You're a rabbit, then. Some people have naturally high HRs (a teammate of mine averages 180+ for a 45 min race), and your genetics coupled with the asthma leads to results like that. I did 76 miles yesterday, averaged a hair over 20mph, and my HR never exceeded 175 and averaged around 150. I've generally got a lower HR than the people around me, and it's not because I'm necessarily more fit, that's just the way it is.