It certainly sounds like she's got an upper respiratory infection. You've got an excellent idea with the hot, steamy bathroom to help alleviate her discomfort until she can be seen.
Upper Respiratory Infections may have underlying causes of bacteria or virus (even fungal or parasitical) and there could easily be a secondary infection happening. Since left untreated it could spread to other systems, turn into pneumonia and possibly become life threatening, it's always the safest suggestion to have a vet evaluate the cause so it can be properly treated. By waiting, the condition may just become more complicated and a great deal more costly to treat.
The typical viruses are Rhinotracheitis, sometimes Calicivirus; these may be chronic and recurring. The importance of having each occurrence seen by a vet is not only for immediate treatment, but to avoid any possible secondary infections (which can be significantly worse).
When the problem is fungal infections (Sporothrix, Aspergillus, Cryptococcus, etc), parasitic (Capillariasis and others), the treatment is different and the possible complications and secondary reactions are also varying.
Look here for more info about what happens http://www.marvistavet.com/html/body_feline_upper_respiratory.html
If the cat is still using the litter box relatively normally, does not have a fever (how to take the temp and what is normal) http://www.petplace.com/cats/how-to-take-your-cat-s-temperature/page1.aspx , if there is no labored or difficulty breathing, loss of balance, persistent vomiting (or any vomiting/diarrhea with blood), you can probably wait until the next day to bring kitty to the doctor. If any of the symptoms mentioned here are present or you just ‘know' (trust your instincts) something's not right - make an emergency visit. Most vets have emergency care numbers, will come in especially for emergencies or direct you to an emergency care clinic.
Albuterol use should be carefully prescribed and monitored by a vet when it comes to cats because dosage is vital to get right. This link will help you with making your own decision http://www.petplace.com/drug-library/albuterol-proventil-volmax-ventolin/page1.aspx
You might also try setting up a vaporizer (no meds in it) and providing a warm sleep area (ie: heating pad set on low under some blankets).
I hope this works out well for you and this very special cat. She's lucky to have you.
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