In
post #49 I replied to
String, talking about CO2 retainers in the sense that doctors usually talk about them. These are people with chronic obstructive pulmonary disease (COPD) who have disrupted breathing-control mechanisms.
Subsequent posts and articles cited in this thread made me realize that people were talking about CO2 retention in more than one context. This included questions about CO2 in the light of altered breathing patterns among divers (skip-breathing and breath-holding). Duh!
The OP inquired about what level people use as their cut-offs for O2 exposure when diving with Nitrox. It seems, though, that this has generated significant interest in gases in general and how they inter-relate while diving.
I am not a technical diver, and I'm still riding the learning curve as to how some of the things I learned in medical training relate to diving. Nevertheless, since people are tossing out hypotheses, let me clarify and expand upon my earlier remarks with respect to CO2 retention, for whatever it is worth.
In Physiology 101 we all learned that the purpose of breathing is primarily to (1) take in oxygen which all tissues need to survive, and (2) to get rid of CO2 which is a by-product of normal metabolism. Keeping the CO2 in check is also important in helping to regulate blood pH balance.
In Diving 101 we learned that the air we breathe is mostly composed of nitrogen, a bystander which may or may not be so innocent when consumed in increasing quantities.
Now, ventilation (breathing) is basically the process which moves all these gases in and out of our lungs. Cells in our brains and the tiny little sacs in our lungs monitor all these gases and work together to facilitate the exchange of the gases to and from the blood.
Although breathing itself seems simple: breathe in, breathe out, repeat -- its control is actually quite complicated. There are many reflexes, feedback loops, and control systems that match gas exchange with metabolic needs. The respiratory system is primarily regulated by the brainstem which monitors CO2-induced pH changes in the blood. Breathing is also controlled by the level of O2 in the blood, but to a much lesser degree. In normal people, CO2 is the predominant controller of the drive to breath, and this is regulated by both neural and chemical sensors that attempt to maintain CO2 levels within a very tight range.
While the metabolic rate influences CO2 production, the most influential factor which affects CO2 levels is ventilation. Increased ventilation enhances the elimination of CO2, leading to lower blood levels. Conversely, decreased ventilation impairs the elimination of CO2, leading to higher blood levels. In medicine, we talk about hyperventilation as a way to "blow off CO2," and when we see high blood CO2 levels, we look to see whether the cause is decreased ventilation.
Things that decrease ventilation are (1) slower breathing rate and (2) increasing dead space. (
Dead space is space where air moves back and forth, but where gas exchange does not take place.)
In divers, slow breathing may be promoted by diving at depth, but it is usually intentional.
Increased dead space among divers can occur when the distance between the mouth and inhaled air is lengthened (such as with a snorkel or old-style back-mounted regulator). Dead space can also be "created" by shallow breathing which moves air back and forth within the bronchial tubes but not enough beyond them to get adequate air exchange.
Does the level of CO2 affect one's sensitivity to nitrogen or oxygen toxicity? Maybe. Increasing CO2 lowers blood pH (makes it more acidic) which, in turn, can cause all kinds of problems at the cellular level.
Here's the problem in a nutshell as I see it: Diving is not normal. We were not made to live under water. When we force the issue by creating devices that allow us to spend time in a world that is unnatural to us, bad things can happen. We may think we have thought about all the things that can go wrong, but there is much that we do not know and much that we do not understand. Even if we did, I think the processes going on simultaneously are so complicated and intricate that trying to control them all would be like those people who try to spin more and more plates on sticks. It can get to be just too much.
It is nice that smart (and, I suppose, a few dumb) people get curious and figure out ways to extend the boundaries. As for me, I'm content to stay within the limits of what all those human guinea pigs have found works.
Still, one of the neatest things about diving is the challenge of figuring out how the physics and the physiology mesh. That's why I, too, love discussions like this thread where everybody chips in.
I'll say one thing: The more I read on this board, the more I realize how much I do
not know about diving, medicine, and life. So I
appreciate all the contributors who take the time to share their knowledge and experience with relative newcomers like me.
