Mandy L. Cha, BS, BVSca , Lais R.R. Costa, MV, MS, PhDb, *
INTRODUCTION
Inhalation therapy has been practiced in humans since ancient times in many cultures, including Chinese, Indian, Greek, Egyptian, Roman, and Hebrew. In all of these cultures, inhalational therapy was performed using condensation of vapor by steaming liquids and combustion of plants or their derivatives, creating a smoke containing aerosolized droplets and solid particles.
Inhalation therapy is the administration of aerosols into the airway and is a means of delivering topical pulmonary therapy. An aerosol is a suspension of liquid or solid particles dispersed in gas. The physical characteristics of the particles, including size, hydrophobicity, and shape, affect their ability to travel within the airways. The size of the particles is regarded as one of the most critical physical characteristics of aerosol therapy. Fig. 1 depicts a schematic representation of the aerosol deposition throughout the airways of the horse. Large aerosols (>10 mm) are filtered out in the upper respiratory tract (URT) or deposited in larger airways in association with turbulent airflow and do not effectively reach the lower airways. Midsized particles (10 to 6 mm) deposit in the larynx, trachea, bronchi, and large-caliber bronchioles. Particles 5 mm or less are deposited in smaller diameter bronchioles and in alveolar acini by gravitational sedimentation. Very small particles (<1 mm) tend to remain in suspension, and approximately 50% are deposited in alveoli and 50% are exhaled. Small particles may coalesce to make larger particles, which affects deposition. Hydrophilic particles attract water, promoting their deposition deeper in the tracheobronchial tree. More aerodynamically shaped particles are also deposited deeper within the respiratory tract. Patient factors that influence aerosol particle deposition include the depth of breathing, airway patency and reactivity, bronchospasm, and coughing.
Inhalation medications may be in the form of solutions, powders, vapors, or pressurized cartridges, when solutions or powders are administered with a propellant. The onset of action of aerosolized drugs is relatively rapid; however, the effects are usually short-lived; this is because the aerosolized drugs are partially degraded in the lung, cleared from the respiratory tract by the mucociliary escalator, and absorbed into the blood stream where they are disposed of by breakdown and excretion as are systemic drugs.
