Wildly fluctuating O2 levels

I don’t know about blood rushing past too fast but a technician did tell me that alveoli can be damaged by things like smoking so that the walls are tougher and the o2 molecules have a harder time getting through.
It would be like punching your fist through drywall-thicker would be harder to punch through. Maybe the blood would have gone past by the time the o2 got through.
I don’t know if diseases like Covid or respiratory infections could damage alveoli, but it might be worth looking into.

Like so many of us, this is such a complicated puzzle and there are so many factors contributing to how are bodies regulates blood oxygen. Why my dr's have all said there isn't much more we can do for you is because all my testing comes back pretty normal and I appear healthy. They say if my 02 numbers really are what I tell them they are you wouldn't be able to run 15 miles like you do. My DLCO was actually 120% of predicted. Also, my a-a gradient (for those who have done CPET you might know about this) is 4 at rest. Because chat gpt can explain it better than I can, here is what an a-a gradient of 4 means: A resting A–a O₂ gradient of 4 mm Hg (≈ 0.5 kPa) means that the partial pressure of oxygen in the alveoli (PAO₂) is only 4 mm Hg higher than the arterial PO₂ (PaO₂). That is very low—well below the usual upper-normal limit of ~10–15 mm Hg in healthy adults—and indicates exceptionally efficient gas exchange with no evidence of ventilation–perfusion mismatch, diffusion limitation, or shunt at rest.

My a-a gradient at peak exercise was also normal as well. However, my pao2 at peak exercise was at the low end of normal, so even though my a-a gradient was normal (meaning good gas exchange) I believe this is where some clues suggest that the issue may be entirely 'ventilation'. Which does makes sense, low tidal volumes at rest, you hit a flight of stairs at 5000 feet and wham, o2 drops to 83% and quickly recovers. The issue becomes, when I go for a run, and my breathing has picked up, why does my 02 fluctuate between 84 and 89.... the harder I run the lower it goes. But I can literally pace my o2 in my runs by knowing what my heart rate is.

Lastly, learning the science behind all these numbers and tests has been quite enjoyable.... unfortunately it comes at the expense or my health! 🙁

Wow. You know a lot more than I do about this. Thanks for the technical explanation; I’ll have to reread it to digest it all.
I’m sure this “adventure” also comes with a cost to your mental health, as mine did.
I even went for a consult at Mayo Clinic, where I was told I do not have restrictive lung disease.
So those numbers I saw remain a mystery. I’ve taken the approach of no longer checking at home as this was really messing me up emotionally.
I am active, exercising and weight training, so I just ignore it.
If you can do half marathons then that’s very encouraging.
You mention a ventilation issue-you mean your lungs just can’t take in enough air at exertion?
Would that have to do with big lungs, normal pipes? Or is there something about the way you’re breathing that a coach could help you with? Perhaps a holistic practitioner? Have you been checked for things like exercise induced asthma?
I find it interesting that your cpap use doesn’t help the o2 levels. I also have sleep apnea, and it does make a difference. I spend much less time below 90 or 88-just minutes as I recall.
Please let me know how your visit in Denver goes.
I wish you the best, and keep on running.

I’ve been dealing with is issue for the past several weeks now. I can sit with 3 liters of O2 with my O2 showing 98/99 and my HR AT 60. As soon as I get up and start walking my O2 drops into the mid 80’s and my HR goes up to the 90s and above 110.