They put him on oxygen and took x-rays, which revealed liquid in the chest cavity completely obscuring the lungs, heart, liver, etc. They ended up draining 10 ounces of liquid out of his chest, and sent me home with diuretics to try and keep the fluids draining until I could raise money for further testing. But the probable diagnoses were grim, including cancer and heart failure.
He was very weak after the procedure, and now sported two shaved patches (one on either side) of fur. He did regain his strength over the next few days, and I continued the diuretics. But after another week or so the hiccup, which I'd now realized was an increased effort of inflating his lungs, was back. After a few more days, I could tell he was losing weight but not changing size due to the liquid in his chest. I knew he wouldn't handle another trip anywhere, and after talking with a home-visit vet, I was pretty sure more procedures might lengthen his life, but it would not fix nor improve it.
We scheduled an at-home euthanasia on Friday, but that morning we found he had already passed on during the night.