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Has any blood work been done (complete blood count, serum chemistry panel, FeLV/ FIV screen)? This would indicate infection, kidneys and liver function, blood protein level and other values that can be used to help diagnose the problem. Has a sample of the ascitic fluid been analyzed?
Ascites is a sign of liver disease and/or low blood protein levels. Often there is fluid in the chest when there is fluid in the abdomen. This would cause labored breathing.
It sounds like your cat is very sick and the vet is working on stabilizing him as well as doing preliminary diagnostic testing. The first priority is life saving emergency treatment. Once an animal is stable, fine tuning the diagnosis and treatment options can be done.
Let me know if blood work has been done and if you know values, and if you have follow up questions.