I have had a lung transplant and have found that when I push myself hard on a treadmill for example, I am unable to breath properly. I find myself gasping for air if I try to talk while doing it. I am doing it at 9.5 speed which is reasonably fast, but I try and raise my heart rate for at least 20 minutes a day. I will be looking into this condition and cross comparing what I have experienced, as in my case it could mean a number of things some possibly life threatening.