Many software features on Apple devices are initially limited to the United States. However, international users have often been able to move their iPhone or Apple Watch region to the United States for access.
This is not the case with the Apple Watch Series 4 ECG app as it only works on models purchased in the US. Those who live abroad in Canada, the United Kingdom or elsewhere and have bought an Apple Watch can not use the region-switching trick to activate the ECG app ̵
Note that the ECG app requires an Apple Watch Series 4 with watchOS 5.1.2 paired with an iPhone 5s or later with iOS 12.1.1.
Both the ECG app and irregular heart rhythm notifications are regulated features of the Apple Watch. Both features have been awarded De Novo by the FDA for US users aged 22 and over. Apple says people who have already been diagnosed with atrial fibrillation should not use the app.
It is not clear when the ECG app will be extended to other countries. However, the internationally sold models of the Apple Watch Series 4 have the required electrical heart sensor. Apple is likely to get approval for the ECG app in other countries, but it has not yet announced availability.