The use of inhaled respiratory medications in dogs and cats is becoming more common. Inhalant delivery of aerosolized medication offers a number of theoretical benefits, including an enormous absorptive surface area across a permeable membrane, a low enzyme environment that results in little drug degradation, avoidance of hepatic first-pass metabolism, and reproducible absorption kinetics. When the target of inhaled medications is the respiratory tract itself, additional benefits include the potential for attaining a high drug concentration directly at the disease site with minimal systemic absorption and toxicity. Often, therapeutic effect can be achieved with only a fraction of the dose required for systemic delivery of the same drug.1
Because of these advantages, inhalant delivery of medication has gained widespread use for treating airway diseases in people. More than 30 drugs licensed for people are available for inhalation, including anti-inflammatory drugs and bronchodilators. An enormous body of evidence in the medical literature exists regarding the efficacy and toxicity of inhalational drug therapy in people. In veterinary medicine, the literature on inhalant therapy to treat naturally occurring disease is sparse. Regardless, aerosol delivery of medication has become popular for treating dogs and, especially, cats with respiratory disease.
Although inhalant drug delivery has many benefits, difficulties in using this route exist as well. Respiratory defenses are efficient at preventing particulates from reaching the lower airways, so it should come as no surprise that only a small proportion of the administered medication reaches the lower airways; a marked amount of drug is lost in the delivery device or deposited in the oropharynx.1 In animals, another portion will be deposited on the fur, especially if an animal is placed in a tent or tank, and can be ingested through grooming.
Another difficulty is that most aerosol drug delivery devices are designed to be used by people on a voluntary basis, and some require purposeful respiration and breath-holding. Adaptations of some devices facilitate their use in animals, and modified systems are now marketed for dogs and cats (e.g. Aerokat—Trudell Medical International; NebulAir Small Animal Chamber—DVM Pharmaceuticals; Breathe Easy—Jorgensen Laboratories).
Drug delivery by the aerosol route depends in part on respiratory depth and rate, tidal volume, and airflow rate, yet all of these may be negatively affected by respiratory disease. Additionally, not all drugs are suitable for aerosol delivery, and the drugs themselves (or preservatives contained in the drug preparation) may cause airway irritation and possible bronchoconstriction that could worsen respiratory function.
AEROSOL DELIVERY SYSTEMS
Two basic types of aerosol delivery systems are used in veterinary practice: nebulizers and pressurized metered dose inhalers (MDIs). The two are distinct devices with different uses. In general, nebulizers deliver much smaller particles, allowing deeper respiratory penetration, and provide fluid along with the drug.1 MDI devices deliver drugs primarily to the larger airways.
Basic nebulizer types include jet nebulizers and electronic nebulizers. Modifications exist (e.g. spinning disk nebulizers, vibrating mesh nebulizers) to improve delivery or modulate particle size.
How they work. Jet nebulizers have a compressed air or oxygen source, a well into which fluid or a drug can be placed, and a baffle that when hit by the drug creates small particles. Jet nebulizers tend to be larger and sturdier than electronic nebulizers. Electronic nebulizers use membrane vibration to produce an aerosol and are much smaller since no air compressor is required. However, in my and other clinicians' experience, electronic nebulizers tend to malfunction more easily than jet nebulizers do.
MDIs are designed for at-home administration of aerosolized drugs and are the preferred devices to deliver maintenance glucocorticoid and bronchodilator medications in people with asthma or chronic obstructive pulmonary disease (COPD). Particles delivered by MDIs are larger than those created by nebulization and, thus, do not penetrate as deeply into the respiratory tract.1
How they work. A traditional pressurized MDI consists of a mouthpiece and an actuator (holder) into which a canister of medication is inserted. Manually depressing the canister (actuation) releases a single dose of medication (sometimes called a puff). People shake the canister, exhale deeply, insert the mouthpiece, and simultaneously depress the canister and inhale as deeply as possible. They then hold their breath for as long as possible, exhale, and rinse the mouth and expectorate to remove most of the drug deposited in the oropharynx (only about 10% of each dose reaches the airways). Since dogs and cats cannot use a MDI in this way, spacer devices designed to fit MDIs have allowed MDIs to be adapted for use in animals.
