Apple has announced that watchOS 5.1.2 will be publicly released today and will enable the long-awaited ECG app on Apple Watch Series 4 models in the United States. The update will also enable irregular heart rhythm notifications on Apple Watch Series 1, Series 2, Series 3, and Series 4 models.

watchOS 5.1.2 should be available through the Apple Watch app on a paired iPhone around 10:00 a.m. Pacific Time as usual.

Apple's COO and Apple Watch team head Jeff Williams:

Apple Watch has helped so many people around the world and we are humbled that it has become such an important part of our customers' lives. With the release of these heart features, Apple Watch takes the next step in empowering people with more information about their health.

Last week, MacRumors was first to report that watchOS 5.1.2 will enable the ECG app on Apple Watch Series 4 models, based on an internal Apple Store training document. ECG data will be available in the Health app and exportable via PDF on an iPhone 5s or newer, after updating to iOS 12.1.1 released yesterday.

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. This is possible thanks to new electrodes built into the back crystal and Digital Crown on Series 4 models.

Apple Watch Series 4 is capable of generating an ECG similar to a single-lead electrocardiogram, according to the FDA. Both the ECG app and irregular heart rhythm notifications are regulated features that have received De Novo classification from the FDA, making them available over the counter.

The ECG app will check heart rhythm every few hours or so, and if there are five consecutive readings that appear to be abnormal, the Apple Watch will alert the user to see a doctor. As for notifications, users will be alerted if an irregular heart rhythm is detected on five checks over a minimum of 65 minutes.

Importantly, in an internal document obtained by MacRumors, Apple cautioned 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."

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 says the ECG app's accuracy was validated in a clinical trial of around 600 participants in the United States:

Rhythm classification from a gold standard 12-lead ECG by a cardiologist was compared to the rhythm classification of a simultaneously collected ECG from the ECG app. The study found the ECG app on Apple Watch demonstrated 98.3 percent sensitivity in classifying AFib and 99.6 percent specificity in classifying sinus rhythm in classifiable recordings. In the study, 87.8 percent of recordings could be classified by the ECG app.

Apple is making good on its promise of making the ECG app available by year's end. watchOS 5.1.2 has been in beta testing since November 7, but the ECG app was nowhere to be found during the pre-release period.

Apple says the setup process for these heart health features will include details about who can use the features, what the features can and cannot do, what results users may get and how to interpret them, and instructions for what to do if users are feeling symptoms that require immediate medical attention.