He went in today for his follow-up visit with the cardiologist.
He was fine in the carrier in the car and then a little agitated but not bad in the waiting room. Unfortunately, the stress of having x-rays and such done apparently caused him to start breathing heavily, and so they're keeping him at least through this afternoon to try to get him stabilized.
The x-rays showed a fair amount of fluid in his lungs, which surprised me. He's been acting virtually normally at home: playing with his toys, snuggling with me, etc. The vet is going to give him a couple of injections of Lasix today to try to get the fluid out, because it's causing his breathing problems, and maybe put him on some oxygen. She'll probably increase the oral Lasix as well, although it's likely to cause kidney problems. That's the real rub here: Treating the heart failure will cause kidney failure at some point.
Crossing my fingers that they can get his breathing stabilized today and he can come home. We'll take it one day at a time from there.