This discussion might provide some insight Reading BLE service in Appinventor
This is for an arduino but some of it might apply to what you are attempting https://appinventor.mit.edu/explore/blogs/karen/2016/08/heart.html
another Heart rate variability viewer (BLE + Arduino)
and here is a Google search that might have articles you have not seen before https://www.google.com/search?q=app+inventor+heart+rate&rlz=1C1CHBF_enUS887US887&oq=app+inventor+heart+rate&aqs=chrome..69i57j69i60l3.6999j0j7&sourceid=chrome&ie=UTF-8