To answer your question about the difference between "clinical grade" and "consumer grade" hardware, consider [1] - where FitBit-measured heartrates differed from actual heartrates by about 30bpm. As a doctor I looked into getting a fitbit but realised they could not live up to the ideal of heartrate tracking that I wanted.

To be fair this is a hard problem. Even the pulse oximeters we use in our hospitals have a hard time picking up certain heart rhythms. The only way to be sure is to get an ECG done!

[1] http://www.telegraph.co.uk/technology/news/12086337/Fitbit-h...