Apple Watch not reading heart rate? Here's the fix. - MyHealthyApple
Gurman: Apple Watch Series 9 and Ultra 2 to Feature New Heart Rate Monitor, More Accurate Sensors, U2 Chip, and More - MacRumors
Accuracy of the Apple Watch 4 to Measure Heart Rate in Patients with Atrial Fibrillation - IEEE Journal of Translational Engineering in Health and Medicine (JTEHM)
Here's What Heart Health Experts Think About the Apple Watch Series 7