I've posted this before - Cook reiterated this explicitly in the Q4 earnings call (quote below):
Later this year, the ECG App will be available to Apple Watch Series 4 customers in the U.S., giving them the ability to take an electrocardiogram any time, right from their wrist. And for U.S. customers with Apple Watch Series 1 and later, WatchOS will soon enable periodic checks for irregular heart rhythms that may be suggestive of AFib. These are unprecedented and potentially life-changing features, showing how Apple Watch is not only an indispensable communication and fitness companion but also an intelligent guardian for your health. More broadly, we see this as just one further example of the kind of contribution we can make in the health space and we look forward to making more in the future.