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Discussion Starter #1 (Edited)
So, we have another newsletter, and like Paul R I was looking forward to not featuring. But there I am along with Paul in the rogues gallery.

I am so heartily sick of arguing about this CO2 sensor, but I will not have the factual information I have recently provided on this matter questioned so baselessly without a response. Much of what I would write here is already in the article linked to from this websites home page, so I will restrict myself to some simple facts...

Fact: in patients undergoing general anaesthesia for surgery, end tidal CO2 is measured by sampling exhaled gas at the mouth in every operating room, in every hospital, in every city, in every country in the world, every day.

Fact: nowhere, not in any operating theatre in the world, is end tidal CO2 measured by sampling gas at the end of the anaesthetic machine exhale hose, even though that would be a more convenient sampling point. This is because attempting to measure end tidal CO2 there introduces the potential for a number of errors (see my article linked from the homepage of this site and also my published work in the peer reviewed scientific literature).

Fact: Despite the fact that none of the engineers from the billion dollar companies who design life support equipment for operating rooms use end tidal CO2 monitoring at the end of the exhale hose, this is where OSEL is going to measure it in the Apoc.

Fact: OSEL have proposed an engineering solution to eliminate the potential errors inherent in this method. This solution may produce measurements that are accurate enough to be safely used to drive life support algorithms in their rebreather.

Fact: OSEL have produced NO data demonstrating that this is so.

Fact: If the gold standard for end tidal CO2 is to measure it at the mouth, then the only way to prove that measuring it at the end of the exhale hose will work is to simultaneously make measurements at the mouth and end of the hose over a range of relevant conditions (including using human subjects,)and compare them.

Fact: The fault study document that Alex cites as some sort of reassurance that this has been done clearly shows that measurements were made at the mouth, but not at the end of hose, none of the sites where measurements were made were monitored simultaneously with the mouth, and the study was conducted using a "homemade" (by DL) breathing simulator whose ability to mimic the relevant human gas exchange parameters is not clear to me (even after reading the pdf about this device).

Fact: It follows that this study proves very little of relevance to concerns about accuracy of end tidal CO2 monitoring across a relevant range of tidal volumes (breath sizes) during use of the Apoc by a human.

Fact: I reiterate, as always, that I am not saying this device won't work. I am saying that, for some inexplicable reason, despite the controversy we have not been given any convincing reason to believe that it does.

Fact: I remain happy to pay for Brad Horn to fly to Auckland to run some simple human studies that will answer this question. If it works I would publish it in a heartbeat. Alex can be principle author. I don't give a rats about that. I just want to know.

And since you have dragged me back onto the stage in your newsletter, allow me to say that your attempt to downplay the design fault in your oxygen injector demonstrates breath-taking hypocrisy. Claiming that sustained oxygen injection is not a critical problem because the diver should notice the fault brings to mind DL's visceral criticism of other manufacturers for suggesting that divers should notice if, for example, their rebreather is not switched on. Both events could be missed by a task loaded diver, and both events could kill you. But good on you for being prepared to fix it though.

Finally, I note that you refer to the use of the water dump in the ALBOV by customers who have purchased the O2 Apoc. I found that a little surprising because over on the RBW archives a certain Dan Fountain enquired about the lack of water dump functionality in the accessory ALBOV he purchased in 2010. He reproduced the reply (posted in the Apocalypse Announcement Thread 18 May 2010) from OSEL which said:

"Your ALVBOV passed all testing prior to being shipped, but the reason that the water dump does not operate is that this functionality is only included on the CE certified Apocalypse Type IV iCCR and not the standalone ALVBOV due to liability reasons as that unit requires that divers receive training in its use" (my emphasis).

I dare say that Dan would be somewhat confused now, wouldn't he? In 2010 OSEL required that users had training to use an ALBOV with a water dump: but now users can have a ALBOV with water dump actually connected to a rebreather, without any training. What has changed? For the record, in my opinion, the release of this rebreather without training is very unwise.

Why don't you just leave me out of your next newsletter, please.

Simon M
 

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Simon, you are arguing with a fool. As the saying goes, he will reduce you to his level and then beat you with experience. All you are going to get in response to this is bluster, baseless drivel, and probably a thinly veiled threat of legal action if you are lucky. Everyone else can see it. There must be about 5 people left in the world that still consider anything from Open Sewer to be worth anything more than toilet paper. You are clearly right, however I'm not sure what good it will do to point it out.

