How conserve air when necessary

[EricTheDood]

IMO, the idea of any sort of "special breathing" technique is built on the faulty premise that something requiring additional task loading / focus can help you in an emergency.

 

[EricTheDood]

Does more complete exhales help any? As CO2 drives the urge to breath and other effects, would more effectively flushing it out help?

No, the amount of effort required to exhale that last 10% of breath counters any gains and probably results in a net increase in gas consumption.

 

[MichaelMc]

Regarding bcd rebreathing: I've dove my rebreather with no scrubber on 21% air (no o2 added, strictly semi closed circuit) out of curiosity of this tip. It is a horrible experience of co2 build up and one of the toughest mental battles I've had on a mundane dive

I agree rebreathing stale air does not seem the path to effective problem solving.

But from the mental and physiology standpoint, why did this let you survive, reasonably in control, with less air than OC breath holding? The CO2 your lungs see seems almost the same. The differences I see, from limited knowledge from basic reading, seem to be:
1) You still get to instinctively exhale and breath in more. It just doesn't do you any good until you add fresh air to the bag. From my earlier reading of static apnea, stillness or motion of the diaphragm plays a role in the extreme stages. On OC, this would be using more tank air on each such breath, not just the ones when you allow yourself fresh air.
2) When compared to OC breath holding, when you vent and get fresh air, your first few breath cycles on that air can reduce your body CO2 into the same volume of fresh air, for a short while, while on OC that would take a fresh lung full of air each time.

How does this differ for BC rebreathing:
1) Breathing stale air with high CO2 from a rebreather loop gives you a solid mouth piece attached to you, while your BC manual inflator does not. So its possible to keep doing it longer into panic stages of blood CO2.

 

[MichaelMc]

No, the amount of effort required to exhale that last 10% of breath counters any gains and probably results in a net increase in gas consumption.

That does seem the trade off. Yet the gain would seem higher that 10%. Male residual (1.2) + expiatory reserve (1.2) + tidal (0.5) is 2.9 liters. So 2.9 liters holds my CO2. Normally I only flush 0.5 (20%) of that when I could flush 1.7 (58%) of it. Even going halfway into the reserve, 1.1 (38%) would be a big gain. Assuming I'm breathing relaxed, tidal, to start.

 

[Rred]

Rebreathing from your bc may give you a headache, and if you haven't bothered with the PITA of rinsing it out and drying it, it may also give you lung infections or fungus, sure.

But the point is it is filled with air (if you haven't tripped a CO2 inflator!) and starting with 20% air, you do not metabolize all of that when you breath, so you can and will get SOME additional useful breaths from it. SOME oxygen beats NO oxygen every time, regardless of having to put up with a headache or risk lung complications.

Folks who disagree are of course welcome to try finding other Hail Mary plays, but there are limited choices when there's no other air source.

 

[Diver0001]

In a hypothetical situation because of some problem such as entanglement you find yourself getting low on air without a buddy are there any techniques one can use to make your air last longer?

Would skip breathing help?

I'm looking for fact based answers other than the obvious (stay calm, move slowly, etc.).

Relaxing helps a lot with air usage. I once took a type of freediving course and over a short time I was able to improve my air consumption by quite a bit by getting more in touch with my body and learning how to relax more. Breath holding was part of that course but my conclusion is that while breath holding (or skip breathing) may help you to considerably extend the time between two given breaths, it is NOT a good strategy for improving air consumption in the big picture. What you're doing with breath holding (or skip breathing) is delaying a threshold by seconds or minutes if you are very good, but it's only a temporary delay. No structural change can be expected.

Ergo, when you are entangled and running low on air, then :
a) relaxing to any significant degree isn't going to happen.
and
b) that is not the time to invent a new wheel.

If you are well and truly trapped, running low on air and without a buddy or anyone to help you then there is only so much you can do. Even being able to extend your air by a couple of minutes is only going to delay the inevitable for a short time. It is not a solution to being trapped.

One thing that has been discussed before on the forums, and this is something I admit that I would personally try if it were a matter of life and death, is to breathe off of the BCD. In other words, to put air in the vest and use oral methods to breathe in and out into the BCD like a makeshift "3rd lung". I think this would delay the inevitable a lot longer than skip breathing but of course it is also not a solution to being trapped. The risk in this, of course is that the CO2 build up would be so severe that you may fall unconscious without warning from this.... aside from the fact that even if you survive the incident you might die from a fungal infection from whatever fungus grows in BCD's. In other words, don't try it, even though I might.

R..

