Well, the first think I recognized was the boot-shaped heart, which reminded me of Tetrology of Fallot, but this CXR is obviously of a much older person, so that is ruled out.
I don't see much of the Lung Markings, and the lungs appear hyperinflated bilaterally, suggesting some type of airway disease, most likely COPD.
This appearance would suggest to me congestive heart failure, however, with that I would expect pulmonary edema and fluid at the lung bases, which frankly I don't see. I see a clear costodiaphagmatic angle on the right and probably would see one on the left except that its obscured by the cardiomegaly.
I don't see much in the way of pulmonary vasculature, which makes me suspicious. However, the ascending thoracic aorta looks dilated.
This may be an aortic aneurysm in the area of the aortic arch with related cardiomegaly...........
the theory...I just am not sure!!
Mossjoh