https://www.techradar.com/news/the-apple-watch-4-ecg-is-98-accurate-at-detecting-afib-says-study

Apple revealed the Apple Watch 4 at its event earlier this week, and aside from the obvious screen upgrade, the new smartwatch added additional health features, including the ability to take an FDA-approved electrocardiogram (ECG). To do get this approval, they funded a long-term study that found the smartwatch’s ECG-detecting software could accurately identify a serious condition known as atrial fibrillation (AFib) over 98% of the time.

The Apple Watch 4 uses its wrist sensor to monitor the wearers heart rate. Its ECG feature specifically looks for heart rates that are too fast, too slow or too irregular – activity that’s linked to AFib. If it detects such activity, it alerts the wearer.