https://www.theverge.com/2018/9/12/17850660/apple-watch-series-4-ekg-electrocardiogram-health-2018

The Apple Watch Series 4 will include a more advanced heart-monitoring technology called electrocardiogram. This feature has received clearance from the US Food and Drug Administration, meaning it can be used as a medical device — a move that is part of Apple’s increasing push to brand the watch as more than a fitness device.

The Watch Series 4 is the first major design overhaul of the Watch since its debut more than three years ago. The existing Apple Watch already had a way to measure heart rate, of course. Like most wearables, it monitors heart rate using green LED lights embedded in the device. The lights reflect on the skin to detect the pulse and changes in blood volume; this is turned into the heart rate number.