It’s all happening in the next 5.1.2 update

The ECG feature on the Apple Watch Series 4 could be about to land in the US, according to a new report.

MacRumours has obtained supposed training documents suggesting that the ECG feature has been in beta for a few weeks, and will be added to the Apple Watch in update 5.1.2 – which will land before the end of the year. That puts the release in line with Apple’s own schedule during the September keynote.
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https://www.wareable.com/apple/apple-watch-ecg-feature-coukd-drop-imminently-6779