Principles of Therapy for Respiratory Disease in Animals

Also see Systemic Pharmacotherapeutics of the Respiratory System.

Respiratory disease is often characterized by abnormal production of secretions and exudates and by a decreased ability to remove them. The primary goal of therapy is to decrease the volume and viscosity of the secretions and facilitate their removal. This can be accomplished by the following measures:

• controlling infection and inflammation

• modifying secretions

• improving postural drainage, when possible

• mechanically removing the material, when possible

Therapeutic methods include the following:

• altering the inspired air and administering expectorants (however, expectorants have shown little or no beneficial effects in clinical trials)

• antitussives

• bronchodilators

• antimicrobials

• diuretics

• other drugs as appropriate

Small animals can benefit from sedation when in respiratory distress.Medications such as butorphanol can be used safely for anxiolysis and mild sedation during triage examination and for patients in shock. Additional sedation or anesthesia might be required to assist with intubation if the patient decompensates (eg, cyanosis or respiratory failure).

Hypoxemia caused by most lung disorders must be addressed with supplemental oxygen. Supplemental oxygen provides a fraction of inspired oxygen (FiO₂) of up to 40–60% via nasal cannula, flow-by oxygen, oxygen cage, or nasal prongs. Intubation provides an FiO₂ of 100%, which is needed for patients requiring mechanical ventilation. Hypoxic patients typically have a respiratory rate of > 40 breaths per minute. A PaO₂ < 60 mm Hg indicates severe hypoxemia, and that is one of the criteria for mechanical ventilation (1). Oxygen toxicosis can occur if the patient remains on continuous 100% oxygen for 12–24 hours (2). Arterial blood gas analysis and pH determinations, when practicable, are extremely valuable for monitoring treatment.

Patients with respiratory disease should be kept hydrated. Inhalation of humidified air can help remove airway secretions. Expectorants are sometimes used to liquefy these secretions; however, they should be used in conjunction with ancillary respiratory therapy, such as improved postural drainage, mild exercise, and coupage, which (in addition to coughing) encourages expectoration and secretion removal. Mechanical removal of tenacious and viscid secretions by suction might be necessary in cases of severe airway obstruction.

Antitussive agents may be considered to relieve the discomfort associated with nonproductive coughing; however, they are contraindicated when secretion of airway mucus is excessive. Products that contain atropine also are contraindicated, at least in theory, because atropine increases the viscosity of airway secretions.

Increased airway resistance caused by bronchial smooth muscle contraction can be alleviated with bronchodilators. Methylxanthines (eg, theophylline and aminophylline) are effective bronchodilators in species other than cattle; however, the therapeutic index is relatively narrow, and they are less efficacious than beta-2-adrenergic receptor agonists. For example, albuterol, when given aerosolized via a metered dose inhaler with a species-appropriate face mask and spacer, is highly effective in treating bronchoconstriction related to asthma in cats.

Systemic corticosteroids are highly effective in sterile inflammatory and allergic conditions but can have adverse effects. Aerosolized corticosteroids are efficacious and associated with few or no adverse effects; however, they require an aerosol delivery device (eg, face mask and spacer) for proper administration. Ideally, in emergent situations, patients should receive either injectable corticosteroids (eg, dexamethasone sodium phosphate) or start on oral corticosteroids before transitioning to inhaled corticosteroids.

Antihistamines can alleviate bronchoconstriction caused by histamine release; however, they are of limited value in large animals.

Patients with respiratory disease due to bacterial infection should undergo antimicrobial treatment. The goal is to select either the most effective agent against a specific microorganism or the least toxic agent of several alternatives. For patients with lower airway disease, fluid can be collected via endotracheal wash for bacteriological culture and antimicrobial susceptibility testing; alternatively, bronchoscopic collection can provide a worthwhile, although not infallible, guide to determining the appropriate antimicrobial. Knowledge of the tissue penetration and pharmacokinetic characteristics of various antimicrobial agents is important as well.

The following antimicrobial agents are effective in the listed species:

• cattle—oxytetracycline, cephalosporins, fluoroquinolones, macrolides, florfenicol, penicillins, and sulfonamides

• sheep and goats—oxytetracycline, cephalosporins, macrolides, penicillins, and sulfonamides

• pigs—lincomycin, spectinomycin, penicillins, and sulfonamides

• dogs and cats—cephalosporins, chloramphenicol, amoxicillin-clavulanate, aminoglycosides, trimethoprim-sulfamethoxazole, fluoroquinolones, macrolides, and tetracyclines

• horses—penicillins, aminoglycosides, cephalosporins, fluoroquinolones, sulfonamides, and tetracyclines (the latter to be used with caution because severe diarrhea is an occasional adverse effect)

Aminoglycosides are nephrotoxic but can be considered if indicated and with careful use.

Trimethoprim, usually in combination with a sulfonamide, is useful for respiratory therapy in most species but is not licensed for food-producing animals in the US.

Appropriate antimicrobial stewardship is essential.

Broad-spectrum antimicrobials are recommended if specific bacteria cannot be identified, and once begun, a full course of treatment should be completed. Multiple antimicrobial agents should be administered only with full knowledge of the potential drug interactions. To avoid residues in food-producing animals, veterinarians must use broad-spectrum antimicrobials according to label instructions and provide sound advice to producers. Extralabel use of antimicrobials is permitted in some situations and, in the US, is regulated by the Animal Medicinal Drug Use Clarification Act of 1994.

For treatment of pneumonia caused by fungal infections (eg, blastomycosis, histoplasmosis), antifungal medications such as itraconazole are appropriate.

Diuretics are indicated in cardiogenic pulmonary edema in animals. Osmotic diuretics have a minimal effect on diuresis, carbonic anhydrase inhibitors (eg, acetazolamide) have a moderate effect, and loop diuretics (eg, furosemide) have a profound effect.