How to set up irregular heart rate notifications on your Apple Watch

You can enable notifications from the Heart Rate app on your Apple Watch to alert you to high or low heart rates and irregular heart rhythms.
On order to to enable irregular rhythm notifications, you need Apple Watch Series 1 or later with watchOS 5.1.2 or later paired with iPhone 5s or later with iOS 12.1.1 or lateran reside in the United States, Puerto Rico, Guam, or the U.S. Virgin Islands. Irregular rhythm notifications are not designed for people who have been diagnosed with AFib or intended for use by people under 22 years old.
High and low heart rate notifications
You can turn on heart rate notifications when you first open the Heart Rate app on your Apple Watch, or at any time later from your iPhone:

On your iPhone, open the Apple Watch app.
Tap the My Watch tab, then tap Heart.
Tap High Heart Rate, then choose a BPM.
Tap Low Heart Rate, then choose a BPM.

Irregular rhythm notifications

Make sure that the software on your iPhone and Apple Watch is up to date.
On your iPhone, open the Health app.
Follow the onscreen steps. If you aren’t prompted to set up, tap the Health Data tab, then go to Heart > Irregular Rhythm Notifications.
Once enabled, you can turn irregular rhythm notifications on or off in the Apple Watch app on your iPhone: Open the Apple Watch app, tap the My Watch tab, then go to Heart > Irregular Rhythm.

Apple Watch Series 1 or later with watchOS 5.1.2 sends a notification if an irregular heart rhythm such as AFib, is identified
 
If you receive a notification, Apple Watch identified an irregular rhythm suggestive of AFib and confirmed it with multiple readings. If you have not been diagnosed with AFib by a physician, you should talk to your doctor.
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