Nebulizers

Nebulizer Generation. Clinicians reviewing articles about aerosol delivery must be sufficiently familiar with all aspects of the process to evaluate laboratory findings. Evaluation of nebulizer performance is a multifaceted process. Newer nuclear scanning methods may improve measurements of regional deposition [67]. More meaningful are clinical studies of efficacy measured either by (3) airflow rates or (4) by long-term patient performance studies. One uncontrolled study reports similar survival over five years in COPD patients using MDIs and nebulizers [68], but controlled studies are needed. Similar studies with better controls would be the most meaningful for the practicing physician, but they are rarely done for aerosol medications, especially bronchodilators.

There is great variability in the output of nebulizers, with lung deposition of older devices reported in the 5-15% range, sometimes lower. Newer devices have promise of better performance. Recent study of a new liquid aerosol generator documented that 53% of the loaded dose reached the lung [69]. Variability of performance using nebulizers has been described [70,71]. Using phantom lung and tissue attenuation methods to measure deposition of 99mTc-labeled colloidal albumin aerosol, deposition was 4.3 and 6.1%, respectively, for the two methods [68]. Other data show much intersubject and intersubject variability, both with same-day and different-day testing with regard to lung [70-72].

"Generic" Products. Low-cost devices are marketed but can perform much worse with respect to intrapulmonary deposition than more expensive devices [72]. Common nebulizers are ubiquitous, as are compressors to operate them. Aerosol medications are marketed with recommendations for use with specific nebulizers and compressors for optimal results [73]. Nebulizers are commonly used for many weeks to months, even those devices described as "disposable." When disposable devices are washed using saline, they maintain size distribution and output for at least 100 uses but, when not so cleaned, begin to deteriorate after 40 uses [74].

New Devices and Materials. A couple of newer technologies merit brief comment. Pulse nebulization is a technique of pressurizing nebulizers intermittently for 50-800 ms. Evidence indicates that pulse delivery can improve particle delivery [75]. Such devices can deliver quite consistent doses [76].

Porous microsphere technology is another area with considerable potential. Porous microspheres have a large physical diameter but, because they are not solid, have a relatively low aerodynamic diameter, which is the physical property that determines deposition fraction and site. They also have a very large surface area for carrying therapeutic substances. Other potential advantages of these preparations may be extended release capability, better size control, and less susceptibility to hygroscopic size changes. Tests of such spheres using cromolyn sodium, albuterol sulfate, and formoterol fumerate appear to show improved stability, content uniformity, and efficiency of use [77]. Delivery could be via MDIs, but other delivery approaches could be explored.

MDIs vs. Nebulizers. Total delivered dose among devices is further emphasized when comparing MDIs and nebulizers. The clinician is regularly faced with patients who swear by the efficacy of nebulizers over MDIs. Deposition fraction is low for both devices, about 5% in children younger than 4 years and about 10% in children over 4 years old for each type of device. The delivered dose was approximately fivefold higher for the nebulizer, so the mass in the lung was much greater for that device [78]. Most studies comparing MDI to nebulizer bronchodilator efficacy show similar improvement in forced expiratory volume one (FEV1), and better results with MDIs have been demonstrated in some laboratories [79]. Most such studies compare patient expiratory flowrates. For bronchodilators at least, pharmacodynamic studies reported by many authors for several decades have regularly indicated equivalent efficacy.

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