Well, yesterday, I borrowed a friend's optical sensor HRM and wore it at the same time as my chest strap-type HRM during a run. I tried to push much harder than usual to see if I could make anything happen. I experienced a skipped beat feeling, probably a PVC. Next, I immediately felt some sort of arrhythmia for about a minute, perhaps a brief run of SVT. The optical sensor HRM displayed about 15-20 bpm lower (145-150) during that post-PVC period as compared with the chest strap HRM (about 165 bpm). I know that would be a low HR for SVT. But, I'm nearly 60 years old and my max HR is probably not as high as many others here. My EP previously diagnosed, on EKG, my SVT/AVNRT at only 128 bpm. During yesterday's run and prior to that skipped-heart-beat feeling and the following, higher arrhythmia, the two HRMs had been displaying within 1 bpm of the same HR. So, it does seem that at least during that one event, my pulse rate was lower than my heart's measured electrical activity.
I mainly was interested in knowing only about pulse rate at wrist corresponding 1:1 to heart rate measured at chest; not whether the technology or sensors in those different type of heart rate monitors have equal accuracy.
I thought that perhaps during SVT/AVNRT there may be extra "electrical-only" activity that occurs, without the ventricle pumping blood.
Interesting, and new? I'm an old guy and have had to give up running when I wore a chest strap and wrist display to monitor my HR. The optical monitor on the wrist should give the same reading as the chest monitor, I believe. Of course, this all depends on the relative sensitivity of the detectors used in each...suppose that is your question.
I'll try to remember to check back to see the "answer".