Does higher RMV cause higher DCS risk?

[Duke Dive Medicine]

Thanks.

NP. Does that help? You asked about CO2 as well. Were you thinking of someone who consciously increases his/her RMV?

Best regards,
DDM

 

[doctormike]

^^ . Listen to DDM, he's smart!

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.

 

[scubadada]

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

Thanks @Duke Dive Medicine

Do you have any thoughts or opinions about the Scubapro decompression algorithms, Buhlmann ZH-L8 or 16 ADT, that incorporate "human factors", breathing rate, heart rate, (and skin temperature) in the equation? Breathing rate and heart rate are to reflect workload.

 

[Alastor]

I actually don't know how they works, but my hypothesis is that in case the workload and skin temperature suggesto to raise the conservative factor they just do that dinamically during the dive.
Some computers allow to change the factor in the settings (cold water, obesity, poor sleeping etc), but once done it will not change during the dive.

 

[Duke Dive Medicine]

Thanks @Duke Dive Medicine

Do you have any thoughts or opinions about the Scubapro decompression algorithms, Buhlmann ZH-L8 or 16 ADT, that incorporate "human factors", breathing rate, heart rate, (and skin temperature) in the equation? Breathing rate and heart rate are to reflect workload.

I think that's the next frontier. As to exactly how they function, I'm with Alastor above in that I'm not well versed in how the computers actually account for markers of increased workload. I assume that they pad the algorithm somehow. It would be interesting to see hard data on how well they work.

Best regards,
DDM

 

[scubadada]

I think that's the next frontier. As to exactly how they function, I'm with Alastor above in that I'm not well versed in how the computers actually account for markers of increased workload. I assume that they pad the algorithm somehow. It would be interesting to see hard data on how well they work.

Best regards,
DDM

Thanks @Duke Dive Medicine

The Scubapro algorithms are proprietary, I have a feeling we will not know anymore about them unless they choose to release that information. Are you aware of data supporting how workload (heart rate and breathing rate) and/or skin temperature (taken by the chest strap) should specifically be used to modify a baseline decompression algorithm? It is my impression that the two Scubapro iterations of Buhlmann are already middle of the road in the conservative to liberal spectrum.

It is my impression that all available decompression algorithms are "safe" and that there is no data differentiating their relative safety. Some of these algorithms are more liberal and some are more conservative. I am skeptical that there will ever be data demonstrating superior safety of any of the available algorithms. My initial enthusiasm that DAN Project Dive Exploration might do so, has waned considerably. Study information has said to be coming soon for a long time.

Good diving, Craig

 

[Duke Dive Medicine]

Thanks @Duke Dive Medicine

The Scubapro algorithms are proprietary, I have a feeling we will not know anymore about them unless they choose to release that information. Are you aware of data supporting how workload (heart rate and breathing rate) and/or skin temperature (taken by the chest strap) should specifically be used to modify a baseline decompression algorithm? It is my impression that the two Scubapro iterations of Buhlmann are already middle of the road in the conservative to liberal spectrum.

It is my impression that all available decompression algorithms are "safe" and that there is no data differentiating their relative safety. Some of these algorithms are more liberal and some are more conservative. I am skeptical that there will ever be data demonstrating superior safety of any of the available algorithms. My initial enthusiasm that DAN Project Dive Exploration might do so, has waned considerably. Study information has said to be coming soon for a long time.

Good diving, Craig

Hi Craig, sorry, that's more a Neal Pollock question. Not sure if he still monitors SB but if he sees this maybe he'll jump in.

Best regards,
DDM

 

[boulderjohn]

that's more a Neal Pollock question

Speaking of that...

At about the 16 minute mark in this video, Neal Pollock says that breathing more brings on more inert gas loading. Now, he does say that in conjunction with exercise, but the only explanation he gives is that if you have better buoyancy control, you will be exercising less, breathing less, and taking on less inert gas. He does not mention perfusion.


Now, I would love a more complete explanation of why that would be true.

 

[Duke Dive Medicine]

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

 

[Kay Dee]

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 consistently 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 practitioner experienced with dopplers and diving and hyperbaric medicine, it was the Greek gas guzzling guy that constantly had the lowest doppler score. Make of that what you will, coincidence, or.............................?

Edit. Just for the record re above. Sea bottom is at 120m / 393ft, average depth for divers was approx 110m / 363ft with BT's of on average 20mins.