Lung Fibrosis in the Dog

History. A 3-year-old English setter in respiratory distress is presented at a teaching hospital. The owner first noticed reluctance of the dog to exercise 3 weeks ago. Since that time, the animal has had progressive difficulty in breathing.

Clinical Examination. Inspection reveals a thin dog breathing through its mouth. The respiratory rate is elevated, but the dog seems to be moving little air despite strong inspiratory efforts during which the intercostal spaces sink. Exhalation presents no difficulty; the ribs collapse rapidly, and there is no accentuated abdominal effort.

Examination reveals slightly blue-colored mucous mem­branes. Lung sounds are not remarkable. All other systems are normal.

Radiographs of the thorax show diffuse miliary density (whiteness) over the parts of the lung that are normally air filled. The bronchi are normal. An elevated change in pleural pressure during breathing, normal airway resistance, and decreased static lung compliance are the key findings on lung function testing. Tidal volume (VT) is greatly reduced.

Comment, The history and clinical signs indicate a respiratory problem. The elevated change in pleural pressure during breathing confirms the increased effort necessary to breathe. This could be caused by (1) increased air movement resulting from an increased metabolic rate, (2) airway obstruc­tion, or (3) a decrease in lung compliance (stiffening of the lung). The increased density in the normally air-filled elastic part of the lungs, coupled with the normal air passages, suggests a decrease in lung compliance rather than airway obstruc­tion. This was confirmed when measurements of lung func­tion revealed a normal airway resistance and decreased lung compliance.

The retraction of intercostal spaces indicates that the stiff lung is resisting expansion. Exhalation is not a problem because the lung has an increased tendency to collapse, and the airways are normal.

This dog has a diffuse disease of the exchange area of the lung, which, by decreasing compliance, increases the work of breathing. The blue tinge to the mucous membranes indicates desaturation of hemoglobin as a result of impaired oxygen exchange in the diseased lung. A biopsy reveals diffuse fibrosis around mineral particles in the walls of the alveoli. The prog­nosis for the dog is poor.