The heart rate sensor in a phone is new but as many tech bloggers have pointed out it's inconsistent and inaccurate. You have inaccurate readings in the area like this could potentially be very dangerous
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They anticipated fitness biometrics and came out with it first again I applaud them for that but the implementation is so so.
So you've used it first hand? No - you've just read what a selected few (albeit perhaps trusted) writers have said so far. The product isn't complete yet though right? It's not currently being sold and its entirely possible accuracy could be improved. I have no idea. Possible though.
Further - I imagine that anyone who has a medical condition that requires heart monitoring isn't going to rely (solely) on their phone. At least this is the same argument Apple users gave when the iPad was reported to interfere with some pace makers. Naturally - those with pacemakers would KNOW and avoid putting the area where there are magnets on their chest.