave reality behind, accept the theoretical example a little until we understood the concept and then apply the learned to the real world, if and as applicable... under consideration of its realities.

 

[boulderjohn]

Can we make the assumptions that two people could be in every way identical, except for their RMV?

The difference could be something as simple as the way they breathe. Doing the same dive side by side, one diver may breathe fairly rapidly in and out, while another takes long, slow, lingering breaths. Some divers come very close to the border of skip breathing. Some cross the border.

 

[Sol518]

Granted.
But in a thought example constructed for the explicit purpose of learning / understanding something, we can allow ourselves to leave reality behind, accept the theoretical example a little until we understood the concept and then apply the learned to the real world, if and as applicable... under consideration of its realities.

I agree, but in the discussion it seems like we are trying to isolate a variable that can't be changed without changing other parts of the equation. It makes sense to me that a change to RMV alone does not cause a change in inert gas uptake.

The difference could be something as simple as the way they breathe. Doing the same dive side by side, one diver may breathe fairly rapidly in and out, while another takes long, slow, lingering breaths. Some divers come very close to the border of skip breathing. Some cross the border.

I guess what I'm asking, and maybe one of the MDs can chime in, would the differences in the way they breathe cause any other changes in their body that could be a factor in their DCS risk? How does it affect heart rate, blood chemistry, work of breathing? Could it hypothetical cause you to burn more calories, or cause a slight dehydration, because you are warming and humidifying a larger volume of air?

 

[scubadada]

This has been an interesting thread.

I keep going back to Neal Pollock's figure representing decompression stress. Much of the content of the video posted earlier in this thread is also included in an article in Alert Diver, Fall 2016 Alert Diver | The Many Factors in Decompression Stress

Dr Pollock, in the video and in the article, stresses a very important point, "Even for situations in which all other factors work against decompression safety, if the dive profile does not generate significant decompression stress, DCS will not occur." Following the dive profile, there are several variables divers have some control over, reasonable hydration, thermal protection to prevent undue cold during ascent and stop off-gassing, and perhaps, to some degree, exercise intensity.

Other than as a measurement of exercise intensity, I find it unlikely that RMV contributes independently in a significant way to the risk of DCS. I will continue to concentrate on the 3 or 4 variables that are under my control.

 

[RainPilot]

A quick thought experiment, all numbers made up:

Superman goes diving. He saturates with nitrogen at depth and comes up to a shallow deco stop.

He is breathing air at 10m so 2 ATA, PPN2 of inhaled gas is 1.6. His tissues are loaded with N2 at eg 3 ATA.
This means that there is a 1.4 bar gradient so he will offgas at a specific rate.

Now he holds his breath for an hour. (thats why I used Superman...)
During that time, the PPN2 in his lung-gas and his tissues will gradually come to equilibrium until eventually he wil stop offgassing.

If he flushes his lungs continually by breathing REALLY fast, the gradient will remain at its highest value thus he will offgas optimally.

I believe that this idea of exhaling a nitrogen rich mix and replacing it with fresh gas, thus keeping the gradient high, is where the "common sense" part (@boulderjohn ) says that a higher SAC should lead to faster off gassing.

However, I believe that in any normal human range of breathing, the effect would be too small to notice, thus the experts' answer of "No, it doesn't matter".

 

[Dr Simon Mitchell]

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?

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