My Pug Has Been To The Vet Several Times For

My pug has been to the vet several times for a bad cough. He done exrays on his lungs and heart and they are fine. He has been on antibiotics, steriods, heart meds and a torbasol (sp) cough suppresant. His cough is still bad. I feel bad for him because when a coughing fit comes on it last a minute or so. Please help