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Different approaches have been employed for in vitro assessment of the aerosol particle size generated by inhalation devices. In this study, aerosols from the Omron MicroAir vibrating mesh (VM) nebulizer were measured by cascade impaction (CI) using the MSP Next Generation Pharmaceutical Impactor (NGI), the ThermoAndersen Cascade Impactor (ACI), and by time-of-flight (TOF) analysis with the TSI 3321 Aerodynamic Particle Sizer Spectrometer (APS). The VM nebulizer was evaluated with sodium fluoride (NaF; 2.5%) and with generic albuterol (0.083%). Aerosol particle size (MMAD), respirable fractions (RF < 5 m), and fine particle fractions (FPF < 3.3 μm) were determined with each method at room temperature (RT) and 4°C using 50% average relative humidity. By NGI at either RT or 4°C, aerosol particle sizes were similar for both NaF and albuterol (4.3–4.5 μm MMAD) with 55–61% RF and 27–43% FPF. With ACI, the distribution of particles at RT was similar except at the extremes of the dispersion and the MMAD was smaller (3.3 μm MMAD; p = 0.03). At 4°C, particle sizes determined by ACI results were similar to the NGI (MMAD 4.1 μm; p > 0.05). TOF analysis by APS with albuterol gave significantly larger calculated MMAD (cMMAD) than either CI method (7.2 μm; p < 0.001). TOF measurements of nebulized albuterol at RT and 4°C were equivalent. In summary, the results of VM nebulized NaF and albuterol were more consistent and generally equivalent when determined by NGI (at RT and 4°C) and ACI analysis (at 4°C). In contrast, aerosol particle sizes measured by TOF in the APS at both RT and 4°C were larger than results obtained by CI. Differences in aerosol particle distribution obtained by different analysis methods should be considered while evaluating the in vitro performance of VM nebulizers.