Impaired ventilation at depth due to gas density?

[Bobjr]

To me it seems strange the hydrostatic imbalance would not count into total work of breathing.

What goes up must come down?

So if you exhale easily because the counter lung is above you its harder to inhale - total net is zero. I would think peak work (and the timing of that) is more important than total work of breathing over the whole cycle.

 

[DrHarry]

Quick question for the medics.

...what's going on here to cause the twitchy hand syndrome that's not really seen in sat divers at these depths? Is it the speed of descent and lack of time to acclimatise to the gas mix and depth? or is the WOB and hydrostic imbalance contributing to the onset of HPNS?

Hi Mike. Our descent rates can be anywhere between 15-45m/min (the latter especially on deep wrecks). I have experienced the worst hand twitching on a 135m wreck dive where I bombed to the bottom and then literally could not clip a strobe to the shot line for about 5 mins until it settled. From chatting to other wreck divers about this phenomenon at these relatively shallower depths, I think it is a combination of HPNS and adrenaline. Maybe I am a more susceptible than others also.

Compare these descent rates to your sat divers who descend at ?1-2m/min.

Cheers

 

[Mike Ferguson]

Hi Mike. Our descent rates can be anywhere between 15-45m/min (the latter especially on deep wrecks). I have experienced the worst hand twitching on a 135m wreck dive where I bombed to the bottom and then literally could not clip a strobe to the shot line for about 5 mins until it settled. From chatting to other wreck divers about this phenomenon at these relatively shallower depths, I think it is a combination of HPNS and adrenaline. Maybe I am a more susceptible than others also.

Compare these descent rates to your sat divers who descend at ?1-2m/min.

Cheers

Pesky shallow 135m dives :yawn: If I descended to 135m at 45m/min I'd have trouble clipping my butt cheeks together.

Thanks, I hadn't really thought about the descent rate.

Compression Rate to depth of 135 msw for sat divers would be as follows :

0-10 msw @ 1 metre per min = 10 min
20 min stop @ 10 metres = 20 mins
10-135msw @ 1 metre per min= 125 mins
plus 120 min stabilisation @ 135 metres = 120 mins.
Total compression and stabilisation time before dive can commence is 4 hours 35 minutes

doesn't really compare.

 

[Simon Mitchell]

To me it seems strange the hydrostatic imbalance would not count into total work of breathing.

I don't think anyone said that hydrostatic imbalance did not potentially affect the work of breathing on a rebreather loop. I separated hydrostatic imbalance and work of breathing out because of their different physiological implications.

Paul, who works with an ANSTI machine is probably the better person to answer your questions. However, when testing a rebreather loop, if its position in the water, the gas density and the ventilation parameters are maintained constant then the work of breathing on the loop will be primarily affected by its geometry, hose diameter, mushroom valve design, scrubber design and material, elastance of the counter lungs and other things. To my understanding that is described by the area inside the curve in the diagram you posted (area 1). If you change the unit's attitude in the water and therefore change the hydrostatic balance, it may change the shape of the inspiratory and expiratory curves, and may change the net work of breathing, but you would have to appraise that for individual units. Moreover, I suspect that the factors I have listed are still more important in determining work of breathing.

One of the things you have to remember in evaluating your own experiences is that an ANSTI machine is not a human. Thus, a human may notice changes in work of breathing when the hydrostatic imbalance changes because of physiological changes in them rather than changes in the actual work required to move gas around the loop. This includes changes in the likelihood of dynamic airway compression (as we have discussed at length), congestion of the pulmonary circulation with blood which decreases lung compliance, and shifts in lung volumes so that ventilation is taking place on less favourable parts of the lung compliance curve. None of these things (which might make a human uncomfortable) are registered by an ANSTI machine.

Simon M

 

[Simon Mitchell]

doesn't really compare.

Yep, I think that's your answer Mike. There are pretty good data demonstrating that descent rate makes a difference.

Simon

 

[plazma]

What goes up must come down?

So if you exhale easily because the counter lung is above you its harder to inhale - total net is zero. I would think peak work (and the timing of that) is more important than total work of breathing over the whole cycle.

