CO2 Monitor (End-tidal) - good news

[Clare]

Can members wind back on the personal digs here.

CCRX is a FORUM - ie matters are open to question and debate. It is neither a lecture theatre, a court room or a sparring arena. If people don't understand your argument explain better don't shout louder. If you disagree with someone, say so if you want but it doesn't mean they are a jerk just because they have a different view.

Off the internet i'm sure you are all gentlemen. Bring a little of that to your posts please.

Remember, we started this to build an inclusive, friendly community. Inclusive means everyone who wants to join in as long as they do so in the general spirit if the community. Let's try to keep the discussion hot but tempers cool.

 

[Dave Sutton]

What Clare said: Plus "The Four-way Test":


Of the things we think, say or do, ask:

Is it the TRUTH?
Is it FAIR to all concerned?
Will it build GOODWILL and BETTER FRIENDSHIPS?
Will it be BENEFICIAL to all concerned?

Nobody is perfect, and we do not always succeed, but if we try...


Dave


.

 

[GLOC]

Having had a quick chat with Simon about this in RF3.0 this morning, trans-dermal is certainly used on the surface, but from what I can gather, there are issues with it in a diving environment due to temperature requirements (probe is heated) plus the responsiveness is unlikely to be responsive enough. He didn't mention any papers published using it in the hyperbaric environment.

Regards

 

[matt]

Having had a quick chat with Simon about this in RF3.0 this morning, trans-dermal is certainly used on the surface, but from what I can gather, there are issues with it in a diving environment due to temperature requirements (probe is heated) plus the responsiveness is unlikely to be responsive enough. He didn't mention any papers published using it in the hyperbaric environment.

Regards

That's a pity, sounds like many problems to over come with that method too.

Matt.

 

[Johan Isaksson]

Having had a quick chat with Simon about this in RF3.0 this morning, trans-dermal is certainly used on the surface, but from what I can gather, there are issues with it in a diving environment due to temperature requirements (probe is heated) plus the responsiveness is unlikely to be responsive enough. He didn't mention any papers published using it in the hyperbaric environment.

Regards

Meh, I was hoping someone had done the work already. The responsiveness I don't quite get, current sensors claim a response time of around 30 seconds.

http://dev.ersnet.org/uploads/Document/2c/WEB_CHEMIN_2560_1194522858.pdf

 

[Simon Mitchell]

I hope Dr. Simon Mitchell will tell us more about this peer reviewed scientific study published recently as this is his field.

The silence is deafening, which means it works and there is substantial commercial interest (and military) .

Gian,

I may be misunderstanding you here, but are you linking me in these two statements? If not, forget it, but if so, can you please explain what you mean.

Thank you,

Simon Mitchell

 

[Simon Mitchell]

Meh, I was hoping someone had done the work already. The responsiveness I don't quite get, current sensors claim a response time of around 30 seconds.

http://dev.ersnet.org/uploads/Document/2c/WEB_CHEMIN_2560_1194522858.pdf

Hello Johan and everyone else,

First, Johan, re an earlier reference to emailing me. I must have missed that somehow... I'm very sorry. I was not ignoring you. I do try to be responsive to these things. I have had a lot of travel over the last month or so.

To clear up some confusion: end tidal CO2 measurement and transcutaneous CO2 measurement are just two different ways of trying to measure the same thing. What we are really interested in is arterial CO2. When we measure end tidal CO2 we are measuring the CO2 in the breath at the end of exhalation. We assume this gas comes from the alveoli, and we know that the CO2 in the alveolar gas is in equilibrium with the arterial CO2. When we measure transcutaneous CO2 we are assuming that CO2 is diffusing to the skin at a level that reflects the content in blood within the arterial end of capillaries: in other words, it reflects the arterial CO2. I hope that clears up some of the debate that has been taking place.

Now, transcutaneous CO2 measurement in divers is an interesting idea. There are several versions of this technology. The one I have used in the hyperbaric environment uses a probe that is stuck onto the skin (usually over the chest). It creates a small fluid layer between the probe and the skin. CO2 diffuses into the fluid and is measured there. The probe heats the skin with the aim of causing dilation in the underlying capillaries and thus maximising perfusion of that local area and increasing the chance that the measured CO2 is an accurate reflection of the incoming arterial blood. There is a second technology that i have not used which applies a clothes-peg-like "clip" to a finger or earlobe and makes a direct measurement somehow. I think it also heats the skin. It looks simpler to use.

