ECG functionality will be enabled on the Apple Watch Series 4 in the publicly released version of watchOS 5.1.2, which is currently in beta testing, according to an internal Apple Store training document obtained by MacRumors.

watchOS 5.1.2 will feature an Apple-designed ECG app that can indicate whether your heart rhythm shows signs of atrial fibrillation, a serious form of irregular heart rhythm, or a normal sinus rhythm. Apple Watch Series 4 is capable of generating an ECG similar to a single-lead electrocardiogram, according to the FDA.

ECG and other heart data is collected in the Health app on a paired iPhone. Apple's document lists a minimum requirement of an iPhone 5s or later running iOS 12.1.1, currently in beta testing, or later.

To take an ECG reading from the Apple Watch, users will need to place a finger on the Digital Crown while wearing the watch. The reading is completed in 30 seconds, allowing users to determine whether their hearts are beating in a regular pattern or if there are signs of atrial fibrillation.

Apple's training document states that the ECG app is regulated and only available in regions cleared for use. The app will initially be limited to the United States, where Apple has received FDA clearance, but Apple Watch Series 4 models sold worldwide include the electrical heart sensor for future expansion.

Apple Store employees are instructed to advise customers that the ECG app is "not intended to be a diagnostic device or to replace traditional methods of diagnosis," and "should not be used to monitor or track disease state or change medication without first talking to a doctor," according to Apple's document.

Apple promised that the ECG app will be available later this year, but it's unclear when watchOS 5.1.2 and iOS 12.1.1 will be released. watchOS 5.1.2 has been in beta testing since November 7, while iOS 12.1.1 has been in beta testing since October 31, so both software updates should be released relatively soon.

A separate Apple training document obtained by MacRumors suggests that irregular heart rhythm notifications will also be enabled in watchOS 5.1.2 on Apple Watch Series 1 through Apple Watch Series 4 models.

The ECG app will not be available on previous-generation Apple Watch models, including the original through Series 3.