Apocalypse Rebreather

[ruiner]

That's totally a big day! WELCOME HOME!!!

Did you receive an O2 version or what? It would be nice to see some pictures and hear first impressions if you did. They should have been shipping the O2 version for two months now, yet no one has claimed to have received one....

 

[M_Bipartitus]

If I were them, I wouldn't announce units are shipping until after all the early adopters received units and were diving them, or no one will believe the units are actually shipping.

 

[jimmy71]

are they again claiming units as shipped or about to be shipped or "make sure your mailbox is big enough because it is about to arrive"?
I have gotten exhausted trying to keep up with the realities if this CCR ever becomes a reality and have not kept up with the latest scuttlebut.

 

[Dr Simon Mitchell]

You know in the past 3 odd years, I have heard a lot of negatively about what DL have said and done. BUT, I haven't yet seen anyone actually publish proof that their R&D is wrong or in error.

Try:

Ineson A, Henderson K, Teubner D, Mitchell SJ. Analyser postion for end tidal CO2 monitoring in a rebreather circuit. Diving and Hyperbaric Medicine 2010 (December edition).... I cannot recall the page numbers now but will post them later.

This paper proves a specific flaw in the logic of the CO2 detection system as it was described and debated on line by Alex Deas several years ago. That is not to say that the system has not evolved into something that has eliminated all concerns now, but no one will know that until it is independently tested.

Simon M

 

[Brad_Horn]

G'Day Simon,

Many thanks, will go have a read. pg209-209

Since it would appear that the CO2 detection in the Apoc et al HAS been independently audited and tested through the CE certification process to both 61508 and 14143:2003 standards and been found to work, how will that change the paper?

Kind regards
Brad

Try:

Ineson A, Henderson K, Teubner D, Mitchell SJ. Analyser postion for end tidal CO2 monitoring in a rebreather circuit. Diving and Hyperbaric Medicine 2010 (December edition).... I cannot recall the page numbers now but will post them later.

This paper proves a specific flaw in the logic of the CO2 detection system as it was described and debated on line by Alex Deas several years ago. That is not to say that the system has not evolved into something that has eliminated all concerns now, but no one will know that until it is independently tested.

Simon M

 

[ruiner]

Ineson A, Henderson K, Teubner D, Mitchell SJ. Analyser postion for end tidal CO2 monitoring in a rebreather circuit. Diving and Hyperbaric Medicine 2010 (December edition).... I cannot recall the page numbers now but will post them later.

It is ANZCA journal right? Is the journal available somewhere online? It would make an interesting reading. I don't think Deep Life has published any algorithm how the end tidal CO2 is measured. Under "design validation" in deep life web site there is respiratory rate sensor document which says "The RR sensor is used directly to monitoring the diver for the effects of CO2, and is also used as a compensating parameter in determining end of exhale CO2"

I saw your interview where you said that shallow breaths makes the end-tidal co2 measurement from end of the hose impossible. I would suspect that DL is using some algorithm with RR as parameter to compensate for inaccuracy that is caused by the hose. I can't find any documentation or patent application which would describe any details of that compensation.

It's too bad that original idea "open source approach" for Apocalypse has been diluted to just open documentation....

 

[Dr Simon Mitchell]

G'Day Simon,

Many thanks, will go have a read. pg209-209

Since it would appear that the CO2 detection in the Apoc et al HAS been independently audited and tested through the CE certification process to both 61508 and 14143:2003 standards and been found to work, how will that change the paper?

Kind regards
Brad

Hello Brad,

Even if recent tests revealed that our concerns have been dealt with adequately, it would not change the paper at all. If you read my post again more carefully, the simple study we conducted was a response to comments made by Alex on line almost 2 years ago. If any problem that we debated then has subsequently been fixed that is great.... but it does not change the paper. Having said that, I would be only too happy to publish favourable results in a second paper if given the opportunity to test the actual unit (see below).

That brings me to your claim that CO2 detection in the APOC has been tested to the standards you quote. That is all well and good if that testing addresses the potential inaccuracy we highlighted. So here is a simple question Brad, and all it requires is a simple categorical yes, no, or don't know:

Has CO2 been measured using human subjects and properly calibrated analysers deployed simultaneously at the mouth and pod (end of hose) with the latter compared to the APOC pod reading over a range of tidal volumes from 500ml to 2000ml?

If so, show me the data.

