Ultrasonic Nebulizers-Yesterday, Today, and Tomorrow
Up to 10 percent of US residents use some form of aerosol medication, with most using a pressurized metered-dose inhaler (MDI).1 With the phasing out of chlorofluorocarbon-based MDIs, there has been a tremendous industry effort to provide alternative forms of aerosol therapy that are as effective, convenient, and inexpensive as the MDI. Although the pharmaceutical industry is investing a large amount of resources into the dry powder inhaler (DPI), a surprisingly robust entry in the competition to meet patients’ aerosol needs appears to be the ultrasonic nebulizer.
Most RCPs who have practiced for more than 15 years probably think of ultrasonic nebulizers as large, industrial-strength units composed of multiple components. These were wheeled on stands to the patient’s bedside or included as part of the humidification system in a ventilator circuit. These devices developed a reputation for being expensive and somewhat inconsistent in operation, requiring considerable practitioner intervention to ensure proper operation. Many facilities have limited these ultrasonic nebulizers to use for sputum induction.
The past year, however, has seen the introduction of several new ultrasonic nebulizers oriented toward use both at home and in the intensive care unit. As new uses for this technology develop, it is important for RCPs to understand its operating principles and the technical considerations necessary for device selection.
PRINCIPLES OF OPERATION
The ultrasonic nebulizer uses a piezo-electric crystal that vibrates at a high
frequency (greater than 1 MHz) to create an aerosol. The crystal transducer, composed of substances such as quartz-barium titanate, converts electricity into sound. The beam of sound is focused in the liquid above the transducer, creating waves. When the frequency is high enough, and the amplitude of the signal is strong enough, the waves crest, creating a geyser of droplets at the surface of the liquid.
Most large-volume ultrasonic nebulizers use a water bath as a couplant between the transducer and a cup (usually, a disposable plastic one) containing medication. The medication cup, which has a flexible diaphragm on the bottom, is seated in the couplant chamber and filled with enough water to allow a firm water seal between transducer and cup bottom. This water conducts sound energy from the transducer to the diaphragm; in turn, this vibrates the medication to produce an aerosol. These units tend to have rather high maintenance needs, with the couplant requiring frequent changes and the unit needing regular cleanings to minimize the buildup of any substances on the transducer.
Ultrasonic nebulizers tend to have higher outputs of solution (0.5 to 7 mL/min) and higher mist densities than conventional jet nebulizers. The particle or droplet size, expressed as the mass median aerodynamic diameter (MMAD), delivered by an ultrasonic nebulizer is related to the frequency at which the crystal vibrates. The frequency is usually set by the manufacturer for each individual model of nebulizer and is rarely adjustable by the user. The particle size is inversely proportional to the frequency. For example, one ultrasonic nebulizer that operates at a frequency of 2.25 MHz produces an MMAD of 2.5 mm, while a similar unit operated at 1.25 MHz produces a much less respirable, larger MMAD of 6 mm. The greater the amplitude, the greater the output from the nebulizer, up to the limit imposed by a device’s design (Table 1, page 100). Increases in amplitude beyond the specific upper limit of performance will not improve a device’s output. It should be noted that not all ultrasonic nebulizers produce aerosols that are primarily respirable (smaller than 5 mm).
Particle size and aerosol density are also affected by the source and flow of the gas that conducts the aerosols from the nebulizer to the patient. If the nebulizer is producing a steady output of particles, the greater the flow of gas through the chamber, the more diluted the same number of particles will be in the larger volume of gas. The faster the flow of gas, the greater the chance that large particles will be driven out of the nebulizer before they can coalesce with other particles and settle out of the stream. Low flow rates are associated with smaller particles and a higher density of mist. High flow rates yield larger particles and less density, but a greater direct output from the nebulizer. The larger commercial ultrasonic nebulizers use low-flow blowers to deliver air or other compressed gases through a flowmeter. A blender can be added to the delivery system to control the delivered gas concentration more precisely. Aerosol tubing, a mask, or a mouthpiece can be used to administer the ultrasonically nebulized solution.
Jet nebulizers cool during use, but the temperature of the solution placed in an ultrasonic nebulizer increases during use as a by-product of the energy required to vibrate the piezoelectric crystal. As the temperature increases, the drug may change in consistency or chemical composition, sometimes altering its effect on the patient.
Smaller ultrasonic nebulizers have been designed for individual-patient use. Often, they do not use water-filled couplants between the transducer and the medication, which is placed in the manifold in direct contact with the transducer. The transducer is connected to a power source (often, to increase portability, a battery). Small nebulizers, which incorporate the transducer manifold at the patient’s airway, usually rely on the patient’s inspiratory flow rate to draw the aerosol from the nebulizer to the lung. Consequently, patients should be encouraged to breathe with low to medium inspiratory flow rates for optimal delivery of medication to the lower respiratory tract.
The primary use for the ultrasonic nebulizer in most acute care facilities is sputum induction. Small-volume ultrasonic nebulizers have also been promoted for bronchodilator therapy in nursing homes or other extended care facilities, as an alternative to pneumatically driven small-volume nebulizers (SVNs). The small-volume ultrasonic nebulizer may offer an advantage because it has less dead space than an SVN; this reduces the need for a large quantity of diluent to ensure the delivery of drugs, although this has not been well documented. The self-contained portable power source of some units also adds a great deal of convenience for mobility. Ultrasonic nebulizers have been used to administer undiluted bronchodilators to patients with severe bronchospasm.2 Because the nebulizers have minimal dead space, the treatment time is shortened. Use of undiluted bronchodilators is not new and is typically included in the manufacturer’s product dosing information in the Physicians’ Desk Reference.
