The expansion of nebulizer therapy in the mid to late 20th century for common respiratory diseases has been followed by a focus on use for more specific indications and certain new applications.2 The introduction of metered-dose inhalers (MDIs) in the 1950s allowed portable patient-actuated drug delivery in the home with markedly decreased waste.

Since then, many studies have shown that MDIs have similar clinical efficacy to nebulizers for many pulmonary therapies if used correctly.3?6 In addition, newer devices (such as dry powder inhalers (DPI)) have mitigated some of the variability in drug delivery attributable to patient technique.7 The need for durable medical equipment in the home and hospital has furthered the trend away from nebulizer use; however, several newer indications for nebulizer treatment, both disease- and drug-specific, predict that the need for nebulizer therapy remains. In addition, nebulizers generate continuous and consistent small particle sizes deliverable to the distal lung that are of directly controllable (and, if necessary, large) total quantities.

This concise article provides a summary of the current applications of nebulizer therapy and comparison with alternate drug delivery systems for pulmonary diseases. It also reviews general features of aerosol generation and output by nebulizers?comparison of detailed technical specifications of the range of commercially available nebulizers is beyond the scope of this article. In addition, the discussion will be limited to use of jet nebulizers, despite understanding that ultrasonic nebulizers may have specific compound-/drug-specific applications. Jet nebulizer use is more prevalent, and engineering improvements over the past 15 years have led to their comparable performance with ultrasonic devices.8 Delivery of nebulized agents for systemic non-pulmonary conditions will not be discussed.

Characteristics of Nebulized Aerosols Nebulizers are essentially atomizers with a continuous source of compressed air.8 When passed through a small aperture (venturi)?as a consequence of the Bernoulli principle?high-velocity air produces primary aerosolized particles of a broad range of diameters (1?500µm). A non-obstructive baffle above the jet captures larger particles that condense and return to the reservoir for reatomization; small particles (1?5µm) evade the baffle for inhalation. Conventionally, particle size available for inhalation is expressed as mass median diameter (MMD) or closely related mass median aerodynamic diameter (MMAD), and size range is expressed as geometric standard deviation (GSD) of particle size. The former is the aerosolized particle diameter that divides the total mass of aerosol (versus size distribution).This is more therapeutically relevant as it determines the mass (or dose) of aerosolized drug at or below a certain particle size, rather than the median particle size.

The therapeutic operational characteristics of various aerosol delivery devices are therefore the total amount of drug exiting a device for inhalation and proportion by mass of drug aerosolized in particles of a certain size or less. In general, particles larger than 10µm in diameter deposit primarily proximal to the airways, 5?10µm in the large airways, and 1?5µm in the distal airways and alveoli. Particles smaller than 1µm have poor deposition and are largely exhaled.

While there is a broad range of commercially available nebulizers, most have a MMD of 4?6µm.8 Based on empirical modeling, this results in approximately 30% and 70% of total lung deposition to the central and peripheral airways, respectively.9 This highlights one of two advantages of nebulizer therapy compared with pressurized MDIs (pMDI) and DPI: the absence of variability in MMAD/fine- particle fraction due to patient/MDI timing or inspiratory flow rate. In addition, nebulizers on the whole permit delivery of a larger total dose of drug due to their continuous operation and, in some cases, related to specific drug characteristics, e.g. antibiotics in CF. This is balanced by the inherent increase in Trends in Nebulizer Therapy 1 B USINESS BRIEFING: US RESPIRATORY CARE 2006 Frédéric F Little, MD Martin Joyce-Brady, MD Frédéric F Little, MD, is an Assistant Professor in the Department of Medicine at Boston University.

He attends in the Medical Intensive Care Unit and on the Pulmonary Consultation Service at Boston University Medical Center. His out-patient activity is concentrated on the Adult Asthma Center and Allergy Clinics. Dr Little?s longstanding interest and research efforts are focused on examining the nature of airway inflammation in asthma. He is working with several transgenic mouse lines to investigate factors that regulate and dampen the allergic airway response.

Martin Joyce-Brady, MD, is Associate Professor in the Department of Medicine at Boston University. He attends in the Medical Intensive Care Unit and on the Pulmonary Consultation Service at Boston University Medical Center. He is also the Director of the Pulmonary Care Unit and Respiratory Therapy at Radius Specialty Hospital-Boston.

Dr Joyce-Brady?s bench research interest is the role of glutathione and glutathione metabolism in antioxidant defense during perinatal lung development and injury.

waste from continuous nebulizer therapy. This is particularly relevant for expensive medications and has been partially mitigated by nebulizers that function in a breath-actuated mode.

Nebulizer Use in Specific Pulmonary Disorders Asthma and COPD in Adults Inhalational delivery of drugs for reactive airways disease in adults is a staple of current therapeutic management, whether the reactivity results from a reversible cause such as asthma or a typically irreversible cause such as COPD. The advantages of direct delivery to the airway, including rapid onset of a therapeutic effect, reduced drug dose need, and limitation of systemic side effects,far outweigh those of any enteral or parenteral route of administration. A continual effort is now expended on improving the inhalational drug delivery device and the duration of drug action.7,10 The nebulizer may have a longer history of usage but, in the patient with stable disease, the advantages offered by pMDI or DPI devices in terms of portability, simplicity of components, and independence from a power source are compelling, especially when the objectives include patient mobility and independence.