I am doing a research about wrist wearables in order to understand how they work. They can be used to measure stress levels, just like pupillometry, skin conductance and other related measures can be.
I can understand how they measure heart rate, movement or SpO2, but I can not understand how they count breaths. When it comes to smart T-shirts they are using stretch sensors but on wrist wearable this is not applicable.
Is there an algorithm that can convert HR and SpO2 measurements to an approximate BPM rate? Sorry for the question but I don't have any medical knowledge.
If there is not such an algorithm, I think I am missing a sensor that can do that.