Equipment needed. Several types of spacers are available, from simple tubes inserted between the MDI and the mouth and nose to holding chambers with one-way valves activated by inhalation. Spacers were designed for young children or debilitated adults with less than ideal coordination, so simultaneous canister depression and inhalation is not necessary. The spacer also has the advantage of allowing the largest particles to fall out and not enter the patient's mouth. In people, spacers improve drug delivery by about 10%, nearly doubling the amount of drug reaching the target site.1,2
Recent developments. Until the last several years, most MDIs used chlorofluorocarbons as propellants. Chlorofluorocarbons are being phased out because of concerns about the ozone layer. The drugs previously used in MDIs must be reformulated either to use alternative, ozone-safe propellants or new technologies including breath-actuated inhalers.
Because breath-actuated inhalers are not readily used by all people (e.g. infants) either, some drugs are still available in pressurized MDIs using alternative propellants such as hydrofluoroalkanes. The recent change to newer formulations of MDIs has dramatically increased the cost of some medications formerly available as inexpensive generic preparations.
WHEN AND HOW TO USE NEBULIZATION
In small-animal medicine, nebulizers have predominantly been used to treat respiratory infections. Nebulizers have long been used to humidify the airway or administer antimicrobials directly into the respiratory tract. Mucolytic agents (e.g. N-acetylcysteine) have also been nebulized to treat animals with respiratory infection but are not generally recommended because of irritation and bronchoconstriction after such therapy.3 Sterile saline nebulization administered three or four times a day is safe to treat animals with bronchopneumonia. Although no scientific studies demonstrate utility, it is my impression that saline nebulization is beneficial.
In people, it is common to include antimicrobials in nebulized solutions to treat severe bacterial pneumonia, particularly in patients with compromised defenses such as patients with cystic fibrosis.4 Some drugs that are made especially for delivery by this route do not contain potentially reactive additives or preservatives (e.g. tobramycin inhalation solution [Tobi—Novartis]), but these preparations are prohibitively expensive for dogs and cats. Veterinarians have used drugs made for parenteral administration in nebulized solutions to treat pneumonia or other respiratory infections, including infection with Bordetella bronchiseptica. Not all liquid antibiotics are suitable for nebulization. Aminoglycosides are the most frequently used class of antibiotics for nebulization.
There are no well-established guidelines for dosing or administering drugs not made specifically for aerosol administration in veterinary patients. Typically, the total daily systemic dose of a drug such as gentamicin or amikacin is diluted in saline solution to be delivered in a single daily session with the nebulizer.
Rarely, patients may experience bronchoconstriction in response to such therapies. Pretreatment with bronchodilators may minimize potential reaction to drug carriers and improve aerosolized drug delivery. Bronchodilators may be administered parenterally 15 minutes before nebulization or by an initial nebulization period, with the bronchodilator added directly to the nebulized fluid before the antimicrobial drug is added. Common bronchodilator choices for nebulization are albuterol 0.5% solution for inhalation (5 mg/ml) or the premixed 2.5 mg/3 ml solutions. The concentrated drug should be diluted in about 3 ml sterile saline solution before administration. While a dose has not been established for nebulized albuterol in dogs and cats, the dose for children is 0.1 to 0.15 mg/kg with a maximum dose of 2.5 mg given up to a maximum of four times daily. Delivery of nebulized antimicrobials should never replace systemic antimicrobials in animals with pneumonia. Instead, nebulization should be regarded as a complementary therapy.
Prevent iatrogenic infections
When nebulizers are used to treat pets with contagious respiratory disease, the device itself must be kept meticulously clean to avoid causing iatrogenic respiratory infection. Extreme care should be given to cleaning, and disposable parts of the device should be discarded. Nebulization of a nosocomial Pseudomonas species, for instance, could have devastating consequences for an animal with compromised respiratory function.
WHEN AND HOW TO USE MDIS
MDIs are the preferred delivery device for most asthma and COPD medications in people, and they have been advocated for use in treating feline bronchopulmonary diseases, including asthma, as well as for treating chronic bronchitis or related airway disease in dogs.
The use of inhaled corticosteroids may be particularly helpful in minimizing the systemic effects of glucocorticoids in asthmatic cats with comorbid conditions such as diabetes mellitus or congestive heart failure. For any patient, concomitant use of inhaled and systemic corticosteroids may allow decreased systemic drug dosages. Keep in mind that inhaled corticosteroids take days or weeks to be maximally effective and, thus, should not be relied on for emergent treatment of asthmatic cats.
