Get out the Vicks and a vaporiser, if you have one. If you don't, set her in a small bathroom in a carrier and run the shower very hot, or use a kettle. Get some steam happening.
You mentioned respiratory meds...does this include antihistamines? Many cats can benefit from a small amount of human Benadryl or etc, but check the dosage with your vet! (Someone here might know the correct dose!)
Get an ionizing air filter. They help allergies in humans, and might help the air for Cassie.
If you have a diffuser, use it with Tea Tree oil in it...great antiseptic in the air.
I hope she is better soon!
"When I think of Mick Jagger still singing that he can't get any satisfaction in over forty years of being in the Rolling Stones, I have to conclude that he's either lying or not all that bright." -- Henry Rollins