Does higher RMV cause higher DCS risk?

[Schwob]

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?

 

[boulderjohn]

I asked the same question more than a decade ago.

The answer has always been that no, it does not matter. That is the opposite of common sense, though, so you will get some arguments.

What matters is the gradient between the nitrogen pressure in the air you are breathing and the pressure in your tissues, and the rate at which you inhale that and exhale that gas is not a factor. It is not like you are filling up a tank or a tire.

 

[Gareth J]

Remember, oxygen is metabolised, the rest isn't. One of the results of metabolisation is CO2, which is exhaled. The other gases in the breathing gas are absorbed by the blood and tissues.

At the surface, those other gases are not an issue.
Diving, they they are absorbed into the tissues, how much is a product of time and depth (ambient pressure). It is also a product of gas flow.
So high work loads (more gas consumed), increase the inert gases in the body, increasing the amount of inert gas absorbed.
Decompression algorithms, work with an assumed workload. Increasing the workload (gas consumed/absorbed) can take you outside the table.
Efficient of off gassing is also effected by efficient breathing and circulation. Cold water interfere with the circulation, and therefore effects off gassing.

The more modern dive computers are more punitive in low temperatures, where high gas consumption is measured (air integrated computers).

Also CO2, is a potential Narcosis trigger. So if you are generating a lot of CO2 your susceptibility to Narcosis could be increased. Some people are natural CO2 retainers, some are more efficient, I don't believe this is a product of gas consumed.

So on that basis, I would say those consuming large volumes of gas are more likely to get an in table bend.
The CO2 issue is more difficult - there may not be a relationship between CO2 and gas consumption. Unless the individual is failing to breath properly (hyperventilating), where there would be an increased risk of CO2 build up.

 

[boulderjohn]

So on that basis, I would say those consuming large volumes of gas are more likely to get an in table bend.

Let me be sure I understand--you are saying that a high RMV leads to more gas because the diver is working harder and thus having more perfusion. I agree with that.

Are you also saying that if the person just breathes more while doing the same work and having the same blood flow it would make a difference?

 

[fmerkel]

A lot of breathing, to be specific "ventilating" is a waste. Air goes in, a substantial portion of the same air goes out. At the level of respiration (actual gas exchange across membranes), the spaces are very very very small. There is little 'air flow' so much as Brownian motion moving molecules of air around. But amazingly the process is quite effective, in a large part because the effective surface area of gas exchange is somewhere around the size of a tennis court. All the rest of it is just tubing.
As you go deeper, the air becomes more dense, and the problems of moving denser air around in very very very small spaces becomes exacerbated. This is why.....slow....deep....breathing is even more effective the deeper you go.

If you breathe faster, you are mostly just flushing the tubing more. But doing so also lowers the CO2 level. It doesn't make much difference in O2 or N2. Depending on your inherent metabolism, fitness, dive experience, and a host of other factors, different folks will have different RMV. It's not JUST the lungs, the cardiovascular system and everything it's feeding are every bit as important to the whole equation.

There is no simple answer to your question.

 

[Schwob]

...If you breathe faster, you are mostly just flushing the tubing more. But doing so also lowers the CO2 level. It doesn't make much difference in O2 or N2. Depending on your inherent metabolism, fitness, dive experience, and a host of other factors, different folks will have different RMV. It's not JUST the lungs, the cardiovascular system and everything it's feeding are every bit as important to the whole equation.

There is no simple answer to your question.

All:
Remember the specific (constructed) example is that ALL ELSE is the same - including workload & metabolized gas. EVERYTHING other than RMV is identical.

@fmerkel
Thanks. That's closest to rhe kind of answer I thought I should expect. Just not sure.

So, is the diver 2 (in that constructed example, not in real live) with the higher RMV then safer in regards to a CO2 hit, because he vents better and (within the constructed example) at no additional risk of DCS?

(That's what I would have thought would be the logical answer... is it?)

If so, again, just within that example, could one then also conclude that the more anxious diver, for as long as he dives safely within his gas supply, is no more likely to get a DCS hit, but less likely to get a CO2 hit?

Or is that violating my own example (all else ither than RMV = just more venting) is maybe not the case anymore if more anxious.... maybe the brain draws more "power" ??? ... Don't know...

 

[guruboy]

I havent heard of any deco algorithm taking SAC/RMV into account for NDL.

 

[Duke Dive Medicine]

This topic comes up on SB every once in a while. Higher RMV alone would not appreciably affect DCS risk. Respiratory minute volume is autoregulated in order to maintain tight physiologic parameters. A smaller person may have a lower RMV than a larger person, but their blood gas values (pH, arterial partial pressures of oxygen and CO2, etc) would be similar if they're both healthy. The argument could be made that if one measures the average partial pressure of inert gas at or near peak inspiration it would be higher for a given time period in someone with a higher respiratory rate (all other things being equal), but the dwell time at or near peak inspiration would be shorter as well. It's also worth reiterating Gareth's point, which is that increased RMV usually accompanies increased workload, and increased work at depth definitely does increase the risk for DCS.

Best regards,
DDM



Best regards,
DDM

 

[Schwob]

This topic comes up on SB every once in a while. Higher RMV alone would not appreciably affect DCS risk. Respiratory minute volume is autoregulated in order to maintain tight physiologic parameters. A smaller person may have a lower RMV than a larger person, but their blood gas values (pH, arterial partial pressures of oxygen and CO2, etc) would be similar if they're both healthy. The argument could be made that if one measures the average partial pressure of inert gas at or near peak inspiration it would be higher for a given time period in someone with a higher respiratory rate (all other things being equal), but the dwell time at or near peak inspiration would be shorter as well. It's also worth reiterating Gareth's point, which is that increased RMV usually accompanies increased workload, and increased work at depth definitely does increase the risk for DCS.

Best regards,
DDM



Best regards,
DDM

Thanks.

 

[Alastor]

Let's think in this way: tissues absorb inert gas according with the partial pressure gradient and the time.
If you breath more gas (let's imagine more frequently, or just having larger lungs), it will not change the partial pressure, so it will not affect the amount of inert gas dissolved into tissues.
That's why high RMV does not affect DCS risk.
Using the same tank compared to another diver with lower RMV, the dive would last less, so in that case, higher RMV would even reduce DCS risk shortening the exposure, but this is an indirect effect, not related with the lungs volume neither with the breath frequency.