Albuterol delivery by MDI can be useful during exacerbations of asthma but will not always successfully replace parenteral administration of bronchodilators for cats in asthmatic crisis.
A variety of respiratory drugs are available as MDIs, including corticosteroids (e.g. fluticasone [Flovent—GlaxoSmithKline]), short-acting bronchodilators (e.g. albuterol [Ventolin—GlaxoSmithKline; Proventil—Schering-Plough]), and nonsteroidal anti-inflammatory drugs such as cromolyn or nedocromil. Some inhaled medations—including most long-acting bronchodilators and combination corticosteroid/bronchodilators (e.g. salmeterol [Serevent—GlaxoSmithKline], fluticasone and salmeterol combination [Advair—GlaxoSmithKline], formoterol [Foradil—Schering-Plough])—come as breath-actuated inhalers instead of MDIs and are, thus, not useful in dogs and cats. Even when the drug is available as an MDI, not all MDIs fit the spacers typically used for dogs and cats. For example, triamcinolone acetonide (Azmacort—Abbott Laboratories) has a built-in spacer, so it cannot be adapted to commercial feline spacer devices. Be certain that the drug you prescribe comes in an MDI that will work with the spacer device used by the client.
To administer a dose, turn the animal so that its head faces away and its tail faces into the person delivering the drug. Shake the MDI and fit it into the spacer device, fit the mask at the other end of the spacer device over the animal's face, and depress the canister (Figure 3). When using some types of valved spacers, you can depress the canister immediately before placing the mask if the noise scares the pet. Then allow the animal to breathe into the mask for seven to 10 breaths. In my experience, few owners have trouble administering the inhaled medication in this fashion.
Appropriate dosage regimens have not been clearly established for drug delivery by MDI in dogs and cats. Commonly used medications include albuterol (108 μg albuterol sulfate/puff) for acute signs of bronchoconstriction. A single puff may be effective, or it can be given up to four times daily during an acute exacerbation. More than occasional use of albuterol can lead to paradoxic bronchoconstriction, and the need for frequent treatment (> three times a week) should prompt re-evaluation of other aspects of treating the disease.5 The most commonly used MDI corticosteroid is fluticasone propionate, available in 44-, 110-, and 220-μg/puff strengths. Although no evidence demonstrates the most effective dose, it is often used in the 110- or 220-μg strength as one puff twice daily.
EFFICACY OF AEROSOL DRUG DELIVERY IN SMALL ANIMALS
Few scientific studies report on the safety or efficacy of these aerosol therapies in companion animals. Although there are plentiful descriptions of aerosol therapy in textbooks and at continuing education meetings, there are few controlled published studies of animals with naturally occurring disease. The information that has been published about aerosol drug delivery in dogs or cats includes the following:
Because of the questions still surrounding the efficacy of drug delivery by aerosol, these drugs should generally be used as adjuncts in treating animals with moderate to severe signs of disease. Once signs have improved, inhalant therapy may replace medications traditionally administered by other routes.
AEROSOL DELIVERY OF OTHER THERAPIES
Other uses for inhalation therapy include treating lung cancer or systemic disease. Several reports describe the use of aerosolized drug delivery with chemotherapeutic or immunomodulating drugs in treating spontaneous primary and metastatic cancers in dogs.17-19 Dogs have also been used as a model in the development of insulins to be delivered via needle-free aerosol.20 Such an insulin product was FDA-approved for use in people (Exubera—Nektar) but was withdrawn from the U.S. market because of a lack of consumer demand. Recently, nebulized regular insulin was demonstrated to effectively lower blood glucose concentrations in five healthy domestic cats.21
Small animals have been used as models for other aerosol therapies, including treatment aimed at cardiovascular and hemodynamic perturbations (e.g. inhaled nitric oxide for pulmonary hypertension), vaccination, and even gene therapy.22 As delivery systems are developed specifically for animals and as our knowledge of the efficacy of this system of drug delivery grows, we are likely to use more inhalational therapy in small-animal patients.
Leah A. Cohn, DVM, PhD, DACVIM
Department of Veterinary Medicine and Surgery
College of Veterinary Medicine
University of Missouri
Columbia, MO 65211
1. Labiris NR, Dolovich MB. Pulmonary drug delivery. Part II: the role of inhalant delivery devices and drug formulations in therapeutic effectiveness of aerosolized medications. Br J Clin Pharmacol 2003;56(6):600-612.
