Wow, 85° would be a cool day for me! Here in Bakersfield, CA, we haven't seen daytime highs under 103° in the last two weeks. Our overnight LOW is in the low 80°'s. I'm jealous !
But, heat is relative. Yes, the first hot day this year (the weather service said it was 98°, but the on-the-ride-telemetry said 102°) was definately a high-heart-rate/puke-ride for me. I do use a HRM, and can tell you the heat definately affects heart rate. In the winter, when it's in the 60°'s, my average heart rate is 5-10 bpm lower than in the summer, and my average speed is 1-1.5 mph faster.

My advice (for whatever it's worth ):
1) drink fluids throughout the day before you ride ("hydrate or die!", as they say);
2) replace electrolytes - drink a sports drink in addition to water when you are riding, and also after the ride;
3) it's okay to go slower - you are still getting a good workout - go with your perceived exertion;
4) remember, after a few days in the hotter temps, you will acclimate - it took me three weeks this year (must be 'cause I'm older this year - ha!)
5) lay off the alcohol, both before and after a ride. Before a ride, it'll dehydrate you. And, I've noticed if I have a drink after a ride, I can actually feel my legs start aching.

Happy riding!!!
~~~~~TrekChick