I mostly fly United and have a USB Battery powered knee cuff with heat. It gets pretty hot and will run for around 3 hours on hottest setting on a 10,000mah battery. According to the United website I am not supposed to use it on the plane because it is in the category of "things that heat up" so I don't. I leave it in my bag with battery removed.
https://www.united.com/en/us/fly/bag...c-devices.html
I have not tried getting a note from a Doctor and claiming it is a medical device. YMMV.