Chapter 162 Bronchopulmonary Disease
Diseases of the tracheobronchial tree and pulmonary parenchyma are very common in canine and feline patients and are responsible for substantial morbidity in the pet animal population. Determining the underlying disease responsible requires an understanding of clinical signs, interpretation of laboratory and radiographic findings, and collection and analysis of airway samples. Individualized therapy will be most likely to lead to amelioration of signs. Many respiratory tract diseases are chronic in nature, and the goals of therapy become controlling clinical signs and avoiding progression of disease.
Cats may present with a history of acute or chronic respiratory disease. Cats with chronic respiratory disease may present emergently with acute signs of respiratory discomfort because stress can lead to rapid physical decompensation.
Cervical tracheal collapse occurs on inspiration. During inspiration, intrapleural pressure drops, leading to a decrease in intrathoracic airway pressure and airflow from the glottis into the region of gas exchange in the lung. The cervical trachea is exposed to atmospheric pressure, and a pressure drop also occurs from the glottis down the airway. Dogs that lack rigidity in the cervical trachea will experience collapse on inspiration and will exhibit clinical signs of inspiratory distress. Inspiratory wheezing may also be evident. In dogs with concurrent upper-airway obstruction (everted laryngeal saccules, elongated soft palate, laryngeal paralysis), a larger pressure gradient develops in the cervical trachea and this increases the tendency of the airway to collapse and perpetuates upper-airway inflammation. Prolapse of the dorsal tracheal membrane into the airway lumen accentuates mechanical trauma to the wall of the trachea, leading to further mucosal irritation, inflammation, and cough.
Intrathoracic tracheal collapse or collapse of the mainstem bronchi is encountered on expiration when intrapleural pressure exceeds intra-airway pressures, and poorly supported airways tend to close. In some dogs, generalized airway collapse is encountered. It is unclear whether cartilaginous deficiency underlies this disease as it does with tracheal collapse. Airway collapse can be seen in conjunction with chronic bronchitis or airway inflammatory disease, perhaps because of irritation caused by cough or because of changes in the pressure gradients within the lung associated with obstruction of smaller airways. In addition, airway inflammation stimulates a cough, which can exacerbate signs related to airway collapse.
Small-breed dogs are typically affected with tracheal collapse, and an increased prevalence is generally seen in the Yorkshire terrier, poodle, Pomeranian, and chihuahua. Any breed of dog can be affected by lower-airway collapse, although typically smaller breed dogs are affected. Animals of any age may present with signs related to tracheal and/or lower-airway collapse.
A long-term history of chronic, intermittent cough is typically reported with or without episodes of respiratory distress. These signs have often been present since the patient was young. Stress, heat, humidity, eating, drinking, or external pressure on the trachea can all trigger clinical signs of disease.
The presence of a honking cough in a small-breed dog is often considered diagnostic of tracheal collapse. Consider a full diagnostic workup in order to identify and treat all coincident disorders that complicate the clinical presentation.
Obtain respiratory samples for cytology and culture to rule out infectious or inflammatory conditions (pneumonia, bronchitis, or fibrosis) that can accentuate clinical signs. Endoscopy (bronchoscopy) offers the most complete evaluation of the respiratory tract and has the benefit of confirming the presence or absence of tracheal and airway collapse. Evaluate upper airway structure and function in all animals sedated for tracheal wash or bronchoscopy.
Dogs that have intrathoracic airway collapse and evidence of small airway disease may benefit from the use of bronchodilators (Table 162-1). Bronchodilators have no clinically significant effect on the large airways and do not result in an increased diameter in the tracheal lumen. Instead, these drugs act primarily on small airways, making it easier for air to flow out of the lungs and lessening the tendency for dynamic collapse of large airways.
|Sustained-release theophylline||Dog: 10 mg/kg PO q12h||Multiple drug interactions, gastrointestinal upset, tachycardia|
|Cat: 10 mg/kg PO q24h (PM)|
|Terbutaline||Dog and cat: 0.01 mg/kg SC, IV||Tachycardia, hypotension|
|Dog: 1.25–5.0 mg/dog PO q12h|
|Cat: 0.625 mg PO q12h|
|Terbutaline MDI||200 μg/puff q12h by aerosol|
|Albuterol||Dog: 50 μg/kg PO q12h|
|Albuterol MDI||90 μg/puff q12h by aerosol|
|Epinephrine||20 μg/kg IV, IM, SC, IT||Cardiac arrhythmias, hypertension, vasoconstriction|
MDI, metered dose inhaler
Treat concurrent chronic bronchitis as described in the next section. Occasionally, tracheal inflammation may require a short course of therapy with glucocorticoids for resolution of airway injury; 5 to 7 days of prednisone, 0.25 to 0.5 mg/kg PO q12-24h followed by rapid tapering of the dose is usually sufficient to treat inflammation.
It is uncommon for clinically significant bacterial infection to complicate the course of tracheal collapse. When a bacterial infection is documented, the choice of antibiotics should be based on culture and sensitivity results.
The etiology of chronic bronchitis in the dog is unknown; however, it is likely that chronic mucosal irritation or immunologic stimulation is responsible for airway inflammation and clinical signs of disease.
The history of a long-term cough in an otherwise bright and healthy dog is suggestive of chronic bronchitis, which is largely a diagnosis of exclusion. Pneumonia and pulmonary neoplasia can cause similar signs. Also, dogs often have separate or concurrent cardiopulmonary conditions such as tracheal collapse, bronchial collapse, bronchial compression, or heart failure. A complete diagnostic workup assesses the role each condition might play in the generation of clinical signs and aids in developing a therapeutic plan. Laboratory evaluation (complete blood count [CBC], chemistry panel, urinalysis) should be done to evaluate systemic health, although there are no typical abnormalities for chronic bronchitis. The diagnosis is based on the history, clinical findings, chest radiographs, and airway sampling to rule out other pulmonary causes of cough.
Cytologic characterization and culture of the cellular infiltrate within the airways can identify or exclude infectious causes of cough and allows development of a rational therapeutic plan. Samples may be obtained through endotracheal or transtracheal wash or with bronchoscopy.