Oh that's clever! Garmin's watches[1] estimate stress levels by variability of pulse rate, and they estimate breathing rate by looking for sinus arrhythmia (the benign change in heart rate speeding up when breathing in and slowing down when breathing out) which is cool. I wonder if any pick up ectopic beats with the normal continuous pulse sensor and not the 1-lead EKG thing?
I'm skeptical, even classic wrist blood pressure monitors with a cuff are notoriously inaccurate compared to upper arm monitors, could a watch really come close with even less data?
[1] possibly other brands too, I didn't look