Quite often, it is difficult to explain how fat in the chest of the dog can cause problems with hypertension on the lung side. People do not realize that a dog can carry fat in between the lungs and around them. There is a layer of fat around the heart called the pericardial fat. Between the lungs it is called mediastinal fat. The fatter the dog, the more fat there is between the lungs. All of this; on top of fat that is surrounding the heart, limiting its excursions. That too, the overweight condition reduces the dogs ability to exercise.