When we used transcutaneous OXYGEN probes in the hyperbaric environment they were never a particularly accurate reflection of the arteral oxygen which markedly increases when breathing 100% oxygen under pressure. But they did show important trends. However, CO2 is a little different because, in theory, the pressures of CO2 in the arterial blood should not change much during a hyperbaric exposure. It is thus plausible that if a transcutaneous probe gives accurate answers under normobaric conditions it will also work in hyperbaric conditions. I have accessed some of the papers cited in the article that Johan linked us to. There is certainly data from anaesthetised patients suggesting that the transcutaneous CO2 provides an accurate reflection of arterial CO2 after a short period of equilibration (presumably whilst the skin heats up). This remains true when the patients were rendered hypocapnic and hypercapnic by increasing and decreasing (respectively) their ventilation.

This is all very encouraging. HOWEVER, it is a number of steps away from the technology being validated for use during diving. If you are going to use the "stick on the skin" probes, then it will need to go under a dry suit, and the wire to the "box" will have to come out. There might be some concern about burning the skin with a very long exposure. I would also be reluctant to use a high oxygen fraction in the drysuit inflation gas. Then the analyser will need to be waterproofed, and supplied with adequate power. If you are going to use the clothes peg type probes, eg on an earlobe, then there may be problems with peripheral perfusion during immersion in cold water. This could easily result in seriously inaccurate readings. There would also be all the same problems with waterproofing and providing power etc.

I am certainly not dismissing this technology as potentially useful, but it is not certain that it can be done. Remember, as I have said on other forums, arterial CO2 is a tightly controlled physiological parameter. If you are going to measure it and base dive management decisions on the number, then it has to be accurate.

I will certainly be interested to hear how things go if someone starts experimenting with transcutaneous CO2 measurement underwater.

Simon M

 

[GLOC]

Thanks for the link, the presentation makes for interesting reading, especially the stats at the end.

Response time: <26s
Accuracy: Pt,CO2 accuracy kPa ±0.6
Drift: Pt,CO2: ±10% over interval calibration
Calibration: Auto-calibration. Integrated gas bottle. 1-point, 7.5% CO2 gas, 20.9% O2 and balance N2. 4-h calibration interval recommended.
Weight 4.58-4.78kg.

Having seen some graphs at RF3.0 this weekend, accuracy of +/-0.6kPa might not be good enough, if the response time is around 26s (manufacturers quote as close to the real number), combined with a drift of +/-10% over 4 hours. I am presuming the calibration level is 7.5kPa, therefore the drift could be .75kPa over 4 hours.

Weight and power might be an issue Doesn't mean it won't work, just that it doesn't appear to be that close to be incorporated into an underwater hyperbaric environment. This isn't my game as the technicalities of CO2 monitoring are outside my level of expertise, but as a Requirements Manager looking at avionic integration onto air platforms, I take a systems approach view to many things, and always take what manufacturers publish with some scepticism.

Regards

 

[matt]

Very interesting. 4.5kg, and presumably mains powered?

Simon's excellent article said:

small changes in PCO2 (even ~ 1kPa) can have very important implications for the safety of the diver.

5.2kPa is the average, so 10% is 0.52 so a range of ±1.12 would seem significant.

Matt.

Thanks for the link, the presentation makes for interesting reading, especially the stats at the end.

Response time: <26s
Accuracy: Pt,CO2 accuracy kPa ±0.6
Drift: Pt,CO2: ±10% over interval calibration
Calibration: Auto-calibration. Integrated gas bottle. 1-point, 7.5% CO2 gas, 20.9% O2 and balance N2. 4-h calibration interval recommended.
Weight 4.58-4.78kg.

Having seen some graphs at RF3.0 this weekend, accuracy of +/-0.6kPa might not be good enough, if the response time is around 26s (manufacturers quote as close to the real number), combined with a drift of +/-10% over 4 hours. I am presuming the calibration level is 7.5kPa, therefore the drift could be .75kPa over 4 hours.

Weight and power might be an issue Doesn't mean it won't work, just that it doesn't appear to be that close to be incorporated into an underwater hyperbaric environment. This isn't my game as the technicalities of CO2 monitoring are outside my level of expertise, but as a Requirements Manager looking at avionic integration onto air platforms, I take a systems approach view to many things, and always take what manufacturers publish with some scepticism.

Regards

 

[Johan Isaksson]

Personally I don't believe accuracy to be terribly important when it comes to the actual numerical values as long as it's consistent. Arterial CO2 levels are well regulated and the trends would be more important. If it shows a steady 5.5 kPa instead of the actual 5.0 it is no big deal as long as it correctly registers when it starts to rise. You can get a good baseline before the dive as a reference and when it starts to deviate it's time to start thinking. Bailing out should normalize CO2-levels within a few minutes.

