20 Small Airway Disorders
Bronchitis is defined as inflammation of the conducting airways. The inflammation is primarily neutrophilic, though eosinophilic inflammation may predominate. Macrophages, in various stages of activity, may also be present in the airways. As part of the definition, bronchitis results in a chronic, daily cough, lasting for at least 2 months, with no specific cause identified.
A chronic cough, sometimes lasting for years, is typical. The cough is generally nonproductive, though in some animals, the cough may terminate with gagging or retching. Because dogs cannot expectorate, the productivity of the cough is sometimes difficult to ascertain. Studies indicate that most dogs are bright, alert, responsive, and, other than the cough, normal on presentation. More severely affected animals may show exercise intolerance, syncope, or cyanosis. Physical examination demonstrates diffuse crackles and wheezes. There may be an end-expiratory push with prolonged expiration in more severely affected cases. Tracheal sensitivity may result in an easily induced cough on tracheal palpation. Sinus arrhythmias may be auscultated. Also, the heart rate may be in the normal range or even potentially decreased. This is in contrast to dogs that present in cardiac failure, in which the heart rates may be elevated in an attempt to maintain cardiac output. This is an important distinction to make, because both groups of dogs may present with a primary complaint of coughing.
The minimum database of complete blood count, serum chemistry, and urinalysis is helpful to exclude other causes of coughing. Heartworm serologic and fecal examinations should be included to rule out possible parasitic disease. Some cases of bronchitis will demonstrate a peripheral eosinophilia; polycythemia may be seen as a result of chronic hypoxia; serum alkaline phosphatase and alanine aminotransferase may be increased. Pulmonary function testing is becoming a more commonly used diagnostic tool. Arterial blood gas analysis may demonstrate hypoxemia (oxygen pressure less than 80 mm Hg at sea level). A concurrently elevated carbon dioxide pressure is associated with a poorer prognosis. Thoracic radiographs are an important part of the workup for bronchitis. They also are important for ruling out other differentials for coughing (e.g., congestive heart failure, neoplasia). Normal thoracic radiographs do not rule out the possibility of bronchitis. The most commonly reported changes on radiographs are a result of increased thickening of bronchial walls. These thickened walls are termed “tram lines” when viewed longitudinally and “doughnuts” when viewed in cross section. An increased interstitial pattern is also associated with bronchitis. However, one study found a similar frequency of interstitial patterns in dogs with bronchitis and normal controls. This same study showed only thickened bronchial walls and increased numbers of visible bronchial walls as significantly increased in dogs with bronchitis. Figure 20-1 shows a lateral thoracic radiograph of a dog with chronic bronchitis, demonstrating multiple “doughnuts” and prominent airway markings. Pulmonary hyperinflation and diaphragmatic flattening have been reported, though these are more common in cats. Bronchoscopy with bronchoalveolar lavage is considered the diagnostic test of choice for chronic bronchitis. It is recommended that this procedure be performed early in the course of the disease, when the animal’s respiratory status is not severely compromised and anesthesia is still relatively safe. Bronchoscopic findings in affected airways include: erythema, mucus, nodular proliferation, irregular mucosa, and, potentially, dynamic airway collapse. Cytologic evaluation of airway secretions in dogs with active disease shows primarily neutrophilic inflammation. Other cell lines that may be present include eosinophils and macrophages. Curschmann’s spirals also may be present. Samples should be obtained for culture and sensitivity, and it is recommended to perform a quantitative assessment of bacterial numbers. Cytologic samples should also be examined for intracellular bacteria or degenerate neutrophils to help determine the significance of possible infectious agents. It has been suggested that septic inflammation, single agent bacterial cultures, atypical bacterial resistance patterns, or acute clinical exacerbations (fever, inflammatory changes in the complete blood cell count, and radiographic evidence of pneumonia) may represent better evidence for a significant bacterial infection.
(Radiograph courtesy of Dr. E. Riedesel, Iowa State University.)
