CO2 retention question

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wetvet

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Messages
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Location
Drayton, Ontario, Canada
# of dives
200 - 499
Hi DIR folks;

I was diving a couple of weeks ago with a DIR outfit a couple of weeks ago. The dive was to ~85 feet, and because of a problem my buddy had, we cut the dive short with a bottom time of about 15 minutes. I surfaced and asked the captain to unzip my drysuit so I could get out of it. My neck seal is fairly tight, and I can feel the pressure in my head go up when I'm in my suit for a bit. The captain told me that the reason I felt this discomfort had to do with the fact that I was breathing AIR, and that this gas was inappropriate to breathe on ANY dive. I got the "good for flat tires" speech, and I asked him if that was strictly a narcosis issue, or if there was another reason why I should never dive on air. I wasn't worried about narcosis at 85 feet, and had no intention of even approaching NDL times since the water was cold, and I was diving with a guy on an AL80.

I was told that CO2 retention was causing my headache, and that air made it much worse. He warned me of the dangers of passing out, and vastly increased narcotic effect of CO2 vs N, but it didn't make sense. He couldnt give me an answer that made me understand, thus this post.

I understand how CO2 retention can result from work underwater, shallow or skip-breathing, or poor supply of gas due to badly serviced gear, but air does not contribute to any of these.

CO2 is produced by the body under work...it is a byproduct of respiration....the amount in air is negligible, and the normal production of Nitrox means that the majority of gas in my tank is air anyway.

The theory of increased work of breathing doesn't wash with me for 2 reasons....1- the diaphragm and intercostal muscles are so small in comparision with the muscles used for swimming or working that the increase in workload would have to be severe to make a noticible difference in the body's total CO2 production, and 2- since oxygen has a higher molecular weight than Nitrogen, air should be less viscous than the EAN32 he was telling me to use.

I appreciate the vasodilation effect of Oxygen on the brain, and thus the reports of some divers of feeling better, but I have never noticed this response in myself. Is the DIR credo of "never dive on air" just a way to gain that extra bit of narcosis safety, or is there something I am missing??

Thanks for any information..


Wetvet
 
You are correct in your thinking. CO2 is made by your body. IT does not matter if the O2 content of the gas you are breathing is 21% or 32%. The CO2 is still offgassed at the same rate. The thing that 32% helps with is nitrogen, not CO2.

I do not know where he got his info but he is wrong.

If your neck seal is too tight, it WILL give you a headache. You might take a closer look in that direction.

James
 
Thanks James.....

The headache is not serious, I just get a feeling of relief when I take the seal off. The seal is only a weeeee bit too tight, and I am very nervous about cutting it. It wasn't really a problem, it just started the conversation.

Thanks for your reply.

Wetvet
 
i think he's getting helium (less viscous, less breathing effort, less hyperventilation and less CO2 retention in the lungs) confused with nitrox.
 
does DIR recommend using He as shallow as 85 feet?....Not Trimix myself yet, but know some of the basics.....seems fairly shallow to use helium.

Helium makes more sense for some ease of breathing, but at <3ATM, will it make a practical difference? Work of breathing is still very little.

Not questioning the program, just trying to understand the whys.

Wetvet
 
wetvet:
does DIR recommend using He as shallow as 85 feet?....Not Trimix myself yet, but know some of the basics.....seems fairly shallow to use helium.
Not that I have ever seen. Its on the > 100 ft depth that the helium is recommended to be added.

The only CO2 headache that I have had on air/nitrox was when I was working hard...working hard to stay alive. (Needless to say I was breathing pretty heavy).

If you were not working hard, it could be that your neck seal is a little to tight.
 
The wetsuit tightness around your neck could cause other problems. The carotid artery pressure tells your heart to slow down because it "thinks" the pressure in your head is too high. This is exactly the opposite of what you want to happen since your blood flow is too low from the restriction. You should fix that as you could pass out from it.
 
From my GUE education:

Consequences of 32% (and never air) to 100':
- Narcosis: not so much, oxygen is technically more narcotic than nitrogen, but it is metabolized, so things even out. We consider EAN to be equally narcotic to air.
- Nitrogen loading: nitrogen is what our tissues are absorbing and what causes DCS. Reducing this only makes sense. We use 32% consistently rather than best mix to build a personal "track record" of how we feel after dives on a standard mix.

Consequences of 30/30 (30% O2, 30% He, called TriOx):
- Advocated for 60-100' dives by GUE, other helium mixes deeper than this
- Nitrogen loading: Even less nitrogen... good thing.
- Less dense, full tanks weigh less. Means less of a buoyancy swing and buoyancy easier to manage at beginning of dive esp in shallows. Not a reason by itself to use the gas, but cool nonetheless.
- CO2 retention: Helium mixes are significantly less dense than air/nitrox, this causes a dramatically observable difference in the performance of my regs, especially with the exhalation stage. For most modern regs, the greatest amount of work done during a breathing cycle is on exhalation, basically the resistence of the little exhaust flappers (technical term there :wink: ). The resistence is related to the density of the media we are breathing. Less dense breathing medium, less work of breathing, more effective respiration, more CO2 expelled, less CO2 retained, less narcosis, less CO2 rentention symptoms (headache, decrease respiration efficiency, etc.).

CO2 is bad for two main reasons, it is 5x more narcotic than nitrogen and it binds to hemoglobin making decreasing respiration efficiency. Consequences of a "CO2 hit" underwater include: narcosis symptoms, loopiness, impared decision making, headache, tunnel vision, decreasing consciousness, unconsciousness. That last one sucks!

Either the dude on the boat confused triox with nitrox, or it was just hard to hear the difference on the boat. In the real world, few of us will us helium until the 80-90' range unless it is a really hairy dive (cold, low vis, high current). But man is it sweet for the 80-100'+ range. This is the range GUE terms "Recreational Trimix".

Hope this helps, if I confused anyone ask away. (Hope I did it justice Mike :wink: )
 
mer:
From my GUE education:

Consequences of 32% (and never air) to 100':
- Narcosis: not so much, oxygen is technically more narcotic than nitrogen, but it is metabolized, so things even out. We consider EAN to be equally narcotic to air.
- Nitrogen loading: nitrogen is what our tissues are absorbing and what causes DCS. Reducing this only makes sense. We use 32% consistently rather than best mix to build a personal "track record" of how we feel after dives on a standard mix.

There's also the theory (from GI3) that N2 bubbles cause a do-it-yourself sickle cell anemia condition where the RBCs change shape and block in capillaries, causing post-dive fatigue symptoms and ultimately bone necrosis... Both a reason for nitrox shallow and helium mixes deep...

I don't know if the mechanism is accurate, but 1-1-1 on air from a non-agressive NDL dive will leave me falling asleep on the ride home and feverish. EAN32 with 1-1-1 will not give me those subclinical DCS feelings. Whatever the reason, I think air is for tires...

Plus there's the advantages of opening up the O2 window, but that's really for decompression, not so much for NDL diving on nitrox...
 
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