The reason for your dog breathing heavily will depend upon its cause. For example:
Heat Stroke
After exertion, you will see your dog breathe heavily. Certain breeds of dogs are more likely to experience this as a result of having short snouts (Bulldogs, Boston Terriers and Pugs, for example). Heavy breathing or panting can also be a symptom of heat stroke. Other symptoms of heat stroke include thick saliva, a bright red tongue, vomiting, diarrhea or coma.
Pain
Heavy breathing can be a sign that your dog has an injury or illness and is in pain. Should the heavy breathing occur suddenly, it can be the result of an injury. Other symptoms of pain and trauma may also be present, to include a decrease in appetite, anxiety, licking a particular place, and restlessness. While you may not see an injury, the damage can be internal.
Congestive Heart Failure
Should your dog be experiencing congestive heart failure, he may experience respiratory symptoms. Congestive heart failure happens when your dog’s heart is damaged and does not pump enough blood through his body, causing his cells to not receive enough oxygen. His respiratory system will work extra hard to compensate for this and his breathing will be labored. This will cause retention of fluid in his lungs and body cavities. Other symptoms of heart failure in your dog include less tolerance of exercising, a blue or grey tint to his gums, trouble breathing, and coughing.
Heartworms
Heartworms are a parasitic worm that can infest your dog’s heart and lungs; they are spread by mosquito bites. In a severe infestation, your dog may breathe heavily, as well as cough (possibly coughing up blood) and retain fluid in his abdomen.
Chronic Illness
Cushing’s syndrome and respiratory disorders are chronic illnesses that can cause heavy breathing in your dog. In Cushing’s syndrome, your dog’s adrenal glands may begin to produce too much cortisol, leading to heavy breathing, hair loss, excessive hunger and a pot-belly.
Disease Impacting Nose
Heavy breathing may occur in dogs that have certain diseases in their nose, for example: tumors, bleeding and bacterial or viral infections.
Disease Impacting Lungs and Small Airways
Heavy breathing is a symptom of a bacterial or viral infection like pneumonia. This can occur as a result of pulmonary edema, tumors, heartworm infection and bleeding in the lungs. Should your dog experience issues with the small airways in his lungs, like asthma, allergies or tumors, he may also display heavy breathing.