No idea if it is the cause or not, but there are strong physiological links between breath control and heart rate. Do some reading on "heart rate variability", see for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6189422/#s5titl...

vargas nerve, parasympathetic nervous system, etc.

(sourced from a quick google search; it's been 2 years since I was active in this and I don't have my sources in short-term memory).