End tidal CO2 Monitoring

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berk

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Perhaps Mel is right, but I've had 4-5 life threatening situations with CO2 over the past 5 years and only one had anything to do with the sorb or the scubber.

All were retention issues. There was nothing wrong with the scrubber and it was in perfect working condition every time but one. She is probably right there. If you scrubber is gonzo, you'd perhaps be taking in CO2 and feeling the effects before it was reflected in the end-tidal or your exhalations.

How retention would reflect in a monitoring device is also unknown to me. Bad breathing habits, loop management, and WOB issues almost got me. Would that retention and rising blood percentages be reflected instantly in the end tidal numbers or would there be some sort of lag. And what percentage of the normal end tidal numbers would an issue look like?

From what I have read regrading CE tests, the scrubbers clean out ALL the CO2 right from the get go. There is no need to "warm up". Unless they are compromised there should be no CO2 in the inhale lung or side of the loop.

-matt
 

sea2summit

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What unit where you on? My buddy with the issue we believe was a retention issue as well.
 

berk

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I dive a meg, but I believe my habits are to blame and I'd be at risk on any unit.

I'm most likely a bad CO2 retainer. I'm a free diver and did lots of anaerobic conditioning all my life: football; 400m running; started diving at 8 and did lots of skip breathing for 6-7 years to conserve gas.

It's easy to mitigate on OC, you just make sure that you're constantly moving gas in and out, I take really long, slow, full breaths and the same on the exhale side. That's worked well both on the CO2 side and gas conservation. The cracking of the reg and positive pressure on inhalation and my semi conscious habit pushing each small bubble past the exhale valve in the 2nd stage spur good breathing habits.

The rebreather is a different story. Really full breaths are not needed for gas conservation and aren't really the breathing that's espoused for rebreathers. "Just breathe normally" they say. So I did. The fact that minimum loop volume isn't pounded into your head as much these days and has been replaced by "optimum loop volume has helped. I used to dive with the ADV close except during decent. Closing the ADV at depth also puts you at danger. Mine is open all the time other than jumping in the water with hypoxic dil or on ascent/surface.

At any point if you are bottoming out your lungs upon inhale or encountering resistance upon exhale, you are ruining your natural tidal flow at probably the worst point, the point after you've got past the dead space of the bronchus and actually started to ventilate useful lung tissue.

My worst experience was chasing after lobsters with a broken goodman handle, so I had a backup light in one hand and the catch bag in the other. I had gone a little deeper and the lungs were bottoming out. Then I saw a really big guy that was 5-10' deeper. He was stubborn. I was stubborn and then the problems started!

So perhaps I have a little higher level of blood CO2. When I then start to work or don't watch the lungs like a hawk it can become a problem. Who know if that is the mechanism or what is to blame..............or how an end tidal machine would pick that up.

If you scrubber is gonzo, you'd perhaps be taking in CO2 and feeling the effects before it was reflected in the end-tidal or your exhalations.

On the other hand, you'd still be pulling that bad gas from the inhale lung into the dead space of the DSV and pushing it out first upon exhale so a monitor would pick it up.

-matt
 

sea2summit

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Same unit. My buddy dove a KISS for years prior to trying to switch to a Meg and had issues on two dives. He thought there was a relation to BMCL vs front and his retention issues. Would be an interesting study with those suseptable to/demonstrated retention and it seems to me it would tie into tidal flow with the slightly pos/neg pressures of each configuration.
 

down4fun

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You can't fix stupid but there are apparently some cases where the diver was unaware that his/her CO2 levels were too high with a bad result.

If someone wants to use a CO2 monitor to get more life out of a scrubber, you can't stop them but knowledge of this measurement can save a life.

You can't fix stupid, but as with many things, I think hypercapnia comes with a certain amount of denial in a lot of divers. Combine that with the often sudden onset as others have said and the response time can be very limited.

