Apple:

The ECG app on Apple Watch Series 4, a direct-to-consumer product that enables customers to take an electrocardiogram right from their wrist, is available in Hong Kong and 19 European countries, including France, Germany, Italy, Spain and the United Kingdom. In addition to receiving De Novo clearance in the US from the FDA, the ECG app1 and irregular rhythm notification2 are now CE marked and cleared in the European Economic Area.

This is part of the watchOS 5.2 software update, which is rolling out now.