C02 Retention / Decay times

kevin stone

Hi Mike
What planning software('s) do you use?

And what a great thread thank's to all involved.

Kevin

Dsix36

I just want to thank everyone who has been posting on this thread. This is an issue that I have been thinking about for quite a while now and still don't have all the answers to. Thanks to this post, I should be able to make an informed decision now.

But keep posting anyway

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Mark Chase

The question is; Is NOT bailing out too big a gamble?

If the scrubber or a valve has failed, then stopping doing a dill flush and breathing isn't going to work. So you stop do a triple flush with an elevated breathing rate and when it builds up even further you think OK it will calm down in a min and you do another triple flush. Then it becomes uncontrollable and you attempt to bailout.

But at this point it could be too late in terms of available gas.

Whilst it goes against what I was taught in Mod3 My attitude is I carry enough gas to make a 40RMV work on a reducing SAC so why not use it?

Bail out first start heading for home and if you feel the need you could flush the unit and go back on once RMV is back to normal just to see if its fixed or if it rises again.

Personally if I suffered elevated breathing rates and going OC fixed it. Id stay OC and go home to fight another day.

I have lugged those damed tanks around for the last three years i might as well use them

ATB

Mark

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Drmike

I think if you are suffering from retained CO2 there wont be any CO2 in the loop (as the scrubber is working) only have CO2 in loop if you have scrubber failure (which isnt cause of retained CO2)

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Simon Mitchell

Hi Abbo,

You can take your pick from a number of possibilities, some of which you have touched on. CO2 is an anaesthetic gas and has actually been used for this purpose in the past. If the PCO2 in arterial blood rises above about twice its normal (ie from 40mmHg to 80mmHg) the subject will most likely begin to go to sleep. The usual stimulation of breathing associated with increasing PaCO2 is reversed at very high arterial levels as the anaesthetic effect predominates and the subject will stop breathing. Obviously, such events occurring underwater are not good. High CO2 does not "shut down" the cardiovascular system as such, but it can cause cardiac arrhythmias which in turn can lead to unconsciousness and sudden death. So there are a couple of choices.

In theory, this can happen during air breathing, but not breathing elevated PO2s at depth.

The following equation can be used to predict the alveolar oxygen pressure (we call it the alveolar gas equation):

PAO2 = PIO2 - (PACO2/0.8)

Where:

PAO2 = pressure of oxygen in the alveolus (mmHg)
PIO2 = pressure oxygen inspired (mmHg) (which is about 150mmHg breathing air at the surface)
PACO2 = pressure of CO2 in the alveolus (mmHg) (which is essentially the same as the arterial CO2 and normally about 40mmHg)

If we substitute those normal numbers in there for breathing air at the surface, then:

PAO2 = 150 - (40/0.8) = 150 - 50 = 100mmHg

Now consider what happens if we cause sufficient hypercapnia (say twice normal arterial CO2) to be close to putting the subject to sleep:

PAO2 = 150 - (80/0.8) = 150 - 100 = 50mmHg (which is a significant level of hypoxia).

Finally, consider what happens if we have this same degree of hypercapnia whilst breathing at a PO2 of 1 ATA (and we usually use even higher set points in diving).

PAO2 = 700 (approx) - (80/0.8) = 700 - 100 = 600mmHg.

Thus, although the diver would be close to being critically incapacitated by their hypercapnia, they are nowhere near being hypoxic.

This is not quite the full story, because oxygen carriage in the blood is altered by high CO2 (it shifts the oxygen - haemoglobin dissociation curve to the right), but this is a more complex bit of physiology, and it is still largely irrelevant to survival in the context of breathing elevated PO2s at depth.

Hope this helps.

Simon M

Simon Mitchell

Mike has sort of addressed this.

If your problem is that you have retained CO2 by producing more than you "ventilate off", then by resting and producing less but maintaining your ventilation you will eventually lower your CO2. Technically speaking this will lower CO2 in the expiratory limb of the loop (because you are blowing less CO2 into it), but provided your scrubber is working, then there will be no CO2 in the inspiratory limb just as there never was. CO2 levels in the loop are essentially irrelevant in the context of a "CO2 retention" scenario.

