Impaired ventilation at depth due to gas density?

R

Raptor^

Member
I saw a video on Vimeo the other day of some deep cave exploration, and a while in I immideately came to think of the presentation from Rebreather Forum 3.0 by Simon Mitchell called CCR Physiology I saw on the DAN youtube channel.

RF 3.0 - CCR Physiology:
w w w . youtube . com/watch?v=GW1yVFF_FK4

If you watch the presentation by Simon Mitchell first, you find at appox. 21:00 he starts explaining more in detail one of the problems with gas denity in the loop on really big/deep dives that can cause the wind pipe to compress. In an effort to relive this the diver tries to cough, and you can see/hear an example of this from 21:20. The compressing of the wind pipe causes CO2 retention in the body due to impaired ventilation. They found coughing to be present in Dave Shaws video from his fatal 270m dive; with Dave trying to cough gas out and ventilate while/due to having a massive CO2-hit.

The cave exploration video, which is quite spectacular, is from the Pearse Resurgence:
w w w . vimeo . com/39746265

From 7:00 to 9:20 in the video from Pearse Resurgence, when the diver on the dual MEG ties in and runs line from 198m to 221m, you can hear him coughing several times.

I was wondering if the coughing that is caused by this compression of the wind pipe and the coughing that was present in Dave Shaws video, is the same/similar to the coughing in the video from Pearse Resurgence? If so, was the diver in the Pearse Resurgence video building CO2 at a dangerous level due to gas density and problems with ventilation? And if that is/was the case, how close to passing out or being impaired by the CO2 are you or the diver in the video when/if this happens?

I cannot embed or post url's so the links look the way they do. Remove the spaces while copying/pasting the links into a browser window.
 
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Craig Challen and Dr Harry were the two divers in that video, they presented a lecture about it at oZtek with a comparison chart of gas density to respitory effectiveness... It was very interesting... I'll ask Dr Harry if he cares to elaborate more on here.
 
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Hi all, happy to discuss that video and the 2 dives depicted by myself (diver 1) and Craig Challen (diver 2).

I edited the video to demonstrate several issues of deep diving, including the issue you are describing in the opening post. To put the dives in context, they represent where our team of 6 (The Wetmules :-)) are up to on the exploration of the Pearse Resurgence in New Zealand. The cave is a vauclusian spring with mild flow at depth, with water temps of 43F (6 C). The limit of exploration is currently 221m with a horizontal penetration of roughly 300m. Some more videos here ww w. wetmules.com /home/ pearse-resurgence

Due to the depth, the cold and the need to traverse horizontal cave passage at depth, we are completely paranoid about CO2 on rebreathers in this site. One of the early explorers (Dave Apperley from Sydney) had a nasty CO2 hit at 120m. Craig had one when he started sinking in a chimney at 194m because he started finning instead of just waiting for his inflator to do the work (actually he had a bit of trouble finding his inflator quickly in his dry gloves).

Anyway to answer your question there are a couple of significant things you can hear in the video. The flapper valves are really loud due to the gas density. And the end of expiration in both divers (but especially Craig) often ends in a grunt or a cough. Simon's talk and slides demonstrates the issue of effort independent respiratory failure very eloquently and I believe we are seeing some of the early hallmarks of the problem in this video.

To summarise the issue - as gas density increases, resistance to flow increases and so a greater effort is required to inhale and expel the same amount of gas each breath. If you don't maintain the same volume and rate, then you will start to accumulate CO2 (the main determinant of CO2 levels being ventilation and production). Inhaling is mostly not an issue because as you work harder to inhale against the increasing resistance, the calibre of the airways increases (they are dragged open because they are essentially "tethered" to the chest wall). But the opposite occurs with exhalation. Increasing force of exhalation causes greater occlusive pressures on the airways until at some point the pressure around the airway is greater than that of the gas inside the airway...and the airway is compressed. No more gas can be expelled no matter how hard you try. In fact, the harder you try, the more obstructed they become. In this situation you have limited options. If you understand what is happening, you might consciously slow your breathing down and take long relaxed breaths in and out. This strategy is very effective in patients we see with upper airway obstruction (who have the opposite problem where inhaling hard sucks their airway closed). If they calm down, they feel better. Another analogy is a baby with croup whose breathing or cough gets worse when they cry or get upset. BUT...it is very hard to relax at 200m when you feel like you cannot get enough breath, you have a line wrapped around your leg and the viz goes to sh1t! And once it gets bad, you enter a viscous cycle of rising CO2, increased anxiety and attempts to increase your minute ventilation...all producing a worsening of the problem.

Now there is one other physiological response which may help in the early phases, and that is to try and splint the airways open at the end of exhalation (when the airways maximally close). This can be done by coughing or grunting and this is not something we are even aware we are doing for the most part. I believe that is what you are hearing in the video.

