https://www.cnbc.com/2018/09/11/apple-watch-getting-ecg-says-ming-chi-kuo-why-it-matters.html

When Apple Watch was first released in 2015, it came with a built-in basic heart rate monitor. That pushed Apple into the health care sector, as users started reaching out to the company with stories about how the device had saved their life. As the company's COO Jeff Williams told CNBC in 2017, "We've been really surprised with the deluge of letters and emails we've gotten where just the simple heart rate monitor was able to have such an impact on people's lives."

Now, the company is expected to take these health ambitions a step further by introducing an electrocardiogram or "ECG" sensor that measures the heart's rhythm — and not just the heart rate.