If not, your comment about testing is completely irrelevant to this discussion. In addition, I will once more in public repeat my offer to conduct this testing with you present, and my promise to publish the results be they favourable or unfavourable to the APOC. I would like them to be favourable, because we all want to see this system work. All you need to do is provide me with a moutpiece, hose and CO2 pod. What could be fairer than that?

Simon M

 

[Dr Simon Mitchell]

It is ANZCA journal right? Is the journal available somewhere online? It would make an interesting reading. I don't think Deep Life has published any algorithm how the end tidal CO2 is measured. Under "design validation" in deep life web site there is respiratory rate sensor document which says "The RR sensor is used directly to monitoring the diver for the effects of CO2, and is also used as a compensating parameter in determining end of exhale CO2"

I saw your interview where you said that shallow breaths makes the end-tidal co2 measurement from end of the hose impossible. I would suspect that DL is using some algorithm with RR as parameter to compensate for inaccuracy that is caused by the hose. I can't find any documentation or patent application which would describe any details of that compensation.

It's too bad that original idea "open source approach" for Apocalypse has been diluted to just open documentation....

Hello Ruiner,

I'm not sure that "impossible" was the word I used, but difficult for sure. Our study shows that at low tidal volumes (small breaths) a direct "end of breath" measurement at the end of the hose is potentially very inaccurate. Clearly DL believe they have a way of compensating for this, and hopefully they do. However, it has not been demonstrated to me. Moreover, I suspect the testing Brad referred to did not actually address the problem we have raised.

The journal is the combined publication of the South Pacific Underwater Medicine Society and the European Underwater and Baromedical Society. It has been approved for listing on Medline early this year but only abstracts will be available on line. I would be happy to send you a pdf of the article if you pm me.

The reference is:

INESON A, HENDERSON K, TEUBNER D, MITCHELL SJ. Analyzer position for end tidal carbon dioxide monitoring in a rebreather circuit. Diving Hyperbaric Med 40, 206-209, 2010

Simon M

 

[Oskar Frånberg]

Hi this is my first post on this forum. But since I find this subject interesting I can’t resist replying.

Simon I read your paper and found it informative, if I read it correctly the biggest error is with small tidal volumes where the difference with measuring in the mouthpiece and at the end of the exhalation hose is about 1 kPa (95% CI 0,9-1,18) and with the larger tidal volumes the error is smaller 0,2 kPa (95% CI 0,19-0,23).

In the newly released document FAULT STUDY: CO2 BYPASS IN REBREATHER MOUTHPIECES Deep life does disclose a method for estimating what they call Peak end of exhale CO2 which I am quoting below.
“The Deep Life CO2 monitor computes a peak end of exhale CO2 from:
1. Direct measurement of the Volume Weighted Mean Expired CO2 (VWAE CO2) at the
scrubber inlet, minute average O2 flow and Respiratory Rate (RR)
2. Determination of RMV from N * surface minute volume of oxygen metabolised + 2,
where at the surface, N is 26.6 on the surface.
3. Calculation of tidal volume, as Tidal volume = RMV / RR
4. Conversion of the mean CO2 to peak exhaled CO2 by a linear transform correcting
for dead space using Tidal Volume. ”

As for the first point, I do not know the exact flow path but if one is to directly measure volume weighted mean CO2 one needs substantial mixing and I have a hard time seeing how that could be done in a breath by breath fashion.

The second point of determining the RMV from the oxygen consumption. As my professor use to say, all things physiological vary, the reference Deep life is giving is referring to a medium work rate in air at surface. If we look at what the work physiologist Åstrand had to say the spread is much larger and especially with low or high work rates. Looking at divers, they are exposed to a completely different environment and that does effect the breathing tremendously. It is not only the obvious work of breathing but also the hyperoxia and that many divers change their breathing pattern by just immersing in water, Morrisson and Reimers in Bennett and Elliott’s 3ed suggest that the spread is 15-30 liters of ventilation to every liter of oxygen consumed, but they also mention that observations as low as 10 has been recorded.

Continuing to number 3, if the diver is breathing with a ventilation equivalent of 15, with one liter per minute of oxygen consumption and with 20 breaths per minute respiratory rate. Then the calculated tidal volume would be 1.43 L and the real would be 0.75 L, quit a substantial difference.