Digs.
 

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Will the guy that got his apox off ebay get this free injector upgraded,, or will he just have to make do with the old but unique Apox o2 injector / auto Ascender :D
 

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https://www.opensafety.eu/datasheets/Apocalypse_Newsletter_16_120102.pdf

just for those who want to read it from the horses mouth so to speak.


Well I woke up early so decided to read the newsletter, and all 49 pages of the embedded report - an interesting (if a bit heavy going) read.

Now I'd be one of the first to jump on the "you promised me a sub £1k rebreather four years ago and I'm still waiting for it you twat" bandwagon. A classic example of catastrophic marketing on a par with Mr Ratner's epic. Some of the more bizarre AD posts both here and on other forum since have only compounded the problem.

I also have very little sympathy (actually that's zero sympathy) for AD's approach to criticism of other rebreather manufacturers. Some of the early published FMECA was clearly emotive and prejudiced relying on evaluating/highlighting flaws in other designs. In my opinion a FMECA should concentrate on the design as it is not diverge into what might have been or "what we didn't do but the bad guys did", ghosting is a salesman's technique not an engineer's and often backfires.

This time however I'm struggling to see the problem. The fault mode evaluated in the embedded report has been demonstrated to exist (and a layman can imagine for themselves that a "too floppy" valve might allow a small reverse flow to occur) and a fix offered.
Whilst it may be "gold standard" medical practice to measure exhaled CO2 at the mouth I can't see a reason to reject a method of measuring it at an additional six other points in the loop (because report does include mouth measurement) and making the appropriate compensations to the calculations. I don't agree that these measurement need to be made simultaneously if sufficeint confidence in the repeatability of the gases and breathing cycles exists.

There are countless examples where we cannot measure the "actual" element/parameter/point and so devise an acceptable alternative - it's the crux of any calibration.


Hang on a minute - a little further background googling/reading reveals:

"Capnography is a standard method of monitoring during anaesthesia and is increasingly used in the ICU. It describes the continuous measurement of expired carbon dioxide tension at the airway to allow the monitoring of the end-tidal CO2 tension. An analyser with a rapid response time is required and most devices measure CO2 tension from the absorption of infrared light, though other techniques such as mass spectrometry can be used.

Sidestream sampling is most common and describes the technique of continually aspirating a sample of gas from the respiratory circuit which is then fed through the analyser. The gas is either returned to the respiratory circuit or scavenged.

Sidestream capnography is favoured during anaesthesia owing to the convenience of a lightweight attachment to the airwaybut may be troublesome due to sampling line blockage by water vapour following protracted use.

Mainstream sampling – the analyser head is attached directly to the airway and the gas is analysed within the respiratory circuit through a clear window or cuvette. In the ICU where inspired gases are often humidified actively, mainstream may be more reliable than sidestream capnography."

Now I'm well outside my area of expertise when it comes to medicine but reading the above would suggest that the facts 1 through 4 are overstated if not misleading.
Sidestream would very much seem to measure the expired CO2 at the end of a hose. Now I'll grant you that the hose may be short, it may be a much narrower bore but it's not at the mouth as described in mainstream sampling and presumably the manufacturers of this equipment are making similar adjustments to the calibration of the measurement. If so why is this not valid for AD's approach? Why would you believe that manufacturer of medical equipment got it right (presumably you have not scrutinised their data at the same level) and not AD?

http://www.capnography.com/Homepage/HomepageM.htm

Such I guess is the peril of internet debate, passions/convictions lead to overstatement of facts. Grey areas that we accept in every other facet of our life become black and white boundaries.
 

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I believe that Simon has previously explained why it's done at the mouth rather than anywhere else.

I'm not even going to try to explain it cos I'll get out all wrong.

Search this forums and read the scientific paper he is referring to

D

Sent from my GT-I9000 using Tapatalk
 

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Now I'm well outside my area of expertise when it comes to medicine but reading the above would suggest that the facts 1 through 4 are overstated if not misleading. ...
Don, that's the danger, if you only google, but have never seen the actual machines...

when you sample gas at the mouthpiece, you take a very small bore hose (1mm for example) and an extraction pump, that feeds the gas to the analyser: this way you get no dillution of the gas (because of low internal volume in the small bore hose, the low diameter/length ratio and the short time delay between sample point and measuring point) and at the analyser you see exactly what happens in the mouth: the test for this is super simple: you do the same test with a 'zero-lenght' 'capilar' tube, and with a +1 meter hose
 

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I believe that Simon has previously explained why it's done at the mouth rather than anywhere else.
I think you've missed the point - it isn't always done at the mouth as stated by Simon - there are two distinct methods - sidestream and mainstream - as explained in much more detail on the link I provided.
I've no axe to grind on either side of the debate and am looking at it purely as a reasonably experienced engineer.

