Have you paid attention to whether your thoracic spine position affects your symptoms? In my case, I have found that when I “hunch” over, that is, bend my thoracic spine forward, my symptoms increase. That has gotten worse as I have gotten older, as often occurs, probably because the rib cage becomes more rigid and the spine becomes more kyphotic. It seems to me that what is happening when one bends/hunches forward is that the rib cage and sternum bend down and inward (since they are attached to the thoracic spine), pinching the heart between the sternum/cartilages and spine. The time you blacked out was after reading, probably hunched forward. At any rate, consider what’s happening with the heart in addition to the lungs. I think the heart is the more important factor.