Positive static loads generated by the rebreather may also assist i the same way, hence over the shoulder counterlungs may be better in this situation that BMCLs. Rolling on your back in a BMCL unit could help in theory, but I think you are better of using your time and energy pointing your scooter for home if things are getting that bad.

Open circuit bailout may help break the cycle due to the significantly lower equipment resistance offered by OC compared with CCR. But you need a lot of gas at 200m. Changing to a second rebreather clearly will not help in this situation. So my strategy is one of OC bailout, head for shallower water ASAP to lower the gas density rapidly before the OC runs out.

Craig and I both consider it vital to try to perform everything in slow motion at depth, concentrate hard on breathing patterns, use the scooter and never fin.

The bottom line is physics is not your friend at these depths, and such diving cannot be recommended :cuckoo:
 
Great reply DrHarry! Can't beat information directly from the source. Thank you for taking the time to elaborate :)
 
Very observant! 5/90 was the dil we planned but actually mixing gas that accurately is not that easy...hence the slight discrepancy.

From a WOB point of view heliox would be ideal but the issue of HPNS is very real at these depths. The video doesn't show it that well (look at the bit where I am doing the first tie off and then where I am trying to get my Z knife out) but it was quite disabling on my dive. Based on my pretty rapid descent and moderate symptoms, Craig did a slower descent but of course paid the price with a substantially longer deco. However we feel that is a safer way to go...accepting that the dive will be long and pretty much taking as much time you need. This "slow down" approach takes some pressure off and removes the instinct to rush which is dangerous from the respiratory point of view discussed earlier.

Actually the EADD for 5/90at 220m is only 42m according to my software so it makes me wonder what other factors are at work at these depths...
 
DrHarry said:
Very observant! 5/90 was the dil we planned but actually mixing gas that accurately is not that easy...hence the slight discrepancy.

From a WOB point of view heliox would be ideal but the issue of HPNS is very real at these depths. The video doesn't show it that well (look at the bit where I am doing the first tie off and then where I am trying to get my Z knife out) but it was quite disabling on my dive. Based on my pretty rapid descent and moderate symptoms, Craig did a slower descent but of course paid the price with a substantially longer deco. However we feel that is a safer way to go...accepting that the dive will be long and pretty much taking as much time you need. This "slow down" approach takes some pressure off and removes the instinct to rush which is dangerous from the respiratory point of view discussed earlier.

Actually the EADD for 5/90at 220m is only 42m according to my software so it makes me wonder what other factors are at work at these depths...
Click to expand...



I read and interesting snipit on Heleox deco but i dont know the credabuility of the author

Brief History Of Mixed Gas Diving



ATB

Mark
 
Christ I just watched the video and when the last diver went past 200 and started coughing i was just thinking drop the bloody reel and get the hell out of there. Even though I knew it had a happy ending I was on the edge of my seat.

That realy was a close one, memories of dave Shaws last moments all over again.

Amazing dive but please dont do it again. At least not with those CCR on that gas.

ATB

Mark
 
Hi Mark, sorry for stupid question, but why not with those CCR and that gas?

Sent from my PAP4500DUO using Tapatalk 2
 
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Naumachos said:
@ Dr. Harry : the setting of the Predator in salt water is served to decrease the co given the lesser depth of calculation or is there another reason ?
Click to expand...

I am not sure what salinity the Predator was set on. I was doing my deco and measuring depth off 2 x Liquivision X1s, using Gradient Factors. The X1s were set to fresh water.

I am not sure this can even be altered on the Predators?

Harry
 
DrHarry said:
Actually the EADD for 5/90at 220m is only 42m according to my software so it makes me wonder what other factors are at work at these depths...
Click to expand...

Hi Harry,

Yes, excellent question. I doubt there are any new factors... just a confluence of old ones. You were not in the same territory as David Shaw.... he was 40m deeper with a denser gas. I calculate your gas density at 220m as 6.8 g/L whereas David's was about 10.1 g/L. The recommended maximum is sometimes cited at 8 g/L.

For our purposes (Harry I know you know this, but it is for the benefit of everyone) there are 2 key factors that contribute to the development of dynamic airway compression and therefore impaired ventilation of the lungs during a dive: gas density and flow rate of gas through the airways. As both rise, dynamic airway compression becomes more likely and once it occurs, you won't be able to increase your ventilation any more. If you think about it, that means that as gas density rises the amount of work you can achieve falls (because work requires increased ventilation with higher gas flow through the airways in order to get rid of the CO2 produced by the work).

