Odd HR during Bike Workout

My background is competetive cycling so the bike training for me is straightforward. The run and swim, well that's a different matter I am still at the base building stage for those.

The strange thing is I can't get my HR to the 170+ doing the same workouts as I was before I started tri training. I can only get to about 161 as a very hard effort. I'm not ill, I don't feel tired but my RHR has dropped 5 -> 45 over the last few weeks since I've been tri training.

Has anyone any ideas why this might be? - Just curious really :-O

Ta

M