My only CO2 hit was due to exertion. Pulling down a line in strong current and over breathing the scrubber. I had a severe headache, (much like i used to get on o/c due to long breathing cycle not intentional skip breathing). I sat motionless, let my heart rate come down, and completed the dive in a leisurely style without and further trouble, in retrospect i should have bailed out,. It did however, change the way i descend in current. I take my time, if i start working to hard i stop, hang out for a few seconds then proceed. If it takes me 12 minutues to get to the wreck oh well, better that than a CO2 hit that could kill me.
 

berk

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That's interesting. Is that the guy from AL who just sold those pimped out KISSes?

I actually bought a KISS and was thinking of selling the Meg. I bought it 1.75 years ago from a guy I know and he still has it. I have to bug him again to send it up to me from FL.

I wonder if I will like it better. There are all kinds of crazy reasons that your WOB can get messed up. Although OTS lungs are supposed to be better, you can cinch them down too tight etc. In the KISS I imagine that they are always in the same spot and always feel the same.

Just looking at the stats would suggest that there would be a ton more issues on the KISS Classic vs the Meg at 3.5+/- j/L and 2.44j/L WOB. (according to some test I've seen)

I still would take an end tidal CO2 monitor in a second. I'd trade it for a HUD and trimix integrated deco. I don't mind just cutting 1.0 and 1.2 tables onto a slate. I don't really do multi level diving and have really slowed down my diving.

-matt
 

berk

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I get the same symptom every time. I finally figured it out. It always felt like the mask was getting sucked onto my face, almost like a squeeze, as if the machine was failing a negative or I was losing air or pressure out of the loop. I could fill up the loop to the point of gas burping out the OPV and still feel the same thing so obviously is was a physiological response and not the machine (other than perhaps a bad scrubber, but each time the scubber has been fine after I recover and slow down).

It was my diaphragm spasming, trying to get a breath. I don't sense anything else wrong till the sh*t really start to hit the fan with the limbs getting numb and the tunnel closing in around the vision. The first two times I was inexperienced (a good thing) and just bailed immediately before it got bad because something was funny and ended the dive. The next time I flushed and flushed and wasn't moving too fast so in stopping and flushing it reversed course. I almost killed myself when I was much more experienced and thought that I could take care of it due to the success of the last experience. I flushed and flushed and suddenly it was getting worse fast. If I bailed more than 10 seconds later I would have been toast. I got onto OC and it got worse for another 1-2 min. After 5 min and swimming from 140 to 70, I had drained an AL40 with 2700psi......in 5min that's right. I was pushing the purge when inhaling I wanted gas so bad. Luckily I had an AL80 of 50%, but when I got to 70' and put the that reg in my mouth there was nothing but water. That's when the fun started, but a story for another time.

Now I don't fart around. If I feel even the slightest bit funny, that loop is gone and I'm on OC. I still would be fascinated to watch an end tidal CO2 monitor. I don't I would change my habits with it or without it. I'm not very trusting. I still stay on 100% O2 and pause at 33 feet to see if I can get the cells to hit close to 2, then flush back to see if they drop quickly and respond well. I still completely flush 3-4 times during a dive to check the cells again and again at 20' and keep a note of the Mv output. Why not?

I don't think a CO2 monitor would cause people to push the scrubber. Hopefully............:(
 

mummyreturns

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A light is shone through the expired air and the degree of absorption of a certain frequency of infra-red light is proportional to the concentration of CO2. The light may be split with half passing through a reference cell. The light may also be 'chopped' so that it is not continuously heating the gas in the reference cell.
The analyser may be placed in one of 2 places: in-line, and out of circuit at the end of a sampling tube.
 

Supermanwoot

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I disagree with the end tidal reasoning. ETCO2 is for measuring blood flow and has nothing to do with breathing really.

However, would a CO2 monitor be useful in rebreathers? It could be, but as someone noted, not to determine if the scrubber is working properly, but perhaps to determine build up in the line? But isn't your breath pushed through the circuit to the scrubber?

I dont think it would be helpful.
 
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