 

[Rred]

"you might die from a fungal infection from whatever fungus grows in BCD's."
We were taught to unscrew the inflator hose and RINSE the inside of the BC after every dive, because otherwise salt crystals can and will build up, and those are abrasive enough to eventually cut through the bladder. Salt is an antibacterial and antifungal, incidentally. But nowadays there are "BC washes" which will also contain something like quaternary agents, that are better antifungals. A capful in the BC when you are sloshing it around, and it stays nice and clean, as in sanitary.
Yes, another PITA for longer gear cleanup times, but what else is new.

 

[Dr Simon Mitchell]

Hello Crookshanky,

Your answers are great, and draw the correct conclusions in terms of what action is appropriate, but there are a couple of factual errors I think should be corrected.

That is correct Bert. Similar to nitrogen, the deeper you go, the higher the partial pressure and the greater the narcotic effect. That is why CO2 is a potential hazard for deep technical dives. Simple tasks at extreme depths can cause CO2 to build up and quickly incapacitate a diver.

This is a common misconception. The partial pressure of CO2 is essentially dependent on the number of molecules present. In a diver, and assuming there is no extra CO2 in the scuba cylinder (which is virtually impossible), the source of the CO2 is the diver's own metabolism. All other factors being equal it makes no difference if a diver stops breathing or hypoventilates (the scenario being considered here) at 10 feet or 300 feet. The rate of metabolism is the same and the partial pressure of CO2 will rise at the same rate. Thus, in this context depth makes no difference. Extreme depth IS a risk factor for CO2 retention for a variety of reasons which are not really relevant to a scenario in which a diver is forced to hypoventilate because dwindling gas supply. That may be what you are thinking of, but it is not really relevant to this discussion.

CO2 is 130 times more narcotic than nitrogen.

I am not certain about this number. You are completely correct to suggest that CO2 is narcotic (and irrespective of the relative potency, you are also correct to suggest that this is a good reason to avoid high levels). But to my knowledge, no one has ever experimentally defined the relative potency in respect of nitrogen for realistic exposures. Given that this is the subject of one of my current grant applications I would be interested in whether you can recall where you heard or read this?

Simon M

 

[Kevrumbo]

<snip>

CO2 is 130 times more narcotic than nitrogen.

I am not certain about this number. You are completely correct to suggest that CO2 is narcotic (and irrespective of the relative potency, you are also correct to suggest that this is a good reason to avoid high levels). But to my knowledge, no one has ever experimentally defined the relative potency in respect of nitrogen for realistic exposures. Given that this is the subject of one of my current grant applications I would be interested in whether you can recall where you heard or read this?

Simon M

It's a common often quoted relative metric in the Tech Forums that comes from this:

Studies on the narcotic action of various gases have shown that the ratio of narcotic or anesthetic potency of CO2 and N20 approximates 4:1, and that of N2O and N2 30:1. From these figures it can be calculated that CO2 has at least 120 times the narcotic potency of nitrogen. Our data would suggest that the narcotic potency of CO2 is even greater, i.e., several hundred times as great. . .

Undersea Biomedical Research, Vol 5, No. 4 December 1978 Hesser, Fagraeus, and Adolfson.

 

[Dr Simon Mitchell]

It common often quoted relative metric in the Tech Forums that comes from this:

Hi Kev,

Thanks. I had seen that paper some time ago but had forgotten the allusion to several hundred times. I think the misunderstanding arises from the context (actual narcotic effect vs narcotic potential). If you look at the results in the paper, the exposure to high levels of CO2 (which are nevertheless physiologically plausible and what I meant by "realistic exposures") did not produce decrements that were much different than those produced by nitrogen also in realistic exposures. The point being that in any realistic diving scenario (even a bad CO2 hit) CO2 will not produce narcosis that is 130 times worse than nitrogen. I think the conclusion you refer to relates to the narcotic potential of CO2. It is a more potent narcotic. If you were exposed to a PCO2 that approximated that of the PN2 exposures in their experiments the effect might be 130 times (or more) worse, but this is a largely irrelevant because no living organism could tolerate exposures much greater than those used in the experiment without encountering problems much worse than narcosis!!

Also, if I recall correctly, the CO2 limb of that experiment received a little criticism for combining high CO2 and oxygen at 1.7ATA. This is a combination of two potentially narcotic gases (oxygen and CO2), and raises the possibility of sub-clinical cerebral oxygen toxicity also affecting the results. A better design would have been to use a mix of oxygen (at an FiO2 producing an inspired PO2 the same as that at the surface), inspired CO2 sufficient to clamp the end tidal value (body) where you want it for the narcosis experiment, and the balance being helium. Then you really are isolating the effect of CO2. Or more simply, you could just do the experiment with inspired CO2 at the surface during air breathing.

Simon M