C02 Retention / Decay times

Mark Chase

Having once again waded through the usual enough gas for bailout debate i was wondering if anyone knew the answer to my $10,000 question.


My hypothesis on bailout safety is bailout early.

So if my SAC hit 25-30 I bailout as a first response and ask questions latter.
The use of a BOV plummed to a large off board tank is suposed to make this a simple no brainer.

The question is: Will my SAC stabilize and drop immediately i remove the source of the C02 or will my SAC continue to rise after i have bailed out.

The sub question is: having bailed to OC what is a reasonable time frame for the increased breathing rate to subside.

ATB

Mark Chase

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tibby

Hi Mark,
Sorry to reply with a question but, wouldn't this SAC recovery time depend on the length/amount of CO2 exposure? I would imagine it would but have no clue.
Thank you for asking these questions because I've been wondering the same thing since JDicediver posted his experience. It's worrisome thinking that the BO we carry may just not be enough folowing a CO2 hit if deep enough.
Another related question would be:
Does the reducing PO2 while ascending on OCBO delay the breathing rate recovery? In other words, do we recover from CO2 poisoning faster in a rich O2 environment (like EAN36 at 30m for instance) than say by breathing surface air? I sure wouldn't hang around 30m on OC to find out but would be interesting nevertheless.
Tibby

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RichClark

Hi Mark,

I took what i believe to be a 'mild' hit after a partial flood (my fault). In may case I was able to bring my breathing rate back to normal rates within the first 1 - 2 minutes. To the extent that I went back on my loop... and took a mild hit again. Again I went OC and my breathing rate returned to normal after about 5 minutes.

I then completed the dive OC.

Cheers

Rich

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Drmike

My experiences for what they are worth have been like this;


Retained CO2 from over exersion/stress:

RMV probably I would imagine well over 50 before finally stop trying to convince myself im not having a problem

Bail to OC.


RMV will continue to rise it wont drop unless activity is stopped and I calm myself down. (on or off the loop recovery just as fast if retained CO2, key is to stop activity and calm down)

This battle to calm down breathing (retained co2 is almost as much mental as physical)

With all activity stopped and thinking happy thoughts RMV drops to levels where I feel ok again (no panic/stress feeling) after less than say 2 mins (this has been my experience) I go back to the loop and see if it was retained or scrubber failure.

I have had more retained CO2 issues than CO2 hits but the recovery time (at those times) for me was similar in both cases. But I think its really a matter of how bad the hit is (or how bad you let it get before you bail) leave it long enough and recovery would i imagine be much longer too.

But I seriously seriously doubt your RMV will be in the low 20s for quite some time after you bail (other stress raisers like being on limited bail out vol would likely kick in)

If I was to guess Id say maybe expect inital RMV 50+ then recovery (of main CO2 symptoms) after maybe 2-3 mins with RMV reducing steadily to slightly highet than normal levels over say a 10-15min period (this is just my guess based on my limited experience - it may be complete bol1ocks)

YMMV

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Mark Chase

Thanks Rich and Mike

Empirical is very cool by me Empirical unlike theoretical is hardly ever bollocks


ATB

Mark

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Freef

I would have thought that the CO2 would have been eliminated from the body in relation to the difference in pO2 between the blood and the gas in the lungs, in the same way that other inert gasses are eliminated.

From experience [not a CO2 hit] the ammount of O2 I use may raise slightly after a 2 minute 'sprint', possibly due to the body still switching from aneerobic to aerobic and then trying to recover after the excercise [trying to push the bus at Guildy over].

As CO2 is carried in the blood plasma as well/instead of the red cells there might be more capacity to have CO2 out of the cells, and it is possibly eliminated faster?

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Mdemon

It's been a while since I did it, but when we did the CO2 test (Inspo, no scrubber, in water) it took about 30 secs for the breathing rate to be noticeably through the roof. Bailing to BOV, breathing rate was normal again within 30 secs.

Bare in mind that this was only at 1-and-a-smidgen ATM, with zero movement, so this could be taken as "best case" from a recovery perspective.

HTH

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a) "Carry adequate bailout"

b) "If in doubt, bail out"

c) "Always know your PO2"

If you find that hard, stick to crochet.

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RichClark

Mike touches on something that I think potentially could be quite important.

On both occasions I stopped on the spot and was able to hold on to some wreckage to gain support, I then focused on long deep breaths and concentrated on nothing else but my breathing.

On both occasions I bailed via my BOV connected inboard as an intermediate (the paragon work fine... not great... but fine) and then onto my off board OC.

Cheers

Rich

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Simon Mitchell

Mike's answer is near perfect in my view. Importantly, he reminds us that there are two potential causes of hypercapnia: CO2 retention because more is produced than eliminated from the body, and scrubber failure. The former usually arises in the context of over-exertion and we are particularly prone to this problem at extreme depths when the inspired gas is dense.

In scrubber failure the problem cannot be resolved unless you bail off the loop (or invoke some sort of semi-closed mode). In retention, the problem is unlikely to be resolved unless you rest and concentrate all attention and energy on just breathing. In reality, you cannot really tell what is causing your problem (scrubber failure or retention) so as Mike implies the correct approach is to invoke both restorative procedures simultaneously: that is, bail out and rest immediately. As he also suggests, if you are strongly suspicious that overexertion and retention caused the problem, then a cautious return to the loop to see if the problem recurs is sometimes appropriate.

I agree with Mike's estimates for recovery times. The drive to breathe will fall rapidly as CO2 is eliminated, and breathing rates / volumes will quickly fall. However, the potential complication in very deep diving is that the work of breathing itself can become a significant contributor to CO2 production. The more this is so, then the slower CO2 levels will fall because the breathing process itself (which is what gets rid of your CO2) will also be producing it. Indeed, it is possible that one can get into a vicious spiral in which so much CO2 is being produced by the work of breathing that it is impossible to rest and ventilate it off. The CO2 drives greater effort to breathe, which produces more CO2 and so on. This is what we think happened to David Shaw, and anyone interested in learning more about this frightening possibility should read our paper in Aviation Space and Environmental Medicine. Thus, Mike's estimates are fine, but there is some contextual influence of depth, gas density, and almost certainly of the resistance to breathing in the equipment. The deeper you are, the denser the gas, and more resistance in your equipment, the closer you are to an unrecoverable problem.

Warm regards,

Simon M

womble

Having had a very mild CO2 hit caused by overexertion (wrestling with a loose weightbelt whilst descending a shot line in the ripping current) I would agree that it takes a couple of minutes to sort yourself out.

In my case, I sat on the wreck, dil flushed a couple of times and did nothing else. Once my breathing had settled, I then sorted out the weightbelt.

I think the most important thing to do is stop what you are doing first.

The problem is, CO2 has a higher affinity to haemaglobin than O2, so you have to wait for all the CO2 to be flushed out of your system. This depends on so many factors, e.g severity of the hit, depth, ambient temperature etc.

Theoretically this would be better achieved by a higher PPo2, but there is also the increased risk of oxtox if breathing high PPO2 gas with significant CO2 retention . . . So is this an argument for carrying a not-very-hot deep bailout?? Say PPO2 1.5-1.6??

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