Hi Andrea,
I'm wondering if this 4 year old post ever got resolved. I'm a 61 year old ultramarathoner with similar symptoms. Echocardiogram treadmill test showed good heart function, but my O2 crashes into the 80's after 2 to 3 minutes at 150 heart rate (i.e. "running"). At 135 heart rate, I'm a solid 99% all day. No difficulties breathing. I have a pulmonary appointment next week. Low expectations!
Just a quick note to say hi!
Hi Andrea, Have you gone to a pulmonary doctor. You may have something loke asthma. It is worth going to a pulmonary doctor while a regular GP may be just guessing what is going on.