I went running yesterday…the temperate was almost 90 degrees. I wore my heart rate monitor, and am really glad I did! I was amazed at how much harder my heart had to work in the heat. I had trouble keeping it below 80% of my max heart rate, and sometimes I even had to stop and walk. I didn’t feel like I was working any harder than usual – I just felt hotter and thirstier. If I hadn’t been wearing my monitor, I might have kept pushing myself despite feeling hot and thirsty.

Here’s a great article about heart response to heat.
http://www.active.com/story.cfm?story_id=7403