Overshooting NDL and mandatory deco stops

[Graeme Fraser]

Autoregulation of carbon dioxide is extremely precise in the mammalian organism. What should surprise you is how little carbon dioxide production decreases under anesthesia. It would be even less during sleep. Metabolism is primarily affected by body temperature and exercise, with the latter dominating.
Hence, the mandate to relax while diving. Given the various inefficiencies in our equipment, it becomes easier to see why exertion on a rebreather can be fatal.

Like many biologic systems, RMV data follows a bell curve of one shape or another. I wish my SAC were lower, but it just ain't. So I work really hard on streamlining, finning technique and resting. I am really tempted to try skip breathing since I know I won't get hypoxic, but as a physician I know that way leads to catastrophe. And as a diver wanting to start CCR, I absolutely have to resist the temptation.

My comments above were prompted by seeing on a daily basis the adjustment in end-tidal carbon dioxide that occurs when I change ventilation in my patients. In the early days of end-tidal CO2 monitoring, truly frightening instances of hypercarbia were seen with what were assumed to be "adequate" ventilation, along with more than one cardiac arrest.

So do all those things you were taught, to relax and streamline. Breathe fully (not shallowly) to minimized dead-space ventilation, and accept what you get. We just can't all be like @scubadada and Crusader, sad to say.

That's fascinating. Thank you for sharing your expertise.

 

[Ayisha]

I cannot easily add a 2nd x-axis to my graph. Here is a translation to metric units:

1 cu ft = 28.3 liters
1 liter = 0.035 cu ft

< 0.3 cu ft/min = <8.5 l/min
0.3-0.39 cu ft/min = 8.5-11.0 l/min
0.4-0.49 cu ft/min = 11.3-13.9 l/min
0.5-0.59 cu ft/min = 14.2-16.7 l/min
0.6-0.69 cu ft/min = 17.0-19.5 l/min
0.7-0.79 cu ft/min = 19.8-22.4 l/min
0.8-0.89 cu ft/min = 22.6-25.2 l/min
0.9-0.99 cu ft/min = 25.5-28.0 l/min
>1.0 cu ft/min = >28.3 l/min

Now that I see that my .3 cf/m equals 8.5 L/m and BlackCrusader's is 6.85 L/m, I can see why his causes some consternation. I don't think I've been diving with anyone who has a lower consumption than mine, and I'm a petite female. Though I have been diving with even some males who have about the same consumption.

It does sound like, though, from BlackCrusader's videos, that he breathes very shallowly. It was a little painful to listen to him suddenly and quickly take a partial breath, then exhale extra slowly but not deeply, then take a quick partial breath again. I do believe he may be retaining CO2. It might be better to breathe a bit more "normally". It's ok to have 8 or 10 or 20 L/m.

 

[rsingler]

Remember from @Dr Simon Mitchell 's posts, (and I'm working from memory - sorry!) there may be 25-33% of divers who do not get the "usual" sensations of panic and shortness of breath that typically accompany hypercarbia. Therefore they could comfortably operate at significantly lower RMV's, because there are plenty of oxygen molecules at depth.
That group of divers (who wouldn't know of their hypercarbia), just like everyday folks with emphysema, ride constantly closer to the knife edge of acidosis (and changes in mental status) if anything causes their CO2 to increase (e.g., exercise). But they never know, because we can't yet measure end-tidal CO2 in divers outside the lab.

 

[dmaziuk]

Gotta wonder how many "CO2-tolerant" freedivers there are. They train for it specifically.

 

[rsingler]

Brought a patient into Recovery one day. A nurse came over, asking if I'd take a look at her patient, brought in earlier.
Oxygen saturation was 100% on supplemental oxygen. But patient just wouldn't wake up. She says he'd been doing a little better earlier, but now, just unresponsive. Something just didn't look right. Patient was tachycardic, but comatose.
Patient appeared to be breathing, but on closer examination, what appeared to be adequate respiratory effort and 100% oxygen sat was in fact grossly impaired ventilation, that just happened to be making a hissing noise somewhat similar to normal breathing. I can't fault the nurse.
We opened the airway and began to bag the patient. Lab arrived and drew a blood gas a few minutes into the resuscitation.
Even after several minutes of supplemental ventilation, pCO2 came back over 100! Normal is 40. I wonder how high it had been at the beginning of our efforts!
Patient woke up spontaneously a few minutes later, but had been just a few minutes away from a cardiac arrest due to acidosis.

Carbon dioxide is anesthetic.
Some residual drug, plus maybe being a non-responder to CO2, plus someone missing the respiratory impairment, all led to coma, instead of the expected awakening from the pain of recent surgery.

Hypercarbia can kill. Get part-way there by being a non-responder trying to improve your SAC. Now add some CO2 as you get caught in a little current. Make it a little worse with high gas density if the problem occurs at depth, and you can't unload your extra CO2. You become another statistic.

Yes, it's rare, and like most accidents requires a chain of events. But starting in a bad place because you're tracking the little SAC number on your new Perdix, trying for lower, is just a bad idea.
Yes, streamline. Yes to Zen. Get a bigger tank. Or just accept a shorter dive.

 

[scubadada]

I have found the SAC on the Teric screen to be not useful and have deleted it from my home screen. The gas consumption data at the end of the dive is much more useful.

 

[Mike1967]

I have found the SAC on the Teric screen to be not useful and have deleted it from my home screen.

I love it on my Perdix.

 

[uncfnp]

@rsingler and others...So what are the physiological factors that predetermine gas consumption? Things we have little control over. Is it the individual’s metabolic rate (and the resultant need to eliminate CO2) that is the primary factor for gas use and therefore the SAC/gas consumption rate? Why then do women seem to have lower rates?

 

[scubadada]

I love it on my Perdix.

What do you use if for?

 

[Jay]

@rsingler thanks for the very informative posts above. Especially this warning "Hypercarbia can kill. Get part-way there by being a non-responder trying to improve your SAC. Now add some CO2 as you get caught in a little current. Make it a little worse with high gas density if the problem occurs at depth, and you can't unload your extra CO2. You become another statistic."

I have found the SAC on the Teric screen to be not useful and have deleted it from my home screen. The gas consumption data at the end of the dive is much more useful.

I was going to ask why you find it not useful? EDIT / Adding: IIRC your SAC is quite stable. Perhaps that's why. Mine is quite variable ... each 1deg C will bump it by 0.1 approx ... a current will bump it by ~30% ... etc have I guess correctly?

What do you use if for?

I use it to keep an eye on my breathing rate every now and then. I don't focus on it too much, but it's handy to confirm what I suspect I'm doing. And if I get into a 'bind', it tells me how elevated my breathing rate is. (more on that later in another thread as it relates to @rsingler's posts above).