Amazing story from Reddit user edentel:

Ok holy hell.... strap in.

If you have the Apple Watch 4 please please update to the new firmware released yesterday and take your ECG. I did last night and tried it out. Weird. Abnormal heat rate notifications. Ran the ECG app and came back afib. Well...glitchy firmware. Let's try again. Afib. Again and again and again. Piece of crap watch. My wife wakes up and I put it on her. Normal. Normal. Me afib. Try the other wrist, try the underside of the wrist. Every time afib warning.

Ok. So go to Patient First. Parking lot full and I'm going to blow it off and head home. Look at the watch again, afib again. Fine walk in and sign in. They ask what's wrong and I'm embarrassed. 'Ok so there is a new watch feature....hahaha....I'm silly but can we check this?"I did not know that this comment was a quick queue pass for Patient First. I'm taken right back and hooked up. The technician looks at the screen and says "I'm going to get the doctor"

Doctor comes in, looks at the screen, looks at me and says "You should buy Apple stock. This probably saved you. I read about this last night and thought we would see an upswing this week. I didn't expect it first thing this morning." Soooo long story short, heading to a cardiologist right now. Test yourself. I didn't have a clue. I may not ever have.

Hit the link to read the edits added later. But this is going to be one of those features that changes everything and for something that could not be any more important.