What Iphones are compatible with Apple Watches to take ECG

The iPhone model is not really relevant to the function of ECG App.

All you need for the ECG app to work is a compatible Series 4 watch with the most recent watchOS update. The Paired iPhone is not important. Note that the ECG app only works in the U.S. currently. So your Phone's region, your watch's region, and your Apple ID's region must be set to the U.S.

The paired iPhone can be any of the models that support the Series 4, which is an iPhone 5s(GPS), or 6(GPS+Cellular) or later.

More info here: Taking an ECG with the ECG app on Apple Watch Series 4 - Apple Support

Read this

https://alivecor.zendesk.com/hc/en-us

Yes, the "X" is compatible. ECG works with my "7."

I have Kardia and I'm very disappointed. The results are NOT stored locally. The only storage is in the Kardia cloud at a cost of $10/month. That being said, my "7" is compatible and I believe that all iPhones are compatible.

I don't pay for the Kardia storage. Instead, I bought the 40mm GPS Watch-4 and, in three years, I'll break even.

Apple Watch Series 4 (GPS) requires an iPhone 5s or later with iOS 12 or later. Apple Watch Series 3 (GPS + Cellular) requires an iPhone 6 or later with iOS 11 or later.