Bronchitis historically was thought to be a disease limited to small-breed, terrier dogs. It is now known that medium and large-size dogs can also develop bronchitis. There may be an increased incidence in the Cocker Spaniel and West Highland White Terrier breeds. Obesity is a well-known risk factor. Chronic, inhaled airway irritants (smoke, allergens, and other airborne pollutants) are thought to play a role in the development of bronchitis, because it is a recurrent, low-grade infection. Periodontal disease and laryngeal disease may also play a role, allowing bacterial showering of the lower airways.
After this condition develops, the airway changes may be progressive and the disease cannot be cured. Histologic examination of affected airways shows smooth muscle hypertrophy, goblet cell hyperplasia, lamina propria fibrosis, mucus gland hypertrophy, mucosal edema, epithelial erosion with loss of ciliated epithelium, and squamous metaplasia. Some aspects of the disease may be reversible, such as airway spasm or secondary bacterial infection.
Treatment is geared toward control of clinical signs and improvement in quality of life, not cure of the disease. Environmental factors should be addressed and potential airway irritants reduced or eliminated (e.g., smoke, dust, deodorizers). If the animal is overweight, a weight loss program should be instituted. If weight loss is difficult, evaluation for underlying endocrine disease should be performed (e.g., hypothyroidism, hyperadrenocorticism). Medical management generally involves antitussives, antiinflammatories, bronchodilators, and, potentially, antibiotics. Medications commonly prescribed for the treatment of chronic bronchitis is provided in Table 20-1. Antitussives are contraindicated when the cough is productive, because mucus retention can exacerbate airway inflammation and worsen disease progression. After inflammation is resolved and infection has been cleared, antitussives may be used. Sedation and constipation may be seen with these medications. The role of bronchoconstriction in chronic bronchitis is questionable. However, bronchodilators may have the beneficial effects of improving mucociliary clearance, improving diaphragmatic contractility, and allowing reduction in the dosage of glucocorticoids. Adverse effects include tachycardia, gastrointestinal upset, and excitability. Enrofloxacin, which may be a useful antibiotic for respiratory infections, impairs the metabolism of theophylline. This results in toxic levels of theophylline. Plasma levels of theophylline may be measured through human laboratories; the peak goal range is 5-20 μg/ml. If these two medications are used simultaneously, the theophylline dose should be decreased by at least 30%. In some cases, generic alternatives for theophylline may be available. The bioavailability of some of these alternatives has been questionable, so generic substitutions should be avoided in most cases. Antibiotics are indicated with concurrent bacterial infection. Ideally, antibiotic choice is based on culture and sensitivity results. If culture results are not available, and there is sufficient evidence for a bacterial infection, a bactericidal antibiotic with gram negative spectrum and good penetration into respiratory tissues (e.g., fluoroquinolone, amoxicillin/clavulanic acid) should be chosen. There has been recent discussion about the use of inhalant therapy for dogs with chronic bronchitis. There has not been clinical evidence to support the use of inhalant therapy. Other medications that have been used in cats with feline asthma (cyproheptadine, cyclosporine) would not be expected to be of benefit in the treatment of dogs with chronic bronchitis.
|DRUG||ACTIVITY||DOSAGE (ALL DRUGS GIVEN ORALLY)|
|Terbutaline||Beta agonist||1.25-5 mg/dog q 12 h|
|Theophylline||Methylxanthine derivative||20 mg/kg q 12 h (sustained release)|
|Albuterol||Beta agonist||0.02-0.05 mg/kg q 8-12 h|
|Hydrocodone||Opiate agonist||0.22-1.0 mg/kg q 6-12 h|
|Butorphanol||Partial opiate agonist||0.55-1.0 mg/kg q 6-12 h|
|Codeine||Opiate agonist||1-2 mg/kg q 6-12 h|
|Prednisone||Glucocorticoid||0.5-1.0 mg/kg q 12-24 h for 5-7 days, then taper to alternate day dosing|
The initial prognosis is guarded, but with aggressive identification and therapy, progression of the disease may be slow. It is important to educate clients that therapy does not offer a cure. Rather, the intent of therapy is to limit disease progression and slow the onset of complicating factors. Poorly controlled disease can result in airway fibrosis, pulmonary hypertension, cor pulmonale, and bronchiectasis and, when present, these factors worsen the prognosis.