I would get some repeat xrays to see what changes have been made. Also, a heartworm test may be beneficial as sometimes asthma is diagnosed when heartworm disease is present. Only change meds under doctors orders and decrease the dosages slowly. If he hasn't been running a fever then the likelihood of fungal or bacterial infection of the lungs is low. Many cats have done very well on Flovent and albuterol inhalers as a maintenance. Flovent is a corticosteriod but does not carry the same systemic risks that oral prednisone does. Discuss this with your veterinarian. Without the results of an EKG or a repeat xray to see changes in the lungs I cannot say what is going on, only that it does sound like an asthmatic condition which is becoming more and more common in cats.
Good Luck
Theresa
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