Hypercapnia Hit and then planning for BO with a delayed Deco Switch?

[KenGordon]

By the sound of your work around/minor optimisations I think you are trying to compromise in the face of plans that don’t really work.

I am guessing you have found your Ali40 of 50% is completely empty while your Ali80 of 18/45 is 80% full following your bailout scenario.

Yes, have two Ali80s (or perhaps use the 40 for the deep bailout) but really you need to identify the weak link/limiting factor in the ascent plan and fix that.

I have three bailout cylinders, two 80s and a 7l. The 80s have 18/45 and 80% and the 7l 32%. The 80s get used if there is much deco, the 7 for dives as deep as 35m maybe.

 

[Heat Miser]

By the sound of your work around/minor optimisations I think you are trying to compromise in the face of plans that don’t really work.

I'm asking the question legitimately, because whilst I said I would take two S80 and try and have all bases covered. I also like to think of alternatives/hypotheticals under lost gas scenarios, buddy distress, not to over optimise.

I have MultiDeco on my phone and I'm not aware how to set gases for max time. I can see where you can override gas switch depths and I think it might be a useful override in the scenario I'm talking about where you have a lot of bottom gas and not a lot of deco gas. In Multi-Deco it lets you set the SAC rate for bottom gas of 45 ltr/min and assume 20 ltr/min SAC on deco and so you can lengthen out the bottom gas usage by delaying a switch to deco gas. Underwater your Shearwaters are telling you to optimally switch and are assuming you have unlimited amounts of deco gas - which you don't.

For example speaking to one of the people that this occurred to, they went back on their rebreather but continued to dil flush it, essentially running like a SCR in order to conserve gas. I'm not certain I would have had the balls / sense of awareness, to think that way during a Hypercapnia event.

I own 3 x 12 Ltr steel, 2 x 15ltr steel, 3xS80 Ally and 2xS40 Ally in Australia where we do Hydro once a year thats expensive.

 

[Wibble]

Alternatively is it sensible to switch to the richest mix possible as early as you can.

Hypercapnia aka "CO2 hit"...

The cause of the excessive breathing is the CO2 buildup in your body. Switching to any breathable bailout gas (e.g. the bottom bailout) means you will be breathing from a source guaranteed to not contain CO2, therefore it will offgas.

Can't see any reason for any difference with a rich mix vs a "weak" (e.g. bottom) mix.

As @Duke Dive Medicine said, it will abate after a while -- a few mins -- if you're relaxed. The problem when you're deep is that you are getting through a phenomenal amount of gas purely due to the depth (ATA) multiplication.

e.g. 70m/230ft is 8ATA. Breathing at a hypercapnic SAC of 45 litres/min consumes x8 = 360 litres per minute. An ali80 (11 litres wet volume x 200 bar = 2200 gas litres) will be emptied in 6 minutes!

IF you can switch to the bailout, then you need to get shallower to reduce the gas volumes being consumed. For example, getting up to 30m/100ft is 4ATA -- half the pressure of 70m -- meaning you've doubled the amount of time you can breathe from that bailout to 12 minutes. Or 16 mins if you go up to 20m. Hopefully that extra time will allow your breathing to calm down and you're more or less at your gas switch (somewhat ignoring any missed stops though).


Planning for hypercapnia is hugely frightening.

 

[Duke Dive Medicine]

Assuming all other things are equal, depth, resting, not breathing faulty reg etc. Will switching to a richer ppO2 mix reduce tachypnea (rapid breathing) faster? Or is it largely a function of reduced gas density that reduces rapid breathing.

Ah I see. Then, no, the amount of O2 in the mix wouldn't have an influence on the respiratory rate. What would matter is the amount of CO2 in the mix. Excess CO2 in the blood triggers the hypercapnic ventilatory response and until that CO2 level approaches normal, the respiratory rate will tend to be elevated. Gas density is one contributor to hypercapnia, but I don't see where it fits in this scenario. If the initial hypercapnia event is related to gas density, which is directly related to depth, then switching to bailout isn't going to do much.
<EDIT: Got to thinking about this... of course part of the EP for suspected hypercapnia in a CCR is to switch to bailout - in the moment, it could be difficult to discern the cause, and the above isn't a recommendation to not make the switch!>

Great conversation!

