The Series 4 is the first truly big update to the Apple Watch line. It changes everything: the display, size, the shape, the sensors, the processor, the speaker and microphones—you name it. It truly elevated Apple’s wrist-wearable to a new level, but its most exciting new feature has been missing action since the Series 4’s September launch.

With today’s release of watchOS 5.1.2, though, that feature is finally here. Apple has given us the first smartwatch with a built-in electrocardiogram (ECG) feature (and they say Apple doesn’t innovate anymore!).

Here’s what you need to know about ECG on the Apple Watch Series 4.

How to take an ECG reading

Once you have updated your Apple Watch Series 4 to watchOS 5.1.2, you’ll notice there’s a new ECG app. The first time you run that, you’ll be prompted to open the Health app on your iPhone.
apple watch ecg app IDG
The first time you launch the ECG app, you’ll be prompted to complete setup on your iPhone.

In the iPhone app, you’ll be prompted to set up the ECG app by entering your age, as the app is not approved for use by people under 22 years old. You’ll then pass through several screens describing the kinds of results you may see: Sinus Rhythm, Atrial Fibrillation, Low or High Heart Rate, or Inconclusive.

After passing through several more screens with explanations and warnings, you will be prompted to take your first ECG.
apple watch ecg setup IDG
The initial ECG setup process on your iPhone explains the results and warns you about the limitations of the Apple Watch ECG.

To take the ECG, first make sure your Apple Watch is snug—a loose band can cause a bad reading. The open the ECG app on your Series 4 Apple Watch.

When you see the cloud of dots form a heart, touch your finger to the digital crown and hold it there for 30 seconds. You’ll see a countdown on your watch.