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: 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.