I'm saddened to say that I think Simon has undermined his argument by stating four facts at the begining of his response that are not facts - or are you telling me I have misread a completely independent medical website that describes sidestream monitoring?
 

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I registered just so i could say exactly what Paul said but at least now three of the Jousting Competitors from Newsletter 16 are in the same thread.

To summarise, it makes no difference whether you measure exhaled CO2 inline or sidestream via a capillary tube, it is stillm measured at the mouth. Theoretically and experimentally, gas mixing with dead space gas occurs if you measure at the end of a large bore rebreather exhale hose.

Dave T
 

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Don,

The gas is still taken at the mouth, look at the pictures.

(Ooops, Dave just posted the same!)

Regards
 

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Don, that's the danger, if you only google, but have never seen the actual machines...

when you sample gas at the mouthpiece, you take a very small bore hose (1mm for example) and an extraction pump, that feeds the gas to the analyser: this way you get no dillution of the gas (because of low internal volume in the small bore hose) and at the analyser you see exactly what happens in the mouth: the test for this is super simple: you do the same test with a 'zero-lenght' 'capilar' tube, and with a +1 meter hose
Paul, you're right the only time I've seen one I was connected to it and somewhat distracted :) but I have a very vivid imagination.

A tube is a tube (I acknowledged it was likely to be small bore and short length, infact website says 6ft) but common sense says if there weren't some differences then nobody would have bothered with mainstream. In fact a good read of the Capnagraphy website reveals the following disadvantages of sidestream;
Delay in recording due to movement of gases from the ET to the unit
Sampling tube obstruction
Water vapor pressure changes affect CO2 concentrations
Pressure drop along the sampling tube affects CO2 measurements
Deformity of capnograms in children due to dispersion of gases in sampling tubes

Which sort of makes my point, the manufacturers must be making allowances for these, or the users allowing for these innacuracies to derive the true readings.

The bigger point being that sidestream exists.
 

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Don,

The gas is still taken at the mouth, look at the pictures.

(Ooops, Dave just posted the same!)

Regards
But it's not in the mouth - have a look at the website. It's at the end of a 6ft tube. In mainstream the IR light passes from one side of the exhaled gas through to the sensor on the other side. In sidestream it is sampled via a hose.

My first point is - sidestream demonstrates there is an acceptable alternative to in the mouth monitoring.

Now what I don't know but as I say having read all 49 pages of the report, I'm reasonably convinced is that AD measurement method using seven points within the loop including the mouth), seems to me as an engineer (but not a medical expert) a reasonable approach.

What we don't know is of those disadvantages cited for sidestream how do they stack up on the large bore longer hoses of a rebreather.
 

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Don,

Have you read the papers which Simon has posted? He explains quite clearly why it is sampled at the mouth (although not analysed until elsewhere). This is different to sampling somewhere other than the mouth.

Regards
 

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Don,

Have you read the papers which Simon has posted? He explains quite clearly why it is sampled at the mouth (although not analysed until elsewhere). This is different to sampling somewhere other than the mouth.

Regards
Gloc! I'm surprised you'd even ask the question ;-)

I'd hardly enter a debate like this (particulalry as I'm not a rebreather diver) without doing the background reading. Like I said I've no axe to grind I'm interested from purist point of view - "is the AD method valid?". To my mind his paper seems to have made a reasonable attempt to achieve the accuracy required to identify the valve failure mode and its remedy.

Now how well that translates to a level of accuracy for general CO2 monitoring is another level.

I know Simon's well known/respected and popular on the forum - I hope he will not misinterpret my questioning as a personal attack - because it isn't.
 

