Heart echo HCM.
Found on YouTube a video showing a cat in breathing distress due to heart disease. It's only a few seconds but if you look closely, you can see the chest heaving extremely quickly, almost to fast to count. I've seen this in Myrna. Meanwhile, the cat looks unhappy but not miserable. This is why it might be easy to miss and why we must pay close attention to the speed of the breathing/chest heaving.