Use Heart Rate on your Apple Watch to monitor your heart rate throughout the day.
How to pair an external heart rate monitor to Apple Watch
Apple Watch - Official Apple Support
Apple Watch User Guide - Apple Support (CA)
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How to Set Up an Apple Watch for Your Kids
GoolRC DM101 4G Smart Watch WiFi BT Smartwatch 2.41-inch Touch
Record an electrocardiogram with the ECG app on Apple Watch