I have no idea! From the time I found him to the time he died was probably 5-6 hours so it wasn't sudden. He never improved, but stated the same on the oxygen. The x-ray showed very foggy area covering most of the heart. Heart seemed enlarged and there seemed to be some nodules at the base of his heart - on in, but outside. His trachea was also narrow about middle to base as if the heart was pinching it off. Like our other boy's had. I forgot about that till now. I'm going to have our regular vet take a look at them tomorrow. I wanted a necropsy, but hubby said no. The end was dreadful as he gasped and lounged for me (my face) while I held him before they took him back the second time. We knew he wasn't going to make, but everything changed so quickly. I'm not sure why they gave him back to me in the same condition I brought him in. He was still cold! Myco was the other diagnosis she was thinking if it wasn't his heart. I really wish we would of done the necropsy. We had the vet I don't trust and she knows it - she won't even be alone in the room with me. If she messed up again..... right now its just easier for me to blame her.