What Causes Chest Pain Along With Shortness Of Breath?

Hello and good evening. I m emailing regarding my mother in law. It s not am emergency; her PCP, cardiologist and pulmonologist have been working toward a diagnosis for over a year, The primary symptom is that she has chest pain and shortness of breath when she stands. If she s sitting or laying down she has no pain at all. But upon standing she feels as though she can t get a full breath and occasionally feels dizzy. The symptoms are not all the time and there isn t anything that makes it better or worse, other than sitting or laying down when it happens. I m wondering if there may be a nerve in her neck that is affected when she stands. None of the specialists have found anything that would cause it but I don t think it s psychosomatic, either.