The borderline low diffusion capacity is of concern. The first step would be to confirm the validity of the result (68-70%) as this test requires careful attention to detail or the result may be falsely reduced. The information you provide provides no clues regarding the cause of a truly low DLCO. I could find no evidence of a cause and effect relationship between the DLCO and a PFO.
I believe that the cause of your alleged “low blood pressure” deserves further investigation. Unusual causes would include adrenal insufficiency and autonomic nervous system disorders.
Another possibility is that the septal defect repair is no longer intact and that, either with or without pulmonary hypertension you are experiencing right to left shunting of blood with severe exercise. If the right heart cath does not reveal a cause, you might want to request a second opinion at an institution that specializes in the diagnosis of exercise related symptoms such as your shortness of breath and low blood pressure.
I would be most interested in the results of the right heart catheterization.
thank you for your opinion. i posted a new question on the website that gives more details about the testing i have done