|If it IS the heart causing him to pant, is it the hole in his heart or the mitral valve defect that would cause it? Any thoughts?
I am not a vet, and don't claim to be, but I would think the hole in his heart MAY be adding to his panting. A friend of mine was born with two holes in her heart. They had to wait until she was a few years old before they could do the surgery, and she told me by that time she would get winded easily, was tired, and was blue at her extremities. Depending on where the hole is in his heart, your kitten may be experiencing something similar. If the hole allows oxygen-rich blood from the lungs to mix with oxygen-poor blood from his body before it's pumped from his heart, his body may not be getting enough oxygen, and he may be panting to get more oxygen.
I'd contact the cardiologist and tell him about the panting. This may be an early or very mild form of exercise intolerence.