Two classes of bronchodilators are used most commonly in dogs and cats. Methylxanthine drugs (theophylline derivatives) have been used for many years; however, the mechanism of action remains elusive. This drug is classified as a phosphodiesterase inhibitor, which should increase cyclic AMP and lead to smooth muscle dilation; however, clinically effective levels of theophylline do not result in accumulation of cyclic AMP. Theophylline has been proposed to act through adenosine antagonism, a potential effector molecule in asthma. Theophylline also alters calcium handling in cells and may result in mild bronchodilation through this mechanism. Aminophylline is also a methylxanthine drug but is a very weak bronchodilator, requires administration three to four times daily, and parenteral use in an emergency situation is not justified since terbutaline is more likely to be effective. Terbutaline, a beta-2 agonist, is an effective bronchodilator with minimal cardiac side effects. This drug relieves bronchoconstriction by activating receptors on smooth muscle cell membranes that lead to relaxation. Terbutaline is convenient and easy to administer in an emergency situation.
Bronchodilators can be helpful in lessening clinical signs in dogs or cats with chronic inflammatory lower airway disease or intrathoracic airway collapse. Bronchodilators may lessen airway collapse through mild expansion of small airways (<2 mm in diameter), causing a reduction in the pressure gradient within intrathoracic airways that would favor collapse. Theophylline is also reported to improve strength of diaphragmatic contractions and respiratory muscle strength and improves mucociliary clearance in dogs, but not in cats. In chronic lower airway disease in either species, bronchodilators may be of benefit to the patient through lowering of the dose of steroid required to control clinical signs. Therefore, trial therapy with one of these drugs can be considered when steroids alone fail to control clinical signs. Bronchodilator therapy,particularly with a beta agonist, can aid in resolution of respiratory distress in cats with asthma that experience active bronchoconstriction as part of the pathophysiology of disease.
Side effects of either class of bronchodilator include tachycardia, agitation, or gastrointestinal upset. Metabolism of theophylline is altered by many conditions, including the level of fiber in the diet and use of enrofloxacin. In dogs, it is wise to start therapy with extended release theophylline at half the recommended dose: 5 mg/kg orally (PO) twice daily (BID), and increase to the standard dose if the dog tolerates it. One limitation in use of the extended release product is pill size: the smallest tablet is 100 mg. The tablet can be cut in half and retain extended-release capabilities but quartering the tablet negates this property. Nonetheless, this dose can be effective and trial therapy should be considered even in small dogs.
Albuterol can be administered as a metered dose inhaler and can also be nebulized. Some clinicians recommend administering a bronchodilator prior to other inhalational therapy to improve penetration of lower airways. When desired, the product should be given approximately 5 minutes before the steroid or antibiotic being administered by inhalation. Overuse of albuterol can cause airway inflammation, and if an animal is not responding properly to combined use of an inhaled bronchodilator and steroid, the former should be discontinued.
In human medicine, there are many inhalational therapies containing steroids and a long-acting beta agonist, typically salmeterol. These have not been investigated for use in animals.
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