Set up Heart rate notifications

Perhaps this article will help:

Heart health notifications on your Apple Watch - Apple Support

Hi there,

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:

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

You can find more information here:

https://support.apple.com/en-us/HT208931