Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am so sorry to hear that Misty got outside and was outdoors for 48 hours, how upsetting and traumatic for all of you.
I am pleased to hear that she was able to eat normally.
A normal cat has a resting respiratory rate of about 10 to 15 breaths per minute, and one breath is considered the in and out motion of his chest. If she has a rate faster than 40 breaths per minute then she truly has a higher than normal rate.
In general I tell owners if you can notice that she is breathing abnormally then I am concerned for her. But she has just been through a very traumatic time, and we cannot underestimate stress as an underlying cause.
Since she was able to eat and is not open mouth breathing I think giving her some time is reasonable.
If she continues to have a fast respiratory rate then there are some other possibilities.
If she doesn't have a nasal discharge and isn't sneezing
then her difficulty breathing could be related to a problem in her chest or secondary to anemia (decreased numbers of red blood cells).
Problems in her chest leading to an increased respiratory include primary heart disease (including cardiomyopathy and heartworm
), lung disease including asthma, bacterial, viral, parasitic (lungworm) or fungal infections, a mass(es) in the chest including lymphoma, heart based tumors or carcinomas, or fluid around the lungs (pleural effusion) due to a mass, bleeding, chylothorax or an infection.
Anemia can be secondary to poor red blood cell production, either due to kidney
disease as the kidneys make a hormone to stimulate production or primary bone marrow disease, or anemia can be due to bleeding or destruction of red cells due to a tumor, blood parasite or autoimmune disease (body attacks its own red blood cells and destroys them).
It is also possible that your girl suffered some sort of blunt trauma (hit by car and thrown, fall) and has internal bleeding or lung trauma but no observable external signs yet. She may show bruising later.
If your girl's tongue and gum color are a nice bubblegum pink them she is in better shape then if her gum and tongue color is blue or gray, these would signify she is real trouble and this is a true emergency.
The conditions I listed above are serious and I would highly recommend that your girl see a veterinarian promptly if her respiratory rate doesn't slow to have an examination and further testing done based upon her examination. They would likely take radiographs of her chest to evaluate her heart and lungs to start and then further diagnostics based upon those findings. If she appeared anemic then a complete blood count and smear should be done. Further testing may include testing for heartworm and lungworms, aspirating fluid for examination if there is fluid present around her lungs, or blood titers for fungal infections.
In the meantime keep her quiet so she can breathe as easily as possible and check her gum and tongue color periodically to make sure it remains pink.
Please let me know if you have any further questions.