Lacey, I am no expert in paddling but i do know a thing or two about heart rates, and IMHO what you are saying is reverse logic, I suspect you are reaching but keeping a higher stroke rate pushing your heart rate up.
Perhaps, stop bring your HRT back to say 120 bpm, try over a controlled distance, reaching further but slowing your stroke rate down.
Then stop again bring you HRT down to 120 and paddle back with the shorter higher cadence stroke.
You should if i am correct find that over the distance that your HRT is lower in the longer stroke.
When we box people try to punch quickly but get more power out of a controlled slower punch, but their HRT is up in the fast punches. Same theory I suspect.
If this doesn't work, go and see a cardiologist!!!

I did a blast race a few weeks ago and pushed my HRT to 195 in the sprint home (say 1200 mtr) with little choppy strokes after it avgd about 165 for the balance of the 6.8 kms.............but seriously almost puked afterwards (but did take home the medal

)
Good luck with it
Greg