[shawnz]

Isn't it possible to estimate breathing rate with any optical heart rate monitor? See https://en.wikipedia.org/wiki/Photoplethysmogram#Monitoring_..., afaik that's how Fitbit, Apple Watch etc do it

[kbre93]

Yes it's common to estimate breathing rate based on the oscillations in the heart rate pattern. But estimating it from the chest expansion is a more direct method