PAEDIATRIC RESPIRATORY REVIEWS (2004) 5(Suppl A), S103–S106 Inhaled corticosteroids: devices and deposition Bruce K. Rubin Professor and Vice Chair for Research, Dept. of Pediatrics; Section Chief, Pediatric Pulmonary Medicine; Professor of Biomedical Engineering, Physiology and Pharmacology, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1081, USA INTRODUCTION Inhaled corticosteroids (ICS) are the most effective anti-inflammatory drugs used today for the preven- tion and treatment of asthma in children. Recent advances in aerosol delivery including improved formulations of ICS and newer devices present the clinician with an increasing variety of choices when trying to decide what form of ICS therapy is best suited to an individual patient. In this paper I will first briefly review the physics and physiology of aerosol generation and deposition followed by a discussion of the potential advantages and disadvantages of devices currently used. AEROSOL PHYSICS An aerosol is a group of particles that remains suspended in air because of a low terminal settling velocity. The terminal settling velocity of an aerosol can be described as a function of aerosol size and density; the mass median aerodynamic diameter (MMAD) equal to the particle diameter multiplied by the square root of particle density. The MMAD normalises particle size to the behaviour of a spherical droplet of water which by definition has a density of 1. Because therapeutic aerosols are never of uniform diameter, shape, or density, MMAD is generally determined by physical methods which can include laser light scattering, time of flight, or the settling behaviour of aerosol particles on a series of stages graded for size in either a multi-stage liquid impinger or in a cascade (Anderson) impactor. The three major mechanisms of aerosol deposition are inertial impaction, gravitational sedimentation, *Tel.: +1-(336)-716-0262; Fax: +1-(336)-716-9229; E-mail: brubin@wfubmc.edu and diffusion. Inertial impaction is the primary mechanism for deposition of particles greater than 4 mm MMAD. Large particles will fail to be suspended for appreciable periods of time and are more likely to settle within the device or on the patient’s face or oral pharynx. Furthermore, nose breathing will filter aerosol from inspired gas. Impaction is also markedly increased by high inspiratory flow as in children who are distressed or crying. Gravitational settlement describes the effects of gravity on particles not influenced by inertia. This is the primary deposition mechanism for particles less that 2 mm MMAD and can affect larger particles under low flow conditions. Diffusion affects extremely small particles where Brownian motion has a greater influence on particle movement than gravity. This results in coalescence of particles within the airway particularly when they are within a distance from the airway wall of less than 25 times the particle diameter. Another important concept in understanding par- ticle deposition in the lower airway is that of particle size distribution. The MMAD will be the same if the majority of particles are all of similar size or if there is a huge size disparity of particles with a large number of extremely small and extremely large particles but a mass median diameter that is the same. Clearly it is advantageous for a greater proportion of the particles to be in the respirable fraction (MMAD 0.5-5 mm). Size dispersion is measured as the geometric standard deviation (GSD). By definition, a monodisperse aerosol has a GSD less than 1.22. Nearly all therapeutic aerosols can be considered heterodisperse. Particle composition will also influence the mass of drug delivered to the airway in that particles 1526-0542/$ – see front matter © 2004 Elsevier Science Ltd. All rights reserved.