INTRODUCTION

Administration of medications via inhalation has been commonplace in human medicine for decades.¹ Only recently has inhalant therapy begun to emerge in veterinary medicine, and its use remains empiric at best. Although few published peer-reviewed studies exist on inhalant medications in dogs and cats, clinical use is clearly on the rise. This chapter will attempt to summarize the principles behind aerosol therapy, describe the various delivery systems, and depict the common respiratory diseases in dogs and cats for which inhalant therapy is prescribed.

PRINCIPLES OF AEROSOL DEPOSITION IN THE LUNGS

The use of aerosol medication has mainly stemmed from the need to manage a variety of respiratory diseases in dogs and cats effectively and with minimal side effects. Many of the more common respiratory diseases require glucocorticoids and bronchodilators, which can have severe and costly side effects when given systemically. ^2,3 In addition, many owners have difficulty administering medications to their cats or dogs, which results in poor owner compliance and inappropriate dosing. Aerosolization may also allow the clinician to effectively manage a disease that may be difficult to treat with systemic medications (e.g., Bordetella bronchiseptica infection).⁴

Aerosol therapy (also known as nebulization) is the production of a liquid particulate suspension within a carrier gas (the aerosol). Many factors determine whether an inhaled medication will have the desired effect in the correct location. The size of the particle must be small enough to travel to the lower airways. Aerosolized particles generally are described by their aerodynamic equivalent diameter (AED). This is defined as the diameter of a sphere with a standard density of 1 g/cm³ that falls in air at the same rate as the particle in question.⁵ For a particle to be deposited in the small bronchioles and alveoli, it must have an AED of 0.5 to 5 μm. Particles larger than 10 μm usually are deposited in the larynx and nasal turbinates (Table 192-1). The AED is a definition that applies only to aerosols that are homogenous in size, which is not typical of most therapeutic aerosols. Because therapeutic aerosols contain a range of particle sizes (termed heterodisperse), the term that is more widely used is mass median diameter (MMD). This is defined as the diameter above which 50% of the particle mass number resides.⁶

Table 192-1 Site of Aerosolized Particle Deposition in the Respiratory Tree

Other important factors that affect the deposition of aerosolized particles in the airways include the rate of gravitational fall (gravitational sedimentation), the tendency of the particles to resist change in airflow speed and direction (inertial impaction), and the inherent random motion of particles created by collision with gas molecules (Brownian diffusion). Inertial impaction occurs when there is a sudden change in the direction of gas flow. This is most common in the nasal turbinates and bronchial bifurcations, so it tends to have the most impact in the upper airways with large particles that are greater than 5 μm AED. Gravitational sedimentation has more of an impact in the lower airways where smaller particles travel. Brownian diffusion is thought to affect only particles smaller than 0.1 μm and is probably not clinically relevant.^1,5,6 It is important to remember that the degree of particle deposition by these mechanisms will also depend on patient variables, such as inspiratory air velocity, tidal volume, and ventilatory pattern.⁵

DELIVERY SYSTEMS

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