Hopefully Jess and Earl see this post, she works for a veterinary cardiology specialist as a vet tech.
As Hope said, there are many kinds of heart problems. Each might require different drugs. The most common is Hypertrophic cardiomyopathy, which is what my Merlyn had. There is also Restrictive and Dilated Cardiomyopathy.
There is no way your vet could have withdrawn fluid from the lungs, more likely it was fluid around the heart and lungs. Fluid buid up in the lungs is Pulmonary Edema, fluid buildup around the heart and lungs is pleural effusion. Merlyn had fluid in the lungs, and was put on furosemide/lasix. He was also given a Calcium Channel blocker called Diltiazem, but Norvasc(amlodipine) is another. he was also on Fortekor/Benazepril, and Calcitriol (for his kidneys). We also very cautiously gave him sub qs for his kidneys because the lasix dehydrated him so fast, and he was on a small dose of aspirin (we dissolved it in water to get a smaller dose) Aspirin must be discussed with vet, too much is toxic to cats. Plavix is also being used for cats to prevent blood clots, instead of aspirin.
What a cardiac ultrasound will tell you is what kind of heart problem there is, and if there are other meds that might be suitable.
Jess said the same thing, some cats go on treatment and do well, and others fail quickly.. but I wouldnt give up if I were you. I am surprised that Felix isnt on a beta blocker or Calcium channel blocker, but perhaps without a cardiac u/s thats just as well.
Merlyn lived 2.5 years post heart diagnosis, with kidney disease which was stable. It was his heart that gave out first.
Sending lots of cyber hugs.