Heart rate change during exercise

[lmacmil]

Has anyone noticed a significant change in heart rate during exercise?  I see this quite often when I walk but never when I run.  You can see in the plot that my rate and pace were pretty steady until about halfway through my walk.  Then my pace looks to be somewhat erratic though on average it's similar to the first half of  my walk (13:37 for the first mile, 13:54 for the 2nd mile.)  My heart rate drops about 40 bpm for the 2nd half of the walk.  I find it difficult to believe that that slight reduction in pace (less than 3%) could result in that large a reduction in heart rate.

[lmacmil]

Walked the same route as the post above.  Time within 10 seconds but 110 fewer steps.  Heart rate pretty steady, just a couple of short dips of about 1 minute each and averaged 139.  Did not show the fall off like yesterday.  I guess it must just be the tracker moving around on the wrist.  Case closed.