Shortness of breath

79-year-old man who underwent a four-vessel CABG in January 2006. A few months prior to that, he had developed some malaise and ultimately chest pain leading to an angiogram that revealed 95%, 90%, 80%, and 60% occlusion. After the 4-vessel CABG, he reports shortness of breath worse in the supine position that occurs quite quickly after lying down, perhaps after a minute.
He is able to walk a few miles without significant shortness of breath. He is also able to walk up a flight of stairs with only moderate shortness of breath, and he denies any muscle weakness or achiness. He is able to stand from the sitting position in the chair with a bit of shortness of breath, but no frank weakness. The patient denies any fevers, chills, or night sweats. Furthermore, he denies a cough. There is no productive cough except for the occasional cold about once a year. Rest of his complete review of systems is noncontributory.
On exam it is unremarkable with good air entry bilaterally. In the supine position there is bilateral paradoxical abdominal movement with inspiration.

A CXR is here:

I obtained spirometry in the supine position and repeated that in the upright:

FVC Upright: 2.61L (64% predicted)
Supine: 1.18L (29% predicted) -55% change in vital capacity

PI max cmH2O (51%) 100
PE max cmH2O (104%) 187


It appears he has bilateral diaphragmatic paralysis. It coincides with the CABG, but I think this would be fairly atypical, as that is usually associated with unilateral. In terms of muscle weakness, the low PI but normal PE max argues against this.
Thoughts?