Thank you for the further information.
I would love to see her radiographs.
It does sound like her trachea is very narrowed within her chest, such that there is very poor gas exchange, and thus I can understand that she is probably battling to pull air into her lungs. There also appears to be increased tissue density around the base of the heart where the mainstem bronchi (air ways to her lungs) connect to her trachea. That can be due to lymph node enlarge from infection, collapse and inflammation of the bronchi due to infection or the increased amount of force/pressure as she struggles to pull air into her lungs.
I suspect that there is also some mild to moderate heart enlargement as the heart works hard to circulate blood as best as it can to get oxygen out to the body.
Without observing her I cannot say what her quality of life is now, but I assume she cannot do much with those sort of restrictions to her airways.
I would try adding a bronchodilator to the mix of medication, such as theophylline. It can't hurt and may help.
If she cannot tolerate the antibiotic she was on it is worth trying another. There are several that have good efficacy against the bacteria we tend to find in the respiratory tract.
Tracheal collapse can be miserable, and eventually secondary heart disease often develops.
I understand that you are struggling with whether it is appropriate to consider letting her go via humane euthanasia now. I try and guide clients on this very difficult decision by asking them if they believe they would want to live the way their beloved pet is.
I ask if their pet is still able to do the things that they have always enjoyed (albeit for a shorter period of time).
I also ask if they believe that their pet is comfortable most of the time and is living with dignity.
And I ask if there is something else we can do to improve quality of life, in this case perhaps a bronchodilator may work for a bit.
I ask about their socializing compared to normal. Does she seem to be hiding herself away when she always wanted to be in the middle of things, or is she suddenly clingy when she used to be very independent?
If she cannot walk even a short distance without coughing or needing a rest, if she is eating poorly, then we know that she isn't feeling herself.
You live with her day, so if you honestly answer these questions I believe you will come to the best decision for your girl.
I believe humane euthanasia is a gift we can give our pets when their quality of life is poor, and owners are absolutely the best people to evaluate that. As veterinarians we see pets for a short time when they often have an adrenaline rush from being in the clinic. We often don't see a true picture of day to day life.
Please let me know if you have any further questions.