Is it normal for my cat with a heart murmur to pant after running?

Hello and thank you for reaching out to Petco Pet Education Center, formerly Petcoach. When cats pant or breathe with their mouths open, this is abnormal and concerning. Given the information you provided about the Merlin's heart murmur, I am worries that he could have significant enough heart disease to cause heart failure, which leads to the build up of fluid in the lungs. This makes it very difficult for cats to breathe, especially after exertion, and can be life-threatening. Alternatively, cats can pant when they are severely overheated, which is also very dangerous. I would recommend keeping Merlin quiet (no running or playing) until he can be seen by a veterinarian. He/she will want to do a full physical exam and take x-rays of Merlin's chest to look for heart enlargement and/or fluid in the lungs. There is a blood test available called an NT-proBNP that can detect heart failure in cats as well. I hope this information is helpful!