For anyone wanting more details about what apple is doing with ECG and arrhythmia notifications, there’s a nice pdf:
Learn more about arrhythmia detection (PDF)
https://www.apple.com/healthcare/site/docs/Apple_Watch_Arrhythmia_Detection.pdf
Meanwhile, I finally caught an ectopic beat in the act! It looks just like it feels (beat beat nobeat THUMP beat beat). These are normal–almost everyone gets them and most don’t notice. But once you do notice, you can’t unnotice them (at least I can’t) and it’s pretty annoying. I’m hoping that by hunting them down I can get bored enough to stop noticing again.