False arterial fibrillation readings on my Apple watch
I was alarmed to find that I was getting arterial fibrillation warnings on my Apple Watch. I found that it was because the elastic band was too tight.
iPhone 13 Pro, iOS 16
It is possible that a loose or tight fit at the watch band might cause incorrect results from the optical heart rate sensor. The ECG function (put your finger on the Digital Crown) uses electrical signals and will still be correct, so you could use this as a second check, and if there is any cause for doubt then consult a healthcare professional.
Take an ECG with the ECG app on Apple Watch - Apple Support