Hello, thanks for your question today! I am Dr. Drew, and I am a licensed veterinarian. I'll be happy to help you in any way I can.
The medication you are referring to is Lasix (furosemide), which is a diuretic.
This is generally NOT effective for clearing fluid from around the lungs,
rather, it is useful for clearing fluid from inside the lungs, when caused by Congestive Heart Failure.
The fluid that was drained from the chest cavity should have been submitted to a laboratory for evaluation,,
in order to characterize the possible cause.
heart disease is an uncommon cause of this fluid in cats,
more likely are issues such as Chylothorax, Cancer, or unknown in some cases.
Fluid can build up fairly quickly after draining,
within a few days in some cases.
so if it's been a few days since the last draining, and the respiratory rate seems high, a repeat x-ray would be suggested.