Panting is a response to an increased respiratory rate that occurs under abnormal or unusual circumstances such as pain, discomfort and high temperatures, according to MedicineNet. Dogs don't have sweat glands, and panting is a way for them to oxygenate their blood and cool down after exercise.
Dog owners should pay attention to other factors when a dog is panting heavily, including the food or liquid the dog consumed recently, states MedicineNet. Heavy panting can be an indication a dog is overfull or sick and may vomit. It can also be brought on when a dog is suffering from chronic illness such as Cushing's disease, congestive heart failure or chronic renal failure. Side effects from medications such as prednisone can also bring on panting.