Manfred
First, many thanks for all your contributions to 4BX.
I'm surprised your doctor has advised you to measure your SpO2 levels as these will be normal during waking time and the only way to see if they dip in your sleep is to have a continuous monitor all night. The finger one you gave a link to auto-off's after 8s to save battery. They are fine for intermittent checks in patients with acute or chronic lung problems.
There are sleep apnoea sats machines available for around £100 on Amazon that link to smartphones and some vibrate the finger probe if your sats drop.
Your sats drop as a rather late event when you have an apnoeic episode - ie just before you are roused from slumber to shift position! So all the vibration is doing is waking you very slightly earlier and perhaps waking you more than the apnoea would. You will still have a disturbed night's sleep and be tired in the morning.
There is one I've seen on ebay that measures breathing from the nose that might be a bit better as a diagnosis/monitoring tool although it may not detect mouth breathing.
The polysomnography you might have had to diagnose the condition is quite complex. The best ones are done in a hospital environment and measure brain waves, heart rate, breathing patterns, chest movement, O2, CO2, limb movement, sound, etc. Simpler home testing kits provided by hospitals still measure more than just your sats.
There are guidelines to the minimum spec for home polysomnography - it must record airflow, respiratory effort, and blood oxygenation.
The key to diagnosing (and monitoring treatment) is the number of apnoea-hypopnoea episodes per night - ie how often you stop breathing which may or may not cause a drop in sats.
See
https://www.nhsinform.scot/illnesse...e-sleep-apnoea#about-obstructive-sleep-apnoea for more details.
Having said that, there are studies where they have used high resolution sats probes to guide therapy.
See
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6650535/
The paper discusses why their probe was more relevant than a standard probe in picking up the equivalent of an apnoea-hypopnoea event.
If you want to buy one, I'm sure it would be quite interesting but I would caution there are risks.
I wouldn't make one other than for the fun of it as it may not be calibrated.
I'm a cardiologist and see more and more people bringing in their iPhone heart rate readings or similar. Some are useful but a lot are quite inaccurate - perhaps by 20-30 beats/min. Some of these gadgets may be more harmful if it gets you worried or obsessive about the numbers.
NB I'm not a sleep apnoea expert & the above is not medical advice. Just my opinion as a techie doctor.
Another good paper on this technology is
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5883394/
And a great book on sleeping heath is "Why We Sleep: The New Science of Sleep and Dreams" by Matthew Walker.
Allan