I ran a heart rate monitor for years in endurance. In endurance there is a "gate" a spot you can no go past until your horse meets the rides peramiter. It was usually 64. If your horse's heart rate was above 64 you could not proceed.
Resting my guys are all around 32-36. After 12 miles of continued trotting I would get off about 100 yards from one of these gates, loosen the saddle and walk him to the gate and call for a check and he would be below 64.
On trail I ran him about 135, this is a nice working trot, I would not let him sustain 150 or higher since it produced lactic acid and fatique.
Yes on some unhill climbs it did spike around 160 but quickly dropped to the 110 just over the hill and starting back down.
If a horse is kept at say 135 he can go on forever. If it climbs to over 150 he is fatigueing and going down hill.
Hope this gives you more information then you need