ANT or Bluetooth Heart Monitor Strap Compatibility ???
WHat About Compatibility With A Separate More Accurate ANT/Bluetooth Heart Monitor Strap???
Apple Watch
Hi
If and when preferred to using the built-in heart rate monitor, Apple Watch can measure your heart rate via a paired external Bluetooth chest strap heart rate monitor.
More information:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Use Bluetooth accessories with your Apple Watch - Apple Support