I've noticed that if my heart rate goes up 10-15 beats is when I begin to get symptoms and strangely enough if my heart rate drops 10-15 beats I get symptoms. I don't know what is causing it, however if I maintain a hr i seem to be ok, for instance if I ride my bike for 10 minutes at a certain hr, then get of my bike and sit and my hr comes down I get symptoms. If I am walking and I begin to feel exhausted and my hr rises I get symptoms- it's the hr rate changes that really get me. Has anyone else noticed this?