My intention is not to scare you, but to make you aware of the possibilty. One of my dogs, Beethoven, had the same issue. He began panting alot. Very heavily, almost as if he was having a hard time trying to breathe. That or he was coping through pain. We brought him in to the vet after a day of keeping an eye on him. His panting had not weaken. It had gotten worse. It turns out that that was his way in managing his breathing through pain. Kind of like labor pains and the heaving breathing. After taking some blood tests and x-rays, the doctor found that not only does he have an enlarged heart, but he has a brain tumor as well. Heavy panting/breathing is a sure sign something is not right. It's their way in letting you know something is wrong. I would suggest bringing him in to be on the safe side. Better safe than sorry... I hope everything turns out fine. Duke will be in my prayers.