The irregular heart rate monitoring is passive. Apple says that it checks your rhythm every two hours or so (depending on whether you're stationary or not), and if there are five consecutive readings that seem abnormal, it will alert you and suggest you reach out to a doctor. If you have been previously diagnosed with atrial fibrillation, Apple's setup process tells you not to use the feature. Apple tells me these features are most definitely not diagnostic tools. In fact, before you can activate either of them, you will need to page through several screens of information that try to put their use into context and warn you to contact your doctor if needed. They are also not the sort of features Apple expects users to really use on a regular basis. The EKG feature, in particular, should only really be used if you feel something abnormal going on, and then you should only share the resulting report with your doctor, not act on it directly. Angela Chen from The Verge notes that these features have only received "clearance" from the FDA, which is not the same thing as FDA "approval": The Apple Watch is in Class II. For Class II and Class I, the FDA doesn't give "approval," it just gives clearance. Class I and Class II products are lower-risk products -- as [Jon Speer, co-founder of Greenlight Guru] puts it, a classic Class I example is something like a tongue depressor -- and it's much easier to get clearance than approval.
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