The Wet Mules have learned to pay attention to both sides of this equation by minimizing gas density and trying very hard not to work in the deeper portions of their dives. This is a winning formula, which has enabled them to do the dives they do. But they are still coming close to the combinations of work and gas density that promote dynamic airway compression, and because they are very introspective and make videos of themselves, they notice it and we (the watchers) can notice the warning signs (the grunting or coughing at the end of expiration). So why do they not get into the same terminal spiral that Dave Shaw did? It will be because of the fact that because the gas is less dense, the dynamic airway compression is happening at higher flow rates (remember the relationship I described above) and they are still in a zone where stopping / resting / lowering activity can make a difference. Put another way, the gas density is still low enough that they can generate enough flow through the airways to ventilate off their resting CO2 production and maybe a bit more, so resting works. David on the other hand was so deep with such dense gas that he may have been doomed from the moment he arrived there.... once his CO2 started to rise and drove him to try to increase flow, he was unable to get it high enough even to eliminate his resting CO2 production, so resting or doing little would not work. Hope you can see what I am getting at.

David was also wearing a back mounted counter-lung, and if you have a negative static lung load the risk is almost certainly increased.

So, in respect of Harry's question.... why are rebreather divers who use nitrox between 40 and 50m not getting into trouble with dynamic airway compression, especially when working hard. I think the answer is that they probably do. However, like Harry's dives, this is still at a gas density where resting can probably make a difference. So when divers notice themselves getting short of breath they slow down or rest which in most situations works. But they are probably not far from big problems if the gas got any denser or they tried to work any harder. I think "deep air diving" on a rebreather is not a smart move, and even between 40 and 50m divers using nitrox in a rebreather should be very work-shy. I'm sure there will be unexplained rebreather deaths in these intermediate depths where dynamic airway compression, respiratory limitation and CO2 toxicity are major factors.

Finally, I have not even touched on the fact that you don't need dynamic airway compression and limitation of ventilation to get into significant CO2 problems (independent of scrubber failure), because of disturbance of normal respiratory control. But this post is getting too long so that can be a topic for another day. It is covered in that presentation that the OP linked to.

Simon M
 
Some great points and thanks for clarifying Simon.

I think Simon's points emphasise the psychological or behavioural part of the equation. If you are at 50m in OW and you feel a bit short of breath, it is no big deal to relax and slow down without real concerns for the decompression implications, and you are not really thinking about the "David Shaw phenomenon" at that point.

If the same situation occurs at 200m in a cave, there is an overwhelming sense of worry about the impact on deco if you stop for a bit and you are VERY mindful of the potential for the respiratory issue. But with equivalent air density depths much of it is in your mind and so you MUST stop, breathe, think and act the same as you have been trained to do in other settings.

Having said all that I have a strong sense that despite the EADD being the same as a 42m dive, the sounds in that video are not what I would expect to hear on a 42m air dive on CCR. Anyone got a recording we can listen to?

Harry
 
Fascinating topic and discussion. I went back and watched again Simon's RF3 presentation as well as watched Harry's video again, and it really brings things to life for me. I occasionally find myself coughing while diving CCR in deepish water (nothing like Harry's dive mind you!), and I had always assumed that it was due to scrubber material, allergies or slight chest cold or other, but now I am starting to wonder if WOB, gas density and static lung loading are playing a part. This will certainly cause me to pay more attention to what is going on with my body during a dive.

Thanks so much for sharing all of this information. It is highly valuable!

Regards,
Randy
 
Thanks all for the good discussion.

It is not clear to me why negative lung loading would be worse then positive lung loading.
I always learned that to avoid getting into a spiral of CO2 intoxycation, you must be able to ventilate your deeper alveoles.

Now it seems that positive lung loading works against a complete full exhale, while negative lung loading makes exhaling easy, but inhaling mork difficult.

so...?

Please note that this is not a pro or contra agains OTS or back-mounted, as both types of counterlungs allow a sweet spot in orientation where the lung loading is neutral, or can even be shifted from positive to negative. Just for correct understaning
 
Hi Paul. As a manufacturer you would know far better than I how back mounted and front mounted CLs compare in formal WOB tests. My understanding is they both have slightly different sweet spots but overall they are not too different, and perhaps other parts of the rebreathers are equally or even more important overall?

However in this very specific instance, a small amount of positive pressure at the end of exhalation (say 10cmH2O) can help prevent dynamic airways closure and therefore improve ventilation. In fact, it can improve lung compliance in the diver himself by maintaining lung volumes in a more favourable part of the pressure-volume curve. We see this is asthmatics who have trouble exhaling...the last thing you would think to do is adding more pressure for them to exhale against but sometimes it can help (in fact they do it to themselves with a phenomenon called auto-PEEP).

Of course I am not sure that this theory has been formally tested but it seems to wrk in other areas of applied respiratory physiology. I am sure NEDU would have looked at this and no doubt Simon will have some references :-)
 
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