Are you building an app for a covered entity or entities covered by a business associates agreement or are you writing apps for patients directly?
I would recommend you start with this quiz[1] and see what it says. There is a chance you would only be bound to your terms of service.
Also, I'm a huge Health IT geek and if you want talk about your idea I'd love to hear about it. My email is in my profile.
1. https://www.ftc.gov/tips-advice/business-center/guidance/mob...
I attended one of his in-person seminars back when you could go places and meet people face to face, it was eye-opening.
Also please don't hesitate to reach out directly to me (put an @gmail after my username) and I'd be happy to exchange some emails.
Good luck