Apple gave us a hint of where the Watch was headed last year when it announced new heart-tracking features and a Stanford-led study to detect atrial fibrillation. But while they were intriguing, they weren't as potentially life-saving as the world's first consumer ECG device. With that feature alone, Apple has made its latest Watch essential, something practically unheard of in the wearable world. If you own an iPhone, your well-being is easily worth a $399 smartwatch (or $499 if you want cellular connectivity). Good luck beating that, Fitbit and Samsung.

Of course, it's only a matter of time until competitors bundle in ECGs of their own, but Apple will always have bragging rights -- not to mention a dedicated user base -- for doing so first. At this point, it makes sense for wearables to take more cues from sophisticated medical devices than to try to replicate the things we normally do on smartphones. We're also taking Apple at its word so far, since it remains to be seen how well its heart monitoring actually works. (It also would have been nice to see some battery-life upgrades to go alongside such an important new feature.)

This is bold new territory for Apple: It can't afford a Maps-like disaster now that it's handling important personal-health data. Too many false positives could quickly lead to consumers ignoring real issues. Still, there are already several reports about earlier Apple Watch models saving lives, so there's a good chance Apple's claims aren't just marketing bluster.

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