If your problem is that the scrubber has failed and there is some CO2 breakthrough then predicting the effect of rest alone whilst staying on the loop is difficult. It depends on a number of things including the amount of CO2 breakthrough, and whether or not it is flow rate dependent (that is, just resting might stop the breakthrough).

Correct.

Absolutely. Diving provides the potential for situations in which CO2 production increases because of the work of breathing alone, and that same increase in work of breathing prevents an adequate response in VA. Rising CO2 drives greater respiratory effort, which produces more CO2 without generating a significant increase in VA. A spiralling crisis of increasing CO2 can result. And note, it may have nothing to do with the loop.... unless the loop is responsible for a significant component of the breathing resistance.

Hope this helps.

Simon M

ScubaDadMiami

"Carbon dioxide retention . . . renders the diver more susceptible to oxygen toxicity." Howard Packer, Carbon Dioxide and the Rebreather Diver, (RebreatherWorld.com 2007), archived at: http://www.rebreatherworld.com/rebre...tml#post136126. See, Carl Edmonds, Christopher Lowry and John Pennefather, Diving and Subaquatic Medicine, 3rd ed. (Oxford: Butterworth-Heinemann, 1994), pp. 244 and 262. See also, Tom Mount, David Sawatzky and Joerg Hess, Tek Closed Circuit Rebreather (Miami Shores, Florida: IANTD, 2005), p. 48.

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CCRDiveTraining.com

Drmike

This is why good WOB for a rebreather is far higher on my list of unit priorities than most other things, especially if doing deep diving or non scooter use.

Its nice to have a small compact rebreather - but at what cost?

'The spiralling crisis' is not a fun place to be. Its a management game with real life/death consequences. The main players being fittness levels, WOB, ventilation, exersion, and stress. If you cant drop the WOB, improve your ventilation, reduce your exersion, lower your stress, or some combination of them all, sufficiently enough and quickly enough you wont be able to pull back your rising breathing rate and it will continue to spiral out of control to the point of death. I think its very important to be able to recognise very early signs of the beginning of that spiral so as to take effective action - if its spirals to high it can be too late to do anything about it...

One thing that Ive found can help a little to beat the spiral is to change your orientation in the water. Some units breathe better in certain positions. I have found that even the small improvement in WOB by changing orientation (whilst stopped and trying to stop spiral) combined with all the other methods mentioned can be enough sometimes to tip the balance in ones favour.....as can thinking positive thoughts

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

womble

Mike,

Do you have any photos of the polystyrene floats??

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womble

Bailing out could be the fail safe way to get over a CO2 hit, but it is apparent that you could be opening yourself up to other problems, IMO. As discussed IBCD, oxtox.

What I have been tought (rightly or wrongly) is to dil flush in hyper/hypoxia scenarios and in a hypercapnia scenario. Without getting into the unit specifics, on the Meg a 2 second dil flush is enough to replenish the loop volume (or at least a good percentage of it) with fresh dil. So my protocol (Only my protocol, not something that I have been tought) is to dil flush, If this doesnt help then flip onto OC thro the BOV.

Of course, the dil flush will only work if it is CO2 retention, NOT in the case of a failed scrubber or mushroom valve. But if you have packed the scrubber properly, prebreathed properly and havent pushed the scrubber duration then surely the chances of scrubber failure are minimal?? Please correct me if I'm talking jibberish.


Ah ah, the dissasociation curve. That was what I was referring to in one of my earlier posts. All those years at uni weren't a complete waste of time for me then!!

Another interesting point you made Simon, was although CO2 elevation stimulates breathing, (rate and depth??) in vastly elevated CO2 scenarios it acts as an anaesthetic ultimately suppressing breathing. Another confusion I had cleared up. Cheers.

Many thanks to the posters on this thread. I've learned a lot. This is what this forum is all about IMO

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