Human are not mechanical motors. You can not store and reuse the energy from breathing against hydrostatic imbalance.

 

[Bobjr]

Human are not mechanical motors. You can not store and reuse the energy from breathing against hydrostatic imbalance.

Humans are no different from mechanical motors - both are subject to the same forces. Nonetheless, I did say peak work was probably most important.

 

[paulraymaekers]

How much does hydrostatic imbalance increase the work of breathing?
It does increase the energy consumed for breathing, right?

Here is an edited image from Apoc tests.

Is the total work of breathing only the area of the lissajou curve (1)?

Or is the total work of breathing 1+2 during inhale and 2 during exhale?
Total 1+2+2.
Or something else?

To me it seems strange the hydrostatic imbalance would not count into total work of breathing. That curve could be even higher up from X-axis and still have the same lissajou curve surface area and therefore the same WOB. But if you breathe a unit with higher hydrotratic imbalance you can feel the extra work. You have to suck more during inhale and also use your muscless and work against the negative pressure (suction) during exhalation. Only during high breathing rate the exhalation can be done without work and let the suction do the job.

Is the lissajou curve misused in rebreather wob tests?

in general, WOB is the surface inside nr 1
hydrostatic imbalance is determined by the end of exhale/(end of inhale) pressure points, at no flow

now a misconception is that you normally breath from a rebreather where both the end of inhale/end of exhale are so far off the neutral/zero pressure line: divers don't do this, as it is very uncomfortable, divers will compensate by changing gas volume in the counterlungs, changing their position a bit etc, so that they get a 'sweet spot', trying to cross the zero-line between full inhale and exhale (if mechanically possible: for example not possible in an RB80 type breather with the counterlung on your butt, in vertical position)

in general rebreather design, sometimes the only orientation you have both endpoints at one side of the zero-line, is for example in a -90°, heads down position (feet up)
in most rebreather designs then the lung load is always negative

as Simon points out, humans feel hydrostatic imbalance different compaired to ANSTI machines

 

[gianaameri]

So are these ANSTI machine tests worth doing?

Sometimes feel RB manufacturers are in an "arm race" to win ANSTI WOB number on advertising! Often seems to lead to changes that users don't want, need & often inconveniences them.

I've dive an Inspiration to 120m & a Sentinel to 95m, the Sentinel has the better WOB figures according to an ANSTI machine. personally I felt that the Inspo breathed better, maybe because I had more confidence in the Inspo vs Sentinel (psychological possibly)!

To put in practice for safe diving the good theories of Dr. Mitchell, we need the ANSTI machine WOB results.

We can't always feel subjectively what is good or bad for us physiologically, and if WOB is too high for a planned dive we may die of CO2 retention.

For good planning and safe diving we need both Dr. Mitchell good theories and the machine WOB characteristics and limitations.

Psychology alone does not cut it when it comes to CO2 retention (unlike death by hypoxia, dying of CO2 I think is a horrible psychological event).

Sent from my HTC Desire C using Tapatalk 2

 

[ebt]

JP,

I think its just to try to get a proper apples v apples measure. Its a bit like 0 to 60 figures. They dont tell the whole story but are a great starting point and having them measured to a test standard is valuable.

Its good to see the actual ansti results being published by some manufacturers too.

 

[Fireman]

Would the benefit from going SCR be worth the added task loading? If one had an easy way of adding gas, perhaps venting every 3 or 5 breath would help?

 

[Mark Chase]

Would the benefit from going SCR be worth the added task loading? If one had an easy way of adding gas, perhaps venting every 3 or 5 breath would help?

SCR in the event of a C02 hit can only make things worse.

C02 hit because of scrubber failure? SCR = more C02

Co2 hit retained C02? No advantage and its likley to increase WOB from using ADV. Its also its unlikley you will be able to minimum loop flush because your breathing like a train and narked out of your head.


I have never had a C02 hit so maybe I am kidding my self with what id do, but id like to think my first responce to unexplained elevated breathing would be bailout on to a helium rich low PP02 gas on a high performance regulator.

Then go up as fast as possable using as little effort as I can IE hit the wing inflate.

