Background: Cooling the Next Generation Impactor (NGI) is recommended to minimize evaporation due to heat transfer from impactor to aerosols when evaluating nebulized solutions. This methodology increases testing time for serial testing procedures. We hypothesize that after an initial prolonged cooling time, experiments could be repeated after shorter recooling times without sacrificing accuracy.
Methods: Three units of continuous output (HUDSON) and breath enhanced (PARI LC Plus) nebulizers were operated (6 L/min) with albuterol solution (2.5 mg/3 mL) into a cooled (4°C) NGI (internal and external filters) calibrated at 15 L/min. Mass median aerodynamic diameter (MMAD), geometric standard deviation (GSD), % particles <5 μm (P%<5), and % particles 1-3 μm (P%1-3) were compared with three different protocols. Initial cooling of the NGI (90 min for all protocols) was followed by two measurements with recooling intervals of either 90 and 90 (protocol A), 60 and 60 (protocol B), or 30 and 30 min (protocol C). Albuterol was diluted and measured by spectrophotometry (276 nm).
Results: MMAD, GSD, P%<5, and P%1-3 for first measurements of all protocols (n = 9) were: 3.47 ± 0.21 μm, 2.31 ± 0.07, 67.3 ± 2.6%, and 40 ± 2.3% (PARI) and 4.56 ± 0.35 μm, 2.16 ± 0.08, 54 ± 3.7%, and 22.4 ± 2.8% (HUDSON). No differences were found between cooling protocols (p > 0.05). Percentage of variation from first measurement ranged from: -3.9 to +2.1% (PARI) and -4.1 to +2.9% (HUDSON) for MMAD; -5.6 to +2.6% (PARI) and -4.9 to +1.9% (HUDSON) for GSD; 0 to +4.6% (PARI) and -3.7% to +5.7% (HUDSON) for P%<5; and -2.4 to +5.2% (PARI) and -1.8 to +4.9% (HUDSON) for P%1-3.
Conclusions: Aerosol characteristics of nebulized solutions determined by NGI are not affected by performing two repeat measurements after recooling the impactor for either 30 or 60 min after an initial 90-min time.