Not gonna quibble about the usage of the hyper/hypo, other than to say that it is often used differently in diving lingo vs hospital lingo.
BUT, i am gonna beat on the retained CO2 dead horse a bit, since y'all know its one of my favorite topics, and it hasn't been beaten much as of late
What I will point out is that a lot of people confuse what is actually a host of issues (layperson terms here again) "retained" CO2 in blood (CO2 produced is more than CO2 being expired for whatever reason) vs. dead space CO2 (ineffective ventilation, full face mask, CCR design whatever) vs. CO2 scrubber breakthrough (in CCR's).
Generally what I feel impacts divers most often (again, layperson talking here) is nitrogen narcosis + CO2 build up in the body due to either rapid shallow breathing or skip breathing or most likely overwork for the rate CO2 exchange by lungs due either due to skip breathing, holding breath, or increased breathing resistance of equipment. I would argue that there is a tipping point, and I agree that people don't like CO2 build up in our system and even less so at depth. Once it gets to a certain point, (usually caused by trying to swim a bit harder or slow shallow breathing to not move when shooting a photo) coupled with some nitrogen narcosis, then it can hit what feels suddenly like a ton of bricks, I say 'feels' sudden, but if you are paying attention, you can actually feel the niggle coming on. Most people on a regular recreational dive (where the are not chasing to keep up with an instructor or mentor) handle this subconsciously at times by easing off on the swimming pace, stopping to look at something (and showing it to their buddy) hanging out shallower/above their buddy by 10' or gently swimming up slope without even realizing they've essentially 'turned' the dive.
Fritz, I'm happy to go out swimming with you and I'm quite sure we can demonstrate it with pretty much any high performance regulator you want to bring. (of course I'll be on trimix, because it's amazing what a wee bit of helium can do for breathing resistance and narcosis and one of us will want to be clear headed) we can do it swimming the boundary line in Cove 2 or trying to dig some heavy object out of the sediment and drag it around without a lift bag in the 50' zone
Most often, in class, it happens when a diver (student or otherwise) starts chasing their buddy, or trying to keep up. Because of what presents like narcosis, they forget to signal the team to slow down, and just swim harder, which builds up CO2 and they don't efficiently exchange it, and the cycle spirals until they either realize what is going on and stop it, just muddle through and report (or not report) being "really narc'd" on the dive, have a panic attack and bolt, or in a few cases it seems go unresponsive.
As a retired instructor, I believe this happens more often in a class or mentor situation where there is disparity between the buddies and one buddy is trying to keep up to be deemed worthy or pass the class/test, etc.
These experiences are often unreported, underreported, reported as a 'dark narc', not remembered, or 'i don't know what happened, just suddenly had to get out of there'. Even to this day, being narc'd and admitting it is kind of like admitting being a lesser diver. Maybe people won't want to dive with you, maybe they won't want to dive to 100' with you, etc... I think if we drilled down, many of the incidents that occur in that 'deep recreational zone' (the 80-130) on air can be attributed to CO2 compounding narcosis issues.
I did not pay much attention to all this until I started diving CCR, and in learning more about CO2 and its compounding effect with nitrogen narcosis it became clear to me that it is something we should all at least be a bit aware of OC or CCR.
OF COURSE there are also a ton of variables, people who are in shape, people who dive more efficiently, people who put a bit of helium in their mix, high performance regs, vs. rental gear, vs. CCR's... People who tolerate more or less uncomfortable feelings. People who only dive with divers who swim at similar pace so they are never chasing. People who simply don't dive deeper than 80' and don't really know why. Some folks have higher tolerance and have never experienced any of this... So to the readers, don't take my diatribe as gospel or blanket statement, they are just my observations and experience, which may differ dramatically from other peoples.
<we now return you to regularly scheduled programming>