I don't mean to trash HRM's but they are problematic and a proxy useful in training, but overlapping with medical risk assessment and diagnostics in a way that can create big problems.
Tell your doc about your readings and you can end up in a real Donnybrook.
We have made strides in catching congenital heart disease and reducing stroke risk, but screening and responding to odd symptoms has a certain risk that has to be factored in when assessing the good they can do.
We are going to see a lot of problems from the HRM's as they become more prevalent. There are what I call techno-eutopians ( eg. Eric Topol) who overlook the unintended consequences of unrestrained monitoring.
There is low accuracy and lots of inborn errors in the inexpensive HRM's we use to identify target training zones.
There is a rule in medicine, that , in an emergency, the first pulse you should take is your own.
That goes up there with first make sure it's plugged in and the battery is good.
Salt solution in a dropper bottle or even ECG paste for non photic ones, may improve their performance.
After that list of the stuff that can throw these things off gets encyclopedic, but they are either trustworthy or they are not. Kinda binary , if you are lucky.
As my mechanic used to tell me, "They all do that."
As an example , my Wahoo TICKR reads an alarming 150 at times when I am sitting on the couch , with regular apical and radial pulses of 60.
Not worthy of a $1500 + work up