But you can't reliably get beat by beat interval data from a wrist PPG under normal living conditions. Also, to get breathing rate from beat intervals only really works when the parasympathetic nervous system is dominating, and isn't very observable in older people.

Very cool. I had no idea things had gotten that accurate. My grandpa was a cardio-thoracic surgeon so I always had a bit of hobbyist’s interest. Most of my reading materials were obviously a bit dated.

No worries. Wrist measurements are noisy and difficult, but there is a great deal of info to be extracted from heart rate data. Lots of teams, including Apple, are working to figure it out. The better the extraction of heart rate from the data, the more that can be done done with it. For example, there are groups using heart rate variability (HRV) characteristics to predict neonatal sepsis, a leading cause of death in infants.

https://pubmed.ncbi.nlm.nih.gov/31295130/ https://pubmed.ncbi.nlm.nih.gov/11134441/

That work is not with a watch of course, but it is extending the understanding of the relationships of HRV to other physiological issues. Good stuff.