2. Dolovich MB, Ahrens RC, Hess DR, et al. Device selection and outcomes of aerosol therapy: evidence-based guidelines: American College of Chest Physicians/American College of Asthma, Allergy, and Immunology. Chest 2005;127(1):335-371.
3. Dano G. Bronchospasm caused by acetylcysteine in children with bronchial asthma. Acta Allergol 1971;26(3):181-190.
4. Conway SP. Nebulized antibiotic therapy: the evidence. Chron Respir Dis 2005;2(1):35-41.
5. Reinero CR, Delgado C, Spinka C, et al. Enantiomer-specific effects of albuterol on airway inflammation in healthy and asthmatic cats. Int Arch Allergy Immunol 2009;150(1):43-50.
6. Bemis DA, Appel MJG. Aerosol, parenteral, and oral antibiotic treatment of Bordetella bronchiseptica infections in dogs. J Am Vet Med Assoc 1977;170(10):1082-1086.
7. Riviere JE, Silver GR, Coppoc GL, et al. Gentamicin aerosol therapy in 18 dogs: failure to induce detectable serum concentrations of the drug. J Am Vet Med Assoc 1981;179(2):166-168.
8. Miller CJM, McKiernan BC, Hauser C, et al. Gentamicin aerosolization for the treatment of infectious tracheobronchitis (abst). J Vet Intern Med 2003;17:386.
9. Schulman RL, Crochik SS, Kneller SK, et al. Investigation of pulmonary deposition of a nebulized radiopharmaceutical agent in awake cats. Am J Vet Res 2004;65(6):806-809.
10. Reinero CR, Decile KC, Byerly JR, et al. Effects of drug treatment on inflammation and hyperreactivity of airways and on immune variables in cats with experimentally induced asthma. Am J Vet Res 2005;66(7):1121-1127.
11. Kirschvink N, Leemans J, Delvaux F, et al. Inhaled fluticasone reduces bronchial responsiveness and airway inflammation in cats with mild chronic bronchitis. J Feline Med Surg 2006;8(1):45-54.
12. Cohn LA, DeClue AE, Cohen RL, et al. Dose effects of fluticasone propionate in an experimental model of feline asthma (abst). J Vet Intern Med 2008;22(3):706.
13. Bexfield NH, Foale RD, Davison LJ, et al. Management of 13 cases of canine respiratory disease using inhaled corticosteroids. J Small Anim Pract 2006;47(7):377-382.
14. Cohn LA, DeClue AE, Reinero CR. Endocrine and immunologic effects of inhaled fluticasone propionate in healthy dogs. J Vet Intern Med 2008;22(1):37-43.
15. Reinero CR, Brownlee L, Decile KC, et al. Inhaled flunisolide suppresses the hypothalamic-pituitary-adrenocortical axis, but has minimal systemic immune effects in healthy cats. J Vet Intern Med 2006;20(1):57-64.
16. Kirschvink N, Leemans J, Delvaux F, et al. Bronchodilators in bronchoscopy-induced airflow limitation in allergen-sensitized cats. J Vet Intern Med 2005;19(2):161-167.
17. Hershey AE, Kurzman ID, Forrest LJ, et al. Inhalation chemotherapy for macroscopic primary or metastatic lung tumors: proof of principle using dogs with spontaneously occurring tumors as a model. Clin Cancer Res 1999;5(9):2653-2659.
18. Selting K, Waldrep JC, Reinero C, et al. Feasibility and safety of targeted cisplatin delivery to a select lung lobe in dogs via the AeroProbe intracorporeal nebulization catheter. J Aerosol Med Pulm Drug Deliv 2008;21(3):255-268.
19. Khanna C, Anderson PM, Hasz DE, et al. Interleukin-2 liposome inhalation therapy is safe and effective for dogs with spontaneous pulmonary metastases. Cancer 1997;79(7):1409-1421.
20. Cherrington AD, Neal DW, Edgerton DS, et al. Inhalation of insulin in dogs: assessment of insulin levels and comparison to subcutaneous injection. Diabetes 2004;53(4):877-881.
21. DeClue AE, Leverenz EF, Wiedmeyer CE, et al. Glucose lowering effects of inhaled insulin in healthy cats. J Feline Med Surg 2008;10(5):519-522.
22. Laube BL. The expanding role of aerosols in systemic drug delivery, gene therapy, and vaccination. Respir Care 2005;50(9):1161-1176.