Why do dogs have shortness of breath?

It is possible that the dog is panting rapidly because he has just finished a strenuous exercise and his blood oxygen saturation is low, so he needs to pant quickly to increase his blood oxygen level. It is also possible that the dog has some heart disease or abnormalities in the lungs or trachea. If the dog also has a purple or blue tongue or mucous membranes, it is advisable to take the dog to the hospital promptly.