UPDATE December 6, 2018

¹ This feature is available on Apple Watch Series 4 in the US, Puerto Rico, Guam and US Virgin Islands, and requires iPhone 5s or later on iOS 12.1.1. The ECG app does not detect a heart attack, blood clots, a stroke or other heart-related conditions including high blood pressure, congestive heart failure, high cholesterol or other forms of arrhythmia.
² This feature is available for Apple Watch Series 1 and later and requires iPhone 5s or later on iOS 12.1.1 in the US, Puerto Rico, Guam and US Virgin Islands. The irregular rhythm notification feature does not detect a heart attack, blood clots, a stroke or other heart-related conditions including high blood pressure, congestive heart failure, high cholesterol or other forms of arrhythmia.