Hypoventilation in a Bulldog

History. A 5-year-old bulldog is presented to you because it refuses to exercise. Normally the dog is willing to go for short, slow walks. Over the past 6 months, the dog has been making an increasing amount of noise when it breathes.

Clinical Examination. The bulldog is in good condition, but even as you walk into the room, vou notice the loud rattling noises being made by the dog during breathing. You also observe that the mucous membranes of the pendulous lips have a bluish tinge. The dog is standing when you walk into the room, but while you are talking to the owner, the dog lies down and apparently goes to sleep. This causes the breathing noises to become much louder.

Examination of the dog reveals no abnormalities in the heart or the digestive tract, but examination of the respiratory tract reveals multiple abnormalities. The external nares of the dog are extremely small, and it is difficult to introduce a speculum to examine the nasal cavity. When the dogs mouth is opened, an excessive amount of loosely folded tissue is observed in the pharynx, and it is impossible to move this aside to examine the larynx. Listening to the lungs is not helpful because all the sounds being generated by the loose, vibrating tissue in the upper airway are transmitted to the lungs. Radiographs, however, reveal no abnormalities in the lungs, but the trachea is quite narrow. An arterial blood sample is taken for measurement of carbon dioxide and oxygen tensions. Pao₂ is 50 mm Hg (normal, 95-100), and Paco₂ is 75 mm Hg (normal, 40).

Comment. This bulldog’s condition represents an extreme form of the brachycephalic syndrome, which is seen in short­nosed dogs, particularly bulldogs. The syndrome usually includes stenosis (narrowing) of the external nares and obstruc­tion of the pharynx by pendulous folds of excessive soft tissue. In some of these dogs, the trachea is also very narrow. These dogs have difficulty breathing, particularly during inhalation, when the Subatmospheric pressure within the upper airway sucks the loose folds of tissue into the airway lumen. This can result in total obstruction to ventilation. In general, these dogs make a lot of noise during inhalation as the loose folds of tissue vibrate. Exhalation presents less dif­ficulty because the higher-than-atmospheric pressure in the pharynx tends to push back the loose tissue and open the airway. Over time the chronic, excessively Subatmospheric pressure during inhalation can cause deformity of the larynx.

The upper airway obstruction in this bulldog is limiting ventilation so severely that the dog is suffering from alveolar hypoventilation. The elevated Paco₂ indicates this. An elevation in Paco₂ occurs when alveolar ventilation is not suf­ficient to remove the carbon dioxide being produced by the body. The accumulating carbon dioxide in the alveolus and the lack of ventilation also depress the PAo₂, which leads to a decrease in Pao₂, as in this dog. The hypoxemia then leads to hemoglobin desaturation, which accounts for the bluish color (cyanosis) of the mucous membranes of the dog.

Treatment. The treatment for this dog is surgical removal of some of the excessive tissues of the upper airway and enlargement of the external nares. This will alleviate some of the obstruction and may improve ventilation. However, with the narrowing of the trachea observed in this dog, it is unlikely that the dog will ever be able to exercise to a significant degree, although its condition may be improved sufficiently that it can make a suitable pet.