When the Apple Watch Series 4 launched, it was lacking a key feature: electrocardiogram (ECG or EKG) readings.

However, this functionality is rolling out now, thanks to a software update, called watchOS 5.1.2, which was previously in beta testing. Developers had been testing this update since 15 November. It essentially adds the Apple-made ECG app to Apple Watch Series 4. The feature, which has been promised since September, has even received FDA approval. Apple is still working to get approval by the UK and other governments.

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Apple's ECG app can tell whether your heart rhythm has atrial fibrillation (an irregular heart rhythm) or a normal sinus rhythm. Apple Watch Series 4 users will see it once watchOS 5.1.2 is installed on their device. From there, getting a reading is as simple as placing a finger on the Digital Crown of the watch and waiting 30 seconds. ECG data will then be collected from the watch electrodes and displayed in the Health app.

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Users will have access to their ECG information and can see a chart of their heart rhythm. Their chart can even be exported to a PDF document, if they'd like, to share the information with their physician. If the app detects something wrong, such as an irregular heartbeat, it will suggest that users consult with a doctor. Again, the ECG feature will be limited to places where it has been cleared for use.

Apple has only received Type II FDA clearance in the US.