I recently got a Polar heart rate monitor. I use it on a treadmill that has a polar [something] so that I can wear the strap and my heart rate shows up on the treadmill's display.

Here's my issue -- I can run at 6.5 miles/hr on the treadmill. For the first ten minutes or so, my heart rate stays between 140 and 145. After about ten minutes, it starts to climb, slowly, even as I am keeping the same pace. By 20 minutes, it's beating about 160. If I keep at 6.5, it will climb to 170 by 35 minutes.

Is this normal? I've heard that your HR should stay about the same if your effort remains the same. Not true with me.

Any thoughts? Does this happen to anyone else? How would your heart rate act in this sort of condition?

Well if i am normal then yeah it is. My heart works harder when i start to draw more on fats for fuel. If im carbed up and keep refueling the heart rate only creaps up. If i am fasted and just drinking water then about ten mins it does the same as you.
Wilders