Id only go back on loop if i didnt have enough OC gas to get out with.

ATB

Mark

 

[Simon Mitchell]

Simon, you calculated the gas density in one of your previous posts. Can you please show me the math to do this calculation.

And what are the desired limits and how did were they arrived at?

If this is too much of a sidetrack, I can start a new tread or PM Simon.

Hello Don,

Sorry, I didn't mean to ignore your question.

The starting point is the densities of the component gases in g/L at 1 ata.

Helium 0.18g/L
Oxygen 1.43g/L
Nitrogen 1.25g/L

Then, for each gas multiply the density at 1 ata x ambient pressure x fraction of the gas present in mix and add the products for all the gases together.

8g/L is often cited as the ideal maximum. The origins of the recommendation are a little hazy. It is the density of air at 50m (approx). I think it arose out of early work in the US Navy... but obviously it is a somewhat arbitrary line in the sand. I certainly don't think it should be greater.

1. Your comment that "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" made me think of some of the unexplained deaths in folks I have known. Facts are usually not known because the divers were solo, but I am wondering if dynamic compression or CO2 issues could have been involved in a few friends' deaths in the 35-45 meter range based on gas mix. More importantly, would there be any autopsy results that could indicate such issues? We find the COD is usually listed as "drowning," and is very frustrating.

Ken, same to you. Sorry to take a while to address your questions.

Yes, common problem. I think harry might have answered this, but it is impossible to tell at autopsy if CO2 played a role... unfortunately.

2. We have knows for decades about the use of positive end expiratory pressure (PEEP) in ventilating trauma victims to prevent alveolar collapse. Is this the same mechanism at work that you are recommending positive lung load instead of negative in your CL discussions?

Yes exactly... more like constant positive airway pressure (CPAP) but much the same thing in principle. As to building it in to a rebreather.... that would require some careful consideration. It is sort of achieved by having a front mounted lung in a horizontal diver, and there is evidence that this is a better configuration for situations where heavy exercise is a definite possibility or even expected (eg attack swimming in the military). However, there are lots of arguments for and against various counterlung positions, and while exercise capacity is obviously important, it is not the only consideration. I don't want to appear to be arguing against back mounted counterlungs (having used them for years myself).

Simon M

 

[ebt]

SCR in the event of a C02 hit can only make things worse.

C02 hit because of scrubber failure? SCR = more C02

Not sure I agree with that one mark, if your scrubber is on its way out, venting some exhaled gas via your nose would reduce the ammount of work the scrubber has to do.

Id only go back on loop if i didnt have enough OC gas to get out with.

Yep, this I'd agree with. Why SCR if you dont need to.

 

[Mark Chase]

Not sure I agree with that one mark, if your scrubber is on its way out, venting some exhaled gas via your nose would reduce the ammount of work the scrubber has to do.

Yep, this I'd agree with. Why SCR if you dont need to.

I have grave concerns about the concept of a scrubber being "on its way out."


That would sugest a well packed scrubber thats slowly burning through evenly due to over use.


So how many divers out there are doing 6-8 hours + on a scrubber to make this happen?

Onle ones i can think of are GUE bods running RB80s SCRs and using SCR as a way of extending scrubber duration.


To me if the average CCR diver gets a C02 hit its due in order of likleyhood:

1: Retained C02 due to WOB / workload or both

2: Dammaged or missing componant bad asembly etc

3: Channeling

10 Burnt through well packed scrubber


Like most I was taught if i thaught I had a C02 event i should dill flush as a first responce????? I see this as insain. If your scrubber or unit has failed to scrub C02 to that point its not going to magicly heal its self now. The only benifit would be to reduce the C02 getting through, your not going to stop it.


So rather like decoing out on 80% you trying to get rid of C02 dispite having some (all be it a little) in the inspired gas your trying to flush with.

Seeing as C02 is cumlative in the system I can see no situation apart from a lack of bailout OC gas, where this is an advantage or in any way the prefered option.


Then thers the whole issue of flushing the gas.

Possably i am doing it wrong but for me that involves quite a lot of lung work that i wouldent want to be attempting during a C02 event.

