When Apple announced the Watch Series 4 in September a big focus was placed on the smartwatch' ability to perform an electrocardiogram. But the feature was unavailable at launch. Now, Apple is activating it in the US through watchOS update 5.1.2.
The update adds the ECG app to Watch Series 4 devices in the States (where the ECG is currently only available) and also unlocks irregular heart rate warnings on all Apple Watch from Series 1 through to Series 4.
The Apple Watch will passively read your heart rate in the background and will warn you if it spots a suspicious pattern.
As for the electrocardiogram - the Apple Watch Series 4 uses electrodes in the crown of the watch along with the special LEDs in the crystal underneath the watch to read the electrical impulses in your finger.
Both the ECG and the irregular heart rate warning features are intended to serve as health monitors. The Apple Watch Series 4 will give you a detailed PDF with you ECG so that you can forward it to your doctor.
Meanwhile it could be years before the ECG functionality of the Apple Watch makes it outside of the US.