I went for a 4-mile run today and monitored my heart rate much more closely than usual. What I found was surprising. For the first 10-15 minutes of the run, which was gently uphill, my heart rate was only around 88-90 bpm. But, on the slightly downhill trip home (the last 10 min of the run), my heart rate was strangely high – in the mid-160s. Normally, my HR doesn't seem to break 170 even during bouts of intense cardio, such as HIIT on the stationary. Throughout the run, I maintained a pretty steady pace and perceived exertion (not intense, a bit faster than comfortable), averaging 7:45 min/mi.
I first suspected that the heart rate monitor was initially only picking up every other heartbeat, since the initial readings were roughly half the later readings. But I verified many of these readings by checking my pulse manually, so I know they're real.
Also, I've had something similar happen occasionally with other cardio activities. Once I manually took my pulse several times after challenging sets of burpees and was shocked to get languid pulses in the 50s given how hard I felt I was working.
If it's relevant, I am not a runner. I do steady-pace running only infrequently – say, 3-4X/month – although I am otherwise quite active. My resting pulse is around 45 bpm.