Number 4 the linear transformation is to be conducted as cited below,

“If Tidal volume > 300 Then
End of exhale CO2 = Mean_CO2 * Tidal volume / (Tidal volume – 0.230) * √2 “

I do not know if this algorithm holds true, I have never seen it before and if it does hold it would be a slight revolution in allowing us to place the CO2 sensors away from the mouthpiece. But I have some concerns not only to the math itself but also that the constant 0,230 refers to the volume of the dead space where the mouthpiece is measured to 80 ml and the human is estimated to 150 ml. While this is the typical textbook anatomical dead space, in real life this of course varies, but also that it totally ignores the physiological dead space. I am not aware of any studies of ventilation/perfusion inequalities in diving but I suspect that there could be inequalities. Especially if one is varying the hydrostatic load from say negative to positive as could be the case with varying the swimming position.

Going back to the paper in DHM and this is obviously a to small study to draw any large conclusions and the effect of pressure would be interesting to see, but with tidal volumes from 750 ml the difference is 0.3 kPa (0.28-0.34) and maybe that is sufficiently accurate for this application. I ,and I am guessing here, think that the risk of tidal volumes below 750 ml, when the mouthpiece dead space is 80 ml, is low and that the other measured variables respiratory rate and VO2 (not quite sure how or if VO2 is measured) could be used to catch these dangerous situation rather than calculating in accordance with this formula.

Maybe it is better with the devil you know, and just go with the measured peak CO2 values.
Oskar

 

[Dr Simon Mitchell]

Hi this is my first post on this forum. But since I find this subject interesting I can’t resist replying.

Simon I read your paper and found it informative, if I read it correctly the biggest error is with small tidal volumes where the difference with measuring in the mouthpiece and at the end of the exhalation hose is about 1 kPa (95% CI 0,9-1,18) and with the larger tidal volumes the error is smaller 0,2 kPa (95% CI 0,19-0,23).

Correct

In the newly released document FAULT STUDY: CO2 BYPASS IN REBREATHER MOUTHPIECES Deep life does disclose a method for estimating what they call Peak end of exhale CO2 which I am quoting below. etc

Your concerns about the quoted algorithm are similar to mine. It is a series of calculations all involving certain assumptions, and accordingly, there is an opportunity for inaccuracies to have a multiplicative effect. When we are talking about a tightly controlled physiological parameter like arterial PCO2 this is important because small inaccuracies can have very important (and potentially dangerous) effects.

In any event, the key point is this: DOES IT WORK? So far, I have not seen any data that addresses this question, and as mentioned above, I doubt that the testing Brad quotes addressed the issue. And the second key point is: IT WOULD BE EASY TO DEMONSTRATE WHETHER IT WORKS OR NOT, AND I HAVE OFFERED TO DO IT FOR THEM. Any controversy around this issue could be made to go away in a weekend's work for me, yet I have not been taken up on this offer. It makes me slightly suspicious.

But I have some concerns not only to the math itself but also that the constant 0,230 refers to the volume of the dead space where the mouthpiece is measured to 80 ml and the human is estimated to 150 ml. While this is the typical textbook anatomical dead space, in real life this of course varies, but also that it totally ignores the physiological dead space. I am not aware of any studies of ventilation/perfusion inequalities in diving but I suspect that there could be inequalities. Especially if one is varying the hydrostatic load from say negative to positive as could be the case with varying the swimming position.

You are right to have concerns. These are very complex issues. I have recently published a comprehensive review of the relevant physiology (not specifically in relation to the Apocalypse) and would be happy to give you a copy of the paper. If you are interested please email me at sj.mitchell@auckland.ac.nz

Going back to the paper in DHM and this is obviously a to small study to draw any large conclusions

The small number of subjects is acknowledged, but the point of the paper was to demonstrate a principle, not establish population norms. Every mouthpiece and every human has dead space and it follows that even if we did 200 subjects it is highly unlikely that the fundamental conclusions of the study would change.

with tidal volumes from 750 ml the difference is 0.3 kPa (0.28-0.34) and maybe that is sufficiently accurate for this application.

Agreed, and we said that in the paper. I reiterate that the paper was a response to an obfuscation of the fact that low tidal volume breathing could become problematic at some level of breath size. Having said that, and as we point out in the paper, tidal volumes around 500ml could become relevant in some situations in diving.

Maybe it is better with the devil you know, and just go with the measured peak CO2 values.
Oskar

COMPLETELY agree with your inference that the complicated calculation algorithm may introduce more problems than it is worth.... but the fact they are trying to use it makes me suspicious that the dilution effect of mouthpiece and anatomic dead space on the CO2 measurement made directly at the end of hose in the actual Apoc circuit (whic we did not have) may be worse than we showed in our experiment.

Simon M