Having spent quite a lot of time connected to some of these last year (& I’ve even got an embedded ICD now that measures all this stuff from a wire in my heart AND sends it somewhere by wifi but I have no access to that (probably a good thing
). It was frustrating they didn’t have any way of sharing any more than pictures of data with me. (I did ask)
this would certainly be the most useful - strongest and likely most stable signal. It is just an electrical signal measured from one of your limbs to your chest. I wonder if you could actually rig something up with an Arduino type affair (it wouldn’t be medical grade but that’s not what we are aiming for)
This is the one that interested me the most - made lots of interesting sounds when they measured my heart. You can get home ultrasounds for pregnancy - has anyone tried these? (they are a bit expensive for take a punt)
I have no idea what they measured or got when I was in that thing. but kind of unwieldy 
following with interest