Principles of Therapy of Respiratory Disease
Respiratory disease is often characterized by abnormal production of secretions and exudates and by a reduced ability to remove them. The primary goal of therapy is to reduce the volume and viscosity of the secretions and to facilitate their removal. This can be accomplished by controlling infection and inflammation, modifying the secretions, and when possible, improving postural drainage and mechanically removing the material. Therapeutic methods include altering the inspired air and administering expectorants, antitussives, bronchodilators, antimicrobials, diuretics, and other drugs. However, expectorants have shown little or no beneficial effects in clinical trials.
Hydration should be maintained. Inhalation of humidified air may facilitate removal of airway secretions. Expectorants are sometimes used with the intention of liquefying these secretions. However, they should be used in conjunction with ancillary respiratory therapy such as improved postural drainage, mild exercise, and thoracic percussion, which (in addition to coughing) encourages expectoration and removal of secretions. Mechanical removal of tenacious and viscid secretions by aspiration may be necessary in severe airway obstruction.
Antitussive agents are indicated to relieve the discomfort associated with nonproductive coughing but 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, which may be indicated in animals with asthma-like conditions and chronic respiratory disease. Methylxanthines, such as theophylline and aminophylline, are effective bronchodilators in species other than cattle (and possibly dogs); however, the therapeutic index is relatively narrow and they are less efficacious than β2-agonists. Isoproterenol, clenbuterol, and epinephrine are also generally effective, and sodium cromoglycate may be used in horses with inflammatory airway disease. Corticosteroids are highly effective in allergic conditions, but systemic use may result in adverse effects. Aerosolized corticosteroids are efficacious and associated with few to no adverse effects; however, they require an aerosol delivery device (eg, face mask) for proper administration. Antihistamines can be used to alleviate the bronchoconstriction caused by histamine release; however, they are of limited value in large animals. Bronchospasm also can be reduced significantly by removing irritating factors, using mild sedatives, or reducing periods of excitement.
In bacterial infection, antimicrobial therapy should be instituted. The goal is to select either the most effective agent against a specific organism or the least toxic agent of several alternatives. Culture and sensitivity testing of airway secretions provide a worthwhile, although not infallible, guide to determining the appropriate antibiotic. Knowledge of tissue penetration and pharmacokinetic characteristics of the antimicrobial agents is important as well. The following agents have proved 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 with caution due to an occasional adverse effect of severe diarrhea). Aminoglycosides are useful but can be nephrotoxic. 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 USA. Drugs such as enrofloxacin (approved for small animals and cattle but not for horses in the USA) and ceftiofur are effective for pneumonia.
Broad-spectrum antibiotics should be used if specific bacteria cannot be identified, and once begun, a full course of therapy should be completed. Multiple antimicrobial agents should be used only with full knowledge of the potential drug interactions. Because of residues in food-producing animals, veterinarians must use these products according to label instructions and provide sound advice to producers. Extra-label use of antimicrobials is permitted in some situations and is regulated by the Animal Medicinal Drug Use Clarification Act of 1994.
The hypoxemia caused by most lung disorders usually can be corrected by administering oxygen. However, continuous administration of high concentrations increases the tendency for regional resorption atelectasis, thus worsening the hypoxemia, and can cause pneumonitis on its own. Hypoxemia is often accompanied by variable degrees of hypercapnia and acidemia. Endotracheal intubation and mechanical ventilation may be necessary in animals with acute respiratory failure or in animals that are comatose or apneic. Arterial blood gas and pH determinations, when practicable, are extremely valuable to monitor treatment.
Diuretics are indicated in pulmonary edema. The osmotic diuretics have a minimal action on diuresis, carbonic anhydrase inhibitors (eg, acetazolamide) have a moderate effect, and loop diuretics (eg, furosemide) have a profound effect.