While it is natural for a dog to pant after exercise or exertion, excessive panting could be a sign that the dog is suffering heatstroke, has consumed something poisonous, has a heart condition or has some sort of respiratory disorder, according to WebMD. If the dog is panting even after it has been given water and has cooled down, it should be taken into a veterinarian to be assessed.
Heavy panting in dogs means that the dog is either trying to cool itself off or that it is having trouble breathing. If the dog exerted itself in the heat or in direct sunlight for an extended time, the dog could be having heatstroke. If this is the case, give the dog cool water and try to submerge it in cool, not cold, water, recommends WebMD.
If the dog was not out in the sun or heat but is still panting excessively, it could be a sign of illness such as heart disease, Cushing's syndrome or respiratory disorders like pneumonia. Sometimes heart issues will cause a dog's lungs to fill with liquid, making the dog cough and wheeze continuously. If one of these disorders is suspected in an animal, seek professional medical attention to receive medicine, antibiotics or to explore other treatment possibilities.