I don't feel comfortable much over 150bpm. When I had a treadmill test it would go over that for a few seconds when he changed the incline but would settle back to 150. He said it was the most boring test he'd seen.
I climbed with a guy who ran 185 beats all the way up a 3 mile climb while I did 150-ish. Of course he was faster than I was. Friggen hummingbird.
Some weight-loss show I watched they put the participants on a track and had them walk around with monitors on. One woman, who was about 5' tall and 375 pounds, hit about 225bpm before they rushed out there and stopped her. She didn't seem to be in distress and was still smiling.