Effect of Tubing Deposition, Breathing Pattern, and Temperature on Aerosol Mass Distribution Measured by Cascade Impactor

Aerosols produced by nebulizers are often characterized on the bench using cascade impactors. We studied the effects of connecting tubing, breathing pattern, and temperature on mass-weighted aerodynamic particle size aerosol distributions (APSD) measured by cascade impaction. Our experimental setup consisted of a piston ventilator, low-flow (1.0 L/min) cascade impactor, two commercially available nebulizers that produced large and small particles, and two "T"-shaped tubes called "Tconnector_cascade" and "Tconnector_nebulizer" placed above the impactor and the nebulizer, respectively. Radiolabeled normal saline was nebulized using an airtank at 50 PSIG; APSD, mass balance, and Tconnector_cascade deposition were measured with a gamma camera and radioisotope calibrator. Flow through the circuit was defined by the air tank (standing cloud, 10 L/min) with or without a piston pump, which superimposed a sinusoidal flow on the flow from the air tank (tidal volume and frequency of breathing). Experiments were performed at room temperature and in a cooled environment. With increasing tidal volume and frequency, smaller particles entered the cascade impactor (decreasing MMAD; e.g., Misty-Neb, 4.2 ± 0.9 μm at lowest ventilation and 2.7 ± 0.1 μm at highest, p = 0.042). These effects were reduced in magnitude for the nebulizer that produced smaller particles (AeroTech II, MMAD 1.8 ± 0.1 to 1.3 ± 0.1 μm; p = 0.0044). Deposition on Tconnector_cascade increased with ventilation but was independent of cascade impactor flow. Imaging of the Tconnector_cascade revealed a pattern of deposition unaffected by cascade impactor flow. These measurements suggest that changes in MMAD with ventilation were not artifacts of tubing deposition in the Tconnector_cascade. At lower temperatures, APSD distributions were more polydisperse. Our data suggest that, during patient inhalation, changes in particle distribution occur that are related to conditions in the tubing and may reduce the diameters of particles entering the patient. This effect is more significant for nebulizers producing large particles. Changes in ambient temperature did not affect these observations.