 

[matt]

Personally I don't believe accuracy to be terribly important when it comes to the actual numerical values as long as it's consistent. Arterial CO2 levels are well regulated and the trends would be more important. If it shows a steady 5.5 kPa instead of the actual 5.0 it is no big deal as long as it correctly registers when it starts to rise. You can get a good baseline before the dive as a reference and when it starts to deviate it's time to start thinking. Bailing out should normalize CO2-levels within a few minutes.

But the 10% drift is significant, right?

 

[GLOC]

I'll see if I can find the CO2 graph (time/pressure) presented this weekend, to show how quickly it goes from 'just coping' to 'nightmare', from what I remember, it was a matter of minutes...

Normal engineering practice is to measure to an order of magnitude greater than you need to operate to. So if you are looking at
1kPa, the you need a 0.1kPa resolution. You also need your drift to be inside the error budget otherwise you will get false positives or false negatives, neither really acceptable with life support systems.

Something else that came out this weekend was the effect pressure, humidity and temperature have on the calibration of an O2 cell which was designed to operate upto 1bar and 'normal temperatures/humidity. Calibrating in an environment which is different to that being experienced is a flawed concept; the presentation by Nigel Jones was rather enlightening! I was assume that similar issues are likely to be encountered in CO2 monitoring systems.

Regards

 

[Johan Isaksson]

But the 10% drift is significant, right?

The drift could definitely be a problem though the exposures we're talking about usually wouldn't be all that long. A drift should be reasonably easy to check for if you bail out for a minute or two and monitor the values. Arterial CO2 should normally drop rapidly towards normal values if you're indeed suffering from hypercapnia, if it's just drift I expect nothing much would happen.

As for my own experience with these I've found them to be consistently inconsistent. They'll be off by the same amount through the night and keep fairly stable readings. We verify with two arterial samples, both at baseline and during the night and it tends to be off by the same amount at both points of verification.

 

[matt]

Something else that came out this weekend was the effect pressure, humidity and temperature have on the calibration

Calibration specific or actual readings due to pressure, humidity and temperature?

... of an O2 cell which was designed to operate upto 1bar and 'normal temperatures/humidity. Calibrating in an environment which is different to that being experienced is a flawed concept; the presentation by Nigel Jones was rather enlightening! I was assume that similar issues are likely to be encountered in CO2 monitoring systems.

It's OT so we can wait for your separate O2 cell thread. Just 2 points on O2 cells - one reason I calibrate in the afternoons is that wet cells definitely perform differently...but if it was such a big issue then there would be more problems than we see.

Matt.

 

[matt]

A drift should be reasonably easy to check for if you bail out for a minute or two and monitor the values. Arterial CO2 should normally drop rapidly towards normal values if you're indeed suffering from hypercapnia, if it's just drift I expect nothing much would happen.

I'm not sure I agree with that, but it could be because I don't understand properly.

I'm assuming such monitors are not specifically detecting break-thoughts but are aimed at CO2-retaining and WOB generated issues - right?

My understanding that that the basic premise that bailing out always reduces arterial PCO2 _may_ be flawed as the only way to reduce a retained issue is to significantly lower WOB....and that's not necessarily true especially if the valve is a BOV or the breathing technique is at fault.

I may be wrong - I picked this up on the internet.

Matt.

 

[Johan Isaksson]

I'm not sure I agree with that, but it could be because I don't understand properly.

I'm assuming such monitors are not specifically detecting break-thoughts but are aimed at CO2-retaining and WOB generated issues - right?

My understanding that that the basic premise that bailing out always reduces arterial PCO2 _may_ be flawed as the only way to reduce a retained issue is to significantly lower WOB....and that's not necessarily true especially if the valve is a BOV or the breathing technique is at fault.

I may be wrong - I picked this up on the internet.

Matt.

Lowering arterial pCO2 back to normal levels is a quick affair assuming healthy lungs. It can literally be done in the matter of a few deep breaths.

I doubt any modern rebreather, OC regulator or BOV has a bad enough WOB to actually cause rising CO2 levels at rest unless the scrubber or gas pathways are compromised. At high breathing rates it might be possible but then again, the point of a monitor would be to catch this before symptoms would arise. Stopping early, switching to a OC regulator or BOV with good WOB and then evaluate seems to me like a plan that would work if it's done as soon as the monitor indicates rising pCO2.

