Peter Gamma (Physiologist & Director) Meditation Research Institute Switzerland (MRIS)

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Real-time estimation of respiratory rate from a photoplethysmogram using an adaptive lattice notch filter

There are several papers available for respiratory rate estimation in real-time, for instance:

https://biomedical-engineering-online.biomedcentral.com/articles/10.1186/1475-925X-13-170

but I could not find a commercially available device which can do this:

Calculating Respiratory Rate from PPG or ECG signal in Python

Peter H. Charlton told me, that his Matlab Toolbox for respiratory rate:

https://ch.mathworks.com/matlabcentral/fileexchange/55289-respiratory-rate-estimation

can also run in real-time.