Are you sure that's an accurate reading???
My Garmin is prone to giving me false readings - I've had HR's in the 220 range - I KNOW that's not right. I have a max speed somewhere in my database of 375 mph.
Not sure how you felt vs. other more "normal" high HR readings, but from my experience with Garmin HRM's - including second hand reports from others who term it "junk' - I'd take the outlyer reading with a grain of salt....
Look at the points around it....