Either way, someone needs to build it, test it and sell it so that I can buy one

 

[Simon Mitchell]

Hello Johan,

Just picking up on a couple of your posts:

Lowering arterial pCO2 back to normal levels is a quick affair assuming healthy lungs. It can literally be done in the matter of a few deep breaths.

I doubt any modern rebreather, OC regulator or BOV has a bad enough WOB to actually cause rising CO2 levels at rest unless the scrubber or gas pathways are compromised. At high breathing rates it might be possible but then again, the point of a monitor would be to catch this before symptoms would arise. Stopping early, switching to a OC regulator or BOV with good WOB and then evaluate seems to me like a plan that would work if it's done as soon as the monitor indicates rising pCO2.

and this earlier one:

Personally I don't believe accuracy to be terribly important when it comes to the actual numerical values as long as it's consistent. Arterial CO2 levels are well regulated and the trends would be more important. If it shows a steady 5.5 kPa instead of the actual 5.0 it is no big deal as long as it correctly registers when it starts to rise. You can get a good baseline before the dive as a reference and when it starts to deviate it's time to start thinking. Bailing out should normalize CO2-levels within a few minutes.

I am writing this in a jet lagged state in an airport lounge on my way home from RB3, so I reserve the right to sound a bit incoherent and to edit it later!!!

I think you have to be very cautious here. There are two important points I would like to raise.

First: a trend toward CO2 retention is common in diving, and likely to be almost universal whilst exercising during a deep rebreather dive breathing dense gas. Thus, if you are relying on an upward trend to make "dive management" decisions, you are going to be confronted with those decisions on almost every relevant dive. That is why merely detecting a trend is not particularly useful. What you really want to know is an accurate arterial CO2 level because then you can make sensible decisions about when the situation is crossing boundaries that are becoming intolerable. In a similar vein, as Gareth Lock has suggested, there is a narrow "therapeutic index" for CO2. You can go from functional to totally dysfunctional across a narrow range of CO2 dose. Thus, if the CO2 monitor is even 0.5kPa out, (eg the difference between 7.5 and 8kPa) then that can be a significant issue.

Second: As Matthew Outram correctly implies, it would be very unwise to assume that a rising CO2 can be easily corrected by bailing out and hyperventilating. If hypercapnia is due to CO2 scrubber breakthrough then bailing out is likely to correct it very quickly, but if it is due to CO2 retention then bailing out may not be particularly helpful unless the work of breathing on the open circuit supply is significantly less than the rebreather. That is not always the case and it is entirely plausible that bailing out onto a crappy reg could make things worse. In any event, it can take a considerable period of time to lower CO2 levels in a high work of breathing situation.

Simon M

 

[Johan Isaksson]

Perhaps I should clarify that I'm theorizing strictly around the situation of "everything is fine but the reading on the transcutaneous CO2 has started to trend upwards". In this situation, with a clear mind and normal breathing rates, you would hopefully have a reasonable WOB on either bailout option when you stop to assess the situation.


Now, if accuracy in that degree is needed transcutaneous monitoring will have problems. Just the sampling itself means it will be off and temperature conditions during diving is likely to confound it even further. Could still be a fun experiment though

 

[Simon Mitchell]

Perhaps I should clarify that I'm theorizing strictly around the situation of "everything is fine but the reading on the transcutaneous CO2 has started to trend upwards".

Hi again Johan,

I understand, but my point is that upward trends in CO2 whilst everything is fine are common in diving. Say I am swimming around a wreck on my rebreather looking for something to identify it, so it is a typical wreck dive with some degree of exertion... but everything is fine. My transcutaneous CO2monitor goes from reading 4.5 kPa when I arrived on the wreck to 6 kPa say 10 minutes later. At that point I have an upward trend, but if those values are accurate that is not a trend that would particularly worry me because such trends are common. However, if the transcutaneous unit is underestimating my arterial CO2 and the real values are 5.5 and 7 respectively, that is important and would mandate a different response. Without knowing the accuracy of the numbers that reveal the trend it is difficult to prescribe a sensible response to the information.

In summary, prescribing a particular course of action (such as bailing out) based on a trend that is actually common and probably associated with little risk in its early stages, may create unnecessary problems for divers. Don't get me wrong, I am not saying that detecting a trend is valueless. One interpretation might be that it is a warning that you should just slow down. But I'm sure you can see how an accurate estimation of the true arterial CO2 (such as a correctly measured end tidal CO2 value) would be a MUCH more useful piece of data.

Could still be a fun experiment though

TOTALLY agree.

Simon

 

[tecdiverdude]

Loving htis thread... so who has the time,$$ and technical knowledge to start experimenting with it?