R-R Interval definition is ALWAYS via ECG

https://medical-dictionary.thefreedictionary.com/R-R+interval

http://www.meddean.luc.edu/lumen/meded/medicine/skills/ekg/les1prnt.htm

https://en.wikipedia.org/wiki/Heart_rate_variability

https://www.physionet.org/tutorials/hrv/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4848101/

http://medpolicy.ibx.com/policies/MedAdvMPI.nsf/6a827f8172ac2f8185257d3100532b7a/85257d39006e7945852581a10055f195!OpenDocument

https://jamanetwork.com/journals/jama/article-abstract/661268?redirect=true

http://mcgs.bcbsfl.com/?doc=Autonomic%20Nervous%20System%20Testing