Normally dogs take ten to 30 breaths per minute. Dog breathing varies with temperature, activity and size of the dog. In hot weather, a normal dog may pant up to 200 times per minute. A dog breathing that is faster than normal may be taking shallow breaths that aren't as efficient as moving oxygen into the lungs. If your dog seems to be working to breathe, not just breathing naturally, something is wrong.
Difficulty in breathing can have a number of causes that may or may not be related to the respiratory tract. A dog in pain often has quick, shallow breathing. Any abdominal problem that puts pressure on the diaphragm can cause respiratory difficulty as well. A dog that has been hit by a car or had other trauma might have broken ribs or what is called a pneumothorax (free air in the chest, putting pressure on the lungs). Pneumothorax is a serious, potentially life-threatening problem that requires immediate veterinary care.

There are many respiratory causes of difficulty in breathing, starting at the nose and working down to the lungs. A dog with respiratory infection may have purulent (pus-filled) discharge from the nose that blocks his airways. Any obstruction in the nose-blood clots from banging the nose on something hard, inhaled foreign objects or tumors - can reduce the amount of air going into the lungs. The same holds true for all the airways down to the lungs. Dogs can inhale foreign objects into the trachea. Certain dogs, such as many toy and brachycephalic breeds, may have collapsing trachea, which decreases the open airway of the trachea or windpipe. These dogs may cough, gag or struggle to breathe deeply.

Pneumonia is an infection of the lungs by either viruses or bacteria. It can be quite serious, especially in puppies or dogs with poor immune systems. Dogs with pneumonia may cough, gag, retch or just have painful breathing. These dogs usually show other signs of illness such as fever or poor appetite and they may cough up greenish mucus.
Pneumonia can also be caused by parasite migrations, such as with roundworms, or if the dog aspirates or inhales bits of food into the lungs. If your dog has pneumonia, your vet may perform a process known as tracheal aspirate. Tracheal aspirate involves injecting sterile fluid into the trachea, then sucking it back out into a syringe so it can be cultured and examined. X rays are also very important in diagnosing pneumonia.