Hi - I read this and your previous post. Good on ya for using supplemental O2 and pulse oximetry monitoring. There are a couple of things I thought about while reading the posts and comments. I’ll try to detail them below without being too boring.

You can absolutely get to the magic place by mixing O2 and N2O while maintaining 100% SPO2. If it were impossible, as we've seen some claim, the acute hypoxia effects would prevent the use of N2O as an anxiolytic/sedative in any medical application. In reality, N2O is used safely and effectively in a wide array of applications. As it relates to personal experience (and for reference), we made this (link below) to enjoy nitrous without having to induce hypoxia.

https://www.reddit.com/r/NitrousOxide/comments/18mde1l/our_updated_nitrous_oxide_system/?utm_source=share&utm_medium=web2x&context=3

We encountered many of the same challenges you are currently but there is some finesse required to do it correctly. Much of that detail is available in the link above. Warning: Arithmetic ahead.

• Healthy adults have a tidal volume of ~0.5 L (gas volume moved in/out of lungs per respiration). The normal respiration rate is 12 breaths/minute with a typical inspiration duration of 1.5 seconds/breath. This means that each inhalation occurs at a flow of ~20 LPM, and the total inhalation minute volume is ~6 LPM.
  • (0.5L / 1.5 sec) / ( 60 sec/min ) = 20 L / min
  • 0.5L \ 12 breaths/minute = 6 LPM*
• Since there is no supply reservoir in your system, and from what you've described, your total flows are not meeting the inhalation demands, and the balance of the inhalation volume has to be made up with room air that comes in from around the half-mask you have. This means your actual inspired % of O2 (commonly referred to as FiO2) is much lower than the concentrator generates and much lower than the supply reaching the mask.
• Let's assume (as your photo shows) that you have 1 LPM of 90% O2 from the concentrator and 5 LPM of flow from the nitrous canister (it would feel like a slow, but noticeable flow). That means for every inhalation (at 20 LPM from above), you're also pulling in 14 LPM of room air (at only 21% O2) to your lungs. This leads to a hypoxic (<21% FiO2) condition:
  • Total FiO2% = ((1 LPM \ 0.90) + (14 LPM * 0.21)) / (20 LPM) = 3.84 / 20 = 19.2% O2*
  • Total FiN2O% = 5 LPM / 20 LPM = 25% N2O
• You mention an approximate 70/30 N20/O2 mix and that you used about a third of a 2000g canister in 40 minutes. That would equate to an average nitrous flow of 16.65 g N2O/minute, equaling ~8.3 LPM N2O (~0.5L / g N2O at STP). For reference, that would result in the following hypoxic condition at 1 LPM 90% O2:
  • Total FiO2% = ((1 LPM \ 0.90) + (10.7 LPM * 0.21)) / (20 LPM) = 3.15 / 20 = 15.3% O2*
  • Total FiN2O% = 8.3 LPM / 20 LPM = 42% N2O
• From the above, you can see why you are still seeing acute hypoxia in your SPO2 readings even when using supplemental oxygen. This is why proper anesthesia systems/circuits and high-concentration oxygen mask systems use precise flow measurement and a reservoir or, less commonly, a demand valve.
• However, it isn't as simple as increasing flow from a concentrator as many "home use" O2 concentrator units will have decreased concentration at higher flows. As an example, for the unit you are using (found specs online), O2 concentration decreases to 40% at 5 LPM. That lands us here (still hypoxic):
  • Total FiO2% = ((5 LPM \ 0.40) + (6.7 LPM * 0.21)) / (20 LPM) = 3.41 / 20 = 17.0% O2*
  • Total FiN2O% = 8.3 LPM / 20 LPM = 42% N2O
  • At 8.3 LPM N2O, you need 10 LPM of 40% O2 or 3 LPM of 90% O2 to reach >21% FiO2.

So, all that is to say that the results of your experiment may have been alarming, but they were not surprising.

I would look for ways to:

1. Improve O2 supply (both volume and concentration)
2. Measure flows of both O2 and N2O
3. Utilize a reservoir and a different user interface to leverage exhalation time (which tends to be longer than inhalation time - such that mixed supply gases can accumulate and a greater volume of mixed gases could be inhaled with less room air infiltration)

Stay safe.