Best regards,
DDM

 

[JonG1]

It's worth bearing in mind in your deliberations that a co2 event increases blood flow to the brain, which can then trigger an oxygen toxicity issue if a gas with elevated po2 was selected during the ascent before your physiology had calmed down.

Simon Mitchell has observed that in clinical situations during hypercapnia in anaesthetic patients a ventilation rate of 30 litres per minute relatively quickly improves the end tidal co2. This was on a thread on CCRx.

Obviously there will be physiological and differences and variables and rate could exceed 30 litres per minute involuntarily, but it gives a feel for potential volume consumption I guess.

 

[Heat Miser]

Ok so let me throw another one out to cyber land.

Say you have a C02 hit, you have an S80 bottom and an S40 deco, you switch to bottom gas to clear the C02 hit which scenario on average is better.

1) staying on the deep mix as long as you can knowing when you do eventually switch to deco mix you can clear deco; Or

2) switching to deco mix as early as you can, at optimal depth whilst still ascending …. Assuming in breathing like freight train out of the deco gas but you can still switch back to your bottom gas and deco out on a lean mix, with a big headache over a long period.

 

[JonG1]

If you run some bailout plans on 2 bottle dives, often the limiting factor is the volume requiremnts of the deco gas, your obligations increases significantly post bailout due to the dropping po2 and helium content (required to keep densisty down), this then incurs more shallow deco.

Ergo typically an 80 of bottom mix would require an 80 of deco mix, but the BT would be limited by the content of the deco mix bottle.

In your scenario above, I think you would stick with the low po2 low density gas to clear the end tidal co2 and keep the risk of an o2 hit down, when your breathing rate is more normal, you then switch at the optimal depth.

That said a lot of the reports I have read suggest that breathing rates can take some time to normalise post hit.

1 option is to have a back-up GF 90/90 computer or table to get up and shallow fast and then pad as much as you can with whatever gas is available.

 

[KenGordon]

I have MultiDeco on my phone and I'm not aware how to set gases for max time.

on my iPhone It is a button “Maximum Time Plan” two thirds of the way down the bail page.

 

[huwporter]

1) staying on the deep mix as long as you can knowing when you do eventually switch to deco mix you can clear deco; Or

2) switching to deco mix as early as you can, at optimal depth whilst still ascending …. Assuming in breathing like freight train out of the deco gas but you can still switch back to your bottom gas and deco out on a lean mix, with a big headache over a long period.

I'd consider two factors;
A: Reducing work of breathing as much as possible is likely to help with clearing the hypercapnia - this would imply staying on your lowest density available gas for as long as reasonable
B: In a hypercapnic situation, you probably want to reduce the level of PO2 you choose to expose yourself to, assuming that hypercapnia is a predisposing factor for oxygen toxicity.

So that would imply tending towards strategy 1: staying on your highest helium % mix for as long as reasonable while ascending, and delaying switching to deco mixes either until hypercapnic symptoms have cleared, or, if you can't reasonably wait til they clear - running low on deep bailout, for e.g. - then even possibly considering switching to deco mixes shallower than their normal switch depth.

...This assumes you have helium in your deep bailout. If not then getting shallow as fast as possible would be more of a priority, I would think.

From personal experience, some level of hypercapnic symptoms in the water can last a while after removing the 'cause' - 30 minutes plus, 'brain fog' rather than elevated breathing for the bulk of that time in my case. Of course, that brain fog also makes it harder to make rational decisions on the fly like the one above. Hypercapnia is no fun.

 

[Alaskan Scuba Dude]

CNS toxicity (the ‘Paul Bert effect’) acts at higher PO2 levels after short exposures. It may develop within a few to many minutes on exposure to levels of oxygen above 1.8 bars and may have as an end result an epileptic-like convulsion that is not dangerous in itself (under normal circumstances) but that will be quite disruptive in any case and can result in drowning or physical injury. Susceptibility to CNS toxicity is exacerbated by other factors, particularly those that cause an increase in internal PCO2 such as exercise and breathing dense gas, breathing against a resistance, and by other physiological factors such as high body temperatures or thyroidism.


Verbatim.