Here is a GREAT article that explains it:

http://www.myfit.ca/archives/viewan...e=fitness&ID=55

I don't pay attention to my heart rate though - I know that when it feels like I'm going to throw up my heart, I'm doing it right. Hee hee.

DON'T stop doing your intervals though, just throw in a couple of HIIT sessions a week instead of them.