@Oliver1655 about the O2 monitor, we have one inexpensive one here passed from late mother-in-law to wife(used during congestive heart failure/lung fluid issues) and post-heart surgery. With Covid, I decided to get some "spares" as that one still works but has seen a LOT of use. We got 2 of these to have in the "medical pantry". Tested both, both work well.
https://www.amazon.com/gp/product/B07PQ8....

They run on AAA batteries, we have spare alkalines and a LOT of rechargeable ENELOOPs that also are used in our LED headlights(hurricane and freeze power outage use). The ENELOOPs work. I've a pile a AA's used with band and in cameras. The band-use ones have seen over 800 shows(charge, use for 4 hours, recharge prior to next gig, repeat) since about 2008 IIRC. We DO NOT use the "pro" versions as their total life span is shorter and they cost more.
AAA's used here:
https://www.amazon.com/gp/product/B0176D....
AA"s used here:
https://www.amazon.com/gp/product/B00JHK....