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Now what I don't know but as I say having read all 49 pages of the report, I'm reasonably convinced is that AD measurement method using seven points within the loop including the mouth), seems to me as an engineer (but not a medical expert) a reasonable approach.
.
Don, again, read carefully..

the whole report has nothing to do with compairing measurements of end tidal CO2 either at the mouth or either at the end of a large diamter hose... the report talks about back-flow of CO2 when compairing different valves

about the 7 points, yes, one point measured at the mouth, but no measuring at the end of the large diameter exhale hose, (where it is measured in the iCCR apoc), so no-where comparison between the 2 possible points of sampling, and that's what Simon is referring to
 

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What we don't know is of those disadvantages cited for sidestream how do they stack up on the large bore longer hoses of a rebreather.
I think Simon dealt with part of that in his paper - at low tidal volumes, such as those that have been seen in divers, there is sufficient mixing in the hoses to mean that there are significant differences between the CO2 measured at the mouth and that that could be expected to be measured at the Pod on an Apoc.

Remember, the Apoc only measures CO2 in one place - the Pod. They may have measured in 7 for testing, but they only measure in one in the real world.

Simon seemed to be happy that Alex could produce a 'fudging' algorithm based on tidal volume, but we've yet to see any evidence that that has been done in a reliable way.
 

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What we don't know is of those disadvantages cited for sidestream how do they stack up on the large bore longer hoses of a rebreather.
Yes we do.

Sidestream CO2 analysers DO sample gas at the mouth. A pump continuously sucks a small amount of gas from a port near the mouth and analyses it remotely. There is nothing to dilute this tine amount of gas (except perhaps in neonates where the volumes involved are very small anyway) and so it is essentially the same as in line monitoring. There are pros and cons of side stream vs inline monitoring (mostly that side stream has more bits - the pump - and so is more expensive and inline is simpler and cheaper but more of a PITA to use because it is bulky sitting on the patient's face). The main point is that side stream monitoring and the Apocalyse CO2 monitor are not the same thing at all.

Dave T
 

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The answer is simpler than that.

Simon has offered them a free ticket to NZ where the apoc can be "peer tested". If it works... it works.

Wonder why the offer hasn't been taken. I mean... it would shut everyone, right?
 

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Discussion Starter #20 (Edited)
Now I'm well outside my area of expertise when it comes to medicine but reading the above would suggest that the facts 1 through 4 are overstated if not misleading.
Hello Don,

The facts I stated are exactly correct, not overstated, and certainly not misleading. You would be wise to listen to the experts on here telling you that you are wrong. A good start for you would be to read the article I wrote about end tidal CO2. The link is on the home page of this site.

Sidestream would very much seem to measure the expired CO2 at the end of a hose.
Because of the tiny bore of the tube and high flow rate, sidestream sampling is sampling at the mouth; trust me, I use it every day as do several other consultant anaesthetists / intensivists who populate this forum and who would be the first to correct me if my claims were wrong. It has no relevance at all to what Alex is doing.

Such I guess is the peril of internet debate, passions/convictions lead to overstatement of facts.
I grant you that internet debate has its perils, but in this case it is specialist knowledge driving statement of the facts.

This discussion has been long and bitter at times, and I believe there are many things wrong with the way Alex Deas has gone about his business (some of which you allude to). But none of this stops me respecting his ability as an engineer. That is why I would never claim that he can't create a fix for measuring end tidal CO2 at the end of an exhale hose. The report he (and you) cite describes a method that allegedly allows the CO2 pod to predict the same peak exhaled CO2 measured by a mass spec at the "mouth" in a mechanical circuit. This is obviously the beginning of a method. But it has been developed in a mechanically ventilated circuit where all the necessary inputs are known and tightly controlled. That is very different to its use with humans where some of the inputs need to be calculated and the range of potential variables is huge. Human test data where the true end tidal CO2 measured at the mouth is compared to the value measured by the CO2 pod at the end of the hose over a range of breath sizes is required, yet Alex is still referring us to the mechanical circuit "Fault study report" as his proof that it works.

To my mind his paper seems to have made a reasonable attempt to achieve the accuracy required to identify the valve failure mode and its remedy.
I have no problem with that and have never argued otherwise.... but the valve failure mode (the primary subject of that report) has almost no relevance to Alex's ability to measure true end tidal CO2 at the end of an exhale hose. The fact that he has combioned discussion of the two issues in the one report has lead to the confusion that you are now experiencing. I repeat, I have no arguement with his valve failure investigation but it is essentially irrelevant to this discussion.

Please do see the article I wrote. There is not a lot of room for error in this particular game. If you are interested, pm me and I will send you some other material to read.

Simon M
 
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