Today's Veterinary Practice

MAY-JUN 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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PEER REVIEWED 52 ASTHMATIC CAT: MANAGEMENT GUIDELINES Bronchodilators When evidence of airflow limitation exists historically, on physical examination, or on diagnostic testing, bronchodilators should be considered as a component of therapy. They should not be used as a stand- alone therapy because they do not address airway inflammation, which is the driving force behind the asthmatic process and bronchoconstriction. Therefore, using them in combination with anti- inflammatory therapy discussed above is necessary. Several categories and types of bronchodilators are available, including short-acting β2 agonists, long-acting β2 agonists, methylxanthines, and anticholinergics. Numerous options are commercially available, but this article covers those commonly used or investigated in veterinary medicine. Short-acting β2 agonists include albuterol (also known as salbutamol), levalbuterol, and terbutaline. The metered-dose inhalant form of albuterol is a rescue bronchodilator, commonly used to manage acute bronchoconstriction episodes ( TABLE 1 ). Although inhalant albuterol is useful in acute episodes, it should not be used as a long-term therapy. Inhalant albuterol is composed of 2 enantiomers: the R-enantiomer and S-enantiomer. The R-enantiomer is responsible for the bronchodilatory effects of albuterol, and the S-enantiomer was long thought to be inert. However, long-term administration of the S-enantiomer promoted airway inflammation and bronchospasm in an experimental model of feline asthma. 10 Long-term overuse of inhaled albuterol in humans with asthma is also associated with increased risk for death. 11 For these reasons, inhaled albuterol should be limited to management of acute bronchoconstriction, and owners should be cautioned regarding overuse. Levalbuterol is a form of R-enantiomer albuterol. This may be an option for longer-term use in patients requiring such therapy. Terbutaline, which is available in both an injectable and an oral form, is another option for bronchodilators in this class. Injectable terbutaline was discussed as a rescue therapy in the previous section on emergent management. As an injectable medication with quick onset of action, it can rapidly relieve bronchoconstriction and avoid the potential stresses of using an inhaled medication (eg, a cat not trained to accept a face mask). Long-acting β2 agonists are available in inhalant forms but are most widely available in combination with a steroid. These medications are less commonly used in the management of feline asthma. However, salmeterol, alone or in combination with fluticasone propionate, has been evaluated in an experimental model of feline asthma. Although a study evaluating salmeterol alone showed no improvement in measures of airway resistance or airway inflammation, the combination of salmeterol with fluticasone propionate did reduce airway inflammation beyond treatment with fluticasone alone. 5,12 It was suggested that bronchodilation from salmeterol may have promoted better deposition of fluticasone within the airways. These studies evaluated very brief drug administration (4 days); therefore, it is difficult to assess effects seen with longer-term use. Methylxanthines, such as theophylline and aminophylline, are most commonly administered as oral medications in the treatment of feline asthma ( TABLE 1 ). Pharmacokinetics studies have shown that extended- release theophylline administered to cats q24h achieves therapeutic plasma concentrations. 13,14 A follow-up study determined that plasma theophylline concentrations were highest after evening dosing. 15 An ex vivo study documented efficacy of theophylline at producing bronchial relaxation, but the agent was less efficacious than other bronchodilators evaluated. 16 How this finding extrapolates to clinical patients is uncertain, particularly with long-term administration. Although the anticholinergic bronchodilator ipratropium bromide showed promise as an effective bronchodilator in ex vivo experimental conditions, an experimental feline asthma model showed that it failed to improve measures of bronchoconstriction. 16,17 Other Management Clients should be counseled on eliminating or reducing airway irritants, such as cigarette smoke, Glossary ELISA = enzyme-linked immunosorbent assay FiO 2 = fraction of inspired oxygen HPAA = hypothalamic-pituitary-adrenal axis IDST = intradermal skin testing MSC = mesenchymal stem cell PUFA = polyunsaturated fatty acid

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