Drawing gas through a ADV on units like the KISS JJ etc is very bad news.

Manualy flushing via dill inject button sounds great but you end up with over inflated counterlungs you need to vent again to make them breathable (more lung work) and you will tear through diluient that in 99% of cases will be vitle wing inflate gas so if you loose all of it, you will have boyancy issues to add to the drama.


All whilst narked out of your skull on massivly narcotic C02?


Which will make you paranoid / panic / act iraticly / make bad decisions.


God forbit i should have a C02 hit deep on a dive and i went SCR, i would no doubt find little or no instant releife. This would make me parinoid that it wasn't working which in its self would make me breath faster due to panic.


The only way i see for me to be 100% sure I was on the best possable solution to my situation would be on a known safe OC gas.


SCR would come into play if i ran out of said OC gas, and not before.


Obviously just an opinion, but hopfully i have made my thaught process clear.


ATB

Mark

 

[GaryL]

Meg boys I know where taught "open loop" (breath in/out to use ADV as OC reg) which uses diluent but bypasses the scrubber entirely.

The ADV is on the exhale lung...

 

[Mr Walker]

The ADV is on the exhale lung...

Really? Is that to make it harder to trigger the ADV or is it because the dil needs to be run through the scrubber for some reason? :sneeky:

Kidding aside, what are the actual motive(s) behind this design?

/nils

 

[Mark Chase]

I regard SCR as a gas extended to minimize use off deep bailout to get back to a shot line only. Is it any use if you've run out of OC gas?

Meg boys I know where taught "open loop" (breath in/out to use ADV as OC reg) which uses diluent but bypasses the scrubber entirely.

OC bailout gas is a 7ltr or 10ltr off board with 270bar of trimix in.

If that runs out I have to go SCR on inboard 3ltr dill till i can ascend to the next deco gas.

I never get in with less than 150bar dill so worst case ill have about 120bar dill to play with as i use about 50bar dill per dive and most of that is wasted in the wing on ascent.


ATB

Mark

 

[Mark Chase]

Yes, your right, more importantly for me it works on an Inspo!

It's why I don't shut-off ADV until I start the ascent (was taught to shut off at bottom of shot on various courses!). Don't use a BOV might help me get a breath or two while I fumble for my bailout reg

Where ever the ADV is you will still be breathing through the scrubber unless you breath in via Adv and exhale arrond the mouthpiece rather than pushing the inspired gas back through the loop.

How long is your dill cylinder going to last like that?

Lets say a mild C02 hit 30-40lpm SAC at 70m thats 320lpm out of a 3ltr with 120 bar in? so? Less than 60 seconds? Going SCR with fresh gas every forth breath? so 240 seconds (4mins) asuming no wasted gas.

Will you be able to control your breathing enough to switch to OC when its near empty?

Are you confident runnig dry suit for ascent boyancy control, presumably on off board gas?

I just look at the realiaty of SCR on CCR inboard tanks and it dosent add up for me as a responce to C02.

SCR folowing a total loss of electronics and zero panic? Yes i can see that working on my normal 15 SAC but if i can keep to 15 SAC i have a shead load of bailout gas (planned on 25) so why risk it?


SCR has a place in CCR diving and its last place, when all other options are gone.


ATB

Mark

 

[Mark Chase]

Use SCR on ascent? For me ascent is OC only....




Yes, exhale thru nose, breath in to fire ADV, exhale thru nose...... only the ADV, DSV & corrugated hose between them involved. Just a few breaths until I'm on bailout reg.

I'd rather avoid CO2 issues by not pushing scrubber (have done in past), not pushing loop EADD above 40m & not been a working diver at depth smacking hell out of the wreck with a lump hammer!

I know you will never agree but I find your confidance in being physicly able to go OC within a few breaths to be badly misplaced. I base that on on the experiances of divers arround me who have suffered a C02 event.

When Tim had his (on a Vision) he aparently just staired at the OC reg in his hand, decided he just couldent do it and hit his wing inflate button instead.

His was caused by a failed exhale mushroom valve.

How would a failed mushroom valve on the exhale work with your open loop concept?

ATB

Mark