The advantages of the ultrasonic devices can be outweighed by their primary limitation: their high cost, which (at $150 to more than $1,000) may be up to 10 times that of a pneumatic nebulizer or 100 times that of an MDI or DPI. Because the ultrasonic nebulizer manifold is expensive, compared with a disposable jet nebulizer, some practitioners have suggested using a technique that employs a one-way valve between the medication chamber and mouthpiece (so that multiple patients can be treated consecutively without concern of infection). It has yet to be confirmed, however, that a simple one-way valve manifold is adequate protection against contamination of the medication chamber. In addition, contact with infectious secretions on the outside of the nebulizer manifold could result in transmission of pathogens from one patient to another.
The use of ultrasonic nebulizers is associated with a number of potential complications. These include overhydration, bronchospasm, infection, and disruption of the structure of the administered medication.3,4 Overhydration can occur because of a large fluid output from the nebulizer and its potential to deliver small particles directly to the lung parenchyma. Overhydration is of greatest concern after the prolonged treatment of neonates, small children, and other patients with fluid and electrolyte imbalances. Pulmonary secretions can also swell after treatment using an ultrasonic nebulizer.5
Bronchospasm can occur after ultrasonic nebulizer treatment. The delivery of cold, high-density aerosols has been associated with increased airway resistance and irritability in a number of patients.5 In addition, the sterile water administered through an ultrasonic nebulizer is known to be more irritating than normal saline.6
Medications administered by ultrasonic nebulizer can become more concentrated during treatment. This occurs because of the heat generated by the piezoelectric transducer and can result in a situation in which the solvent evaporates at a rate faster than that at which the drug evaporates, leaving the active drug in solution, at a stronger concentration. Nebulizers with acoustic outputs of more than 50 W/cm2 cause changes in the structure of aerosolized medications.6 If the power output of the nebulizer is 50 W/cm2 or less and the aerosol output is less than 2 mL/min, it is reportedly safe to use to deliver medications.7
One novel small-volume ultrasonic nebulizer uses a metering device to inject a small amount of undiluted medication into the nebulizer, which nebulizes the dose over a 60-second period. This handheld, battery-operated device appears to meet the criteria of efficacy and convenience, with the ability to nebulize virtually any solution that it may be safe to inhale. The small, precise dose reduces waste (an important factor for users of expensive medications). The short time needed for nebulization not only reduces treatment time, but decreases the time during which the transducer could create medication-altering heat.
Small-volume ultrasonic nebulizers recently have been developed for use in the administration of aerosols during mechanical ventilation. Pneumatic and jet SVNs have been shown to be inefficient in delivering medication to the patient during mechanical ventilation8 and have the disadvantage of adding a considerable additional flow of gas into the circuit, which can affect both lung volumes and airway pressures. Early reports9 indicate that ultrasonic nebulizers deliver more drug to the lower respiratory tract (in vitro) than SVNs, with virtually no alteration in ventilator parameters or alarms during administration. The greatest problem noted for these nebulizers is their weight, which impairs their ability to be placed in the ventilator circuit without placing too much weight on the patient’s airway. N
James B. Fink, MS, RCP, RRT, is a program analyst, respiratory care, Edward Hines Jr Veterans Administration Hospital, Hines, Ill.
1. The National Lung Health Education Program. Strategies in preserving lung health and preventing COPD and associated diseases. Chest. 1997;113:123S-163S.
2. Ballard RD, Bogin RM, Pak J. Assessment of bronchodilator response to a b-adrenergic delivered from an ultrasonic nebulizer. Chest. 1991:100:410-415.
3. Chatburn RL, Lough MD, Klinger JD. An in-hospital evaluation of the sonic mist ultrasonic room humidifier. Respiratory Care. 1984;29:893-899.
4. Doershuk CF, Mathews LW, Gillespie CT, Lough MD, Spector S. Evaluation of jet type and ultrasonic nebulizers in mist tent therapy for cystic fibrosis. Pediatrics. 1968;41:723-732.
5. Phillips GD, Millard FJL. The therapeutic use of ultrasonic nebulizers in acute asthma. Respir Med. 1994;88:387-389.
6. Boucher RGM, Kreuter J. Fundamentals of the ultrasonic atomization of medicated solutions. Ann Allergy. 1968;26:59-63.
7. Lewis RA, Ellis CJ, Fleming JS, Balachandran W. Ultrasonic and jet nebulizers: differences in the physical properties and fractional deposition on the airway responses to nebulized water and saline aerosols [abstract]. Thorax. 1984;39:712.
8. Fink JB. Placement of an ultrasonic nebulizer at the ventilator circuit does not reduce aerosol delivery during CMV. Respiratory Care. 1998. In press.
9. McPeck M, LeBlanc DS, Smaldone GC. Ultrasonic nebulization with a contemporary flow-triggered ventilator. Respiratory Care. 199;42:1059.