November 2002 / Volume 47 / Number 11 / Page 1257
Design Principles of Liquid Nebulization Devices Currently in Use
IntroductionLiquid nebulization is a common method of medical aerosol generation. Nebulizers are of 2 types: jet (or pneumatic) small-volume nebulizer, and ultrasonic nebulizer. Jet nebulizers are based on the venturi principle, whereas ultrasonic nebulizers use the converse piezoelectric effect to convert alternating current to high-frequency acoustic energy. Important variables for both types of nebulizer are treatment time required, particle size produced, and aerosol drug output. There are several advantages to jet nebulization, including that effective use requires only simple, tidal breathing, and that dose modification and dose compounding are possible. Disadvantages include the length of treatment time and equipment size. Design modifications to the constant-output nebulizer have resulted in breath-enhanced, open-vent nebulizers such as the Pari LC Plus and the dosimetric AeroEclipse. Ultrasonic nebulizers generally have a higher output rate than jet nebulizers, but a larger average particle size. Ultrasonic nebulizers can also substantially increase reservoir solution temperature, the opposite of jet nebulizer cooling. Drug concentration in the reservoir does not increase with ultrasonic nebulization, as it does with jet nebulization. Ultrasonic nebulizers have the same advantages as jet nebulizers. Ultrasonic nebulizers are more expensive and fragile than jet nebulizers, may cause drug degradation, and do not nebulize suspensions well. Neither type of nebulizer meets the criteria for an ideal inhaler: efficient and quick dose delivery with reproducibility, cost-effectiveness, and no ambient contamination by lost aerosol.
Principle of Operation
Variables of Nebulizer Performance
Advantages and Disadvantages of Jet Nebulizers
Design Variations in Traditional Jet Nebulizers
Description and Basic Function
Physics of Ultrasonic Nebulization
Theory of Aerosol Production
Comparison of Ultrasonic and Jet Nebulizers
Advantages and Disadvantages of Ultrasonic Nebulizers
The term "nebulizer" derives from the Latin "nebula," meaning "mist," and reportedly was first used in 1872, followed by an 1874 definition as "an instrument for converting a liquid into a fine spray, especially for medical purposes." The appealing logic of creating a vapor or aerosol for the inhalation treatment of lung disease is at least as old as written records of medicine. The Ayurvedic tradition of medicine in India, which dates back perhaps 4,000 years or more, used inhaled substances for managing asthma. Although inhalation devices were described in the 19th century, the modern precursors of small-volume nebulizer devices appeared with the glass and hand-bulb "atomizers" introduced for asthma treatment in the 1930s, such as the DeVilbiss No. 40 glass nebulizer. The Collison nebulizer became available in the late 1940s; it used a baffle to filter out large particles, thus distinguishing a "nebulizer" from an "atomizer." The Wright nebulizer, which appeared in the 1950s, was engineered from ebonite and perspex; it was much more compact than the Collison and more closely resembled today's pneumatically powered nebulizers. A different method of creating liquid aerosols, the ultrasonic nebulizer, was introduced in the 1960s. It relies on high-frequency sound waves to aerosolize the solution. Today the term "nebulizer," as used clinically in respiratory care, encompasses both gas-powered jet nebulizers and ultrasonic nebulizers. This review considers the physical principles, designs, advantages, disadvantages, and factors affecting performance of jet and ultrasonic nebulizers that are in current clinical use.