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It depends on the type of the fluid...and if it is in the lungs or in the chest...
But I would say most of the issues do carry a guarded prognosis...but to help you further, you would need to get more information.
I wish I could guide you better. The problem is cats can deposit fluid in the chest cavity...which can be removed but withdrawing the fluid...but the causes can be so hard to control...the fluid comes back.
With heart disease and a few other diseases, they can get fluid IN the lungs. Usually medication helps them urinate the fluid out...but again the cause is still present.
I hope they can get control of this, if you find out any more information please let me know.