The effect of unsteady flow rate increase on in vitro mouth–throat deposition of inhaled boluses

A procedure for measuring the effects of flow increase rate (FIR) on mouth–throat and downstream filter (“lung”) deposition was developed. A respiratory aerosol probe (RAP) was used for generation and delivery of small aerosol boluses. This device employs computer controlled, fast opening and closing valves, which allow aerosol injection at a predetermined time during inhalation. Flow was driven by a pulmonary waveform generator (PWG) breathing machine. Monodisperse, 5μm diameter dioctylphthalate (DOP) particles were used. The following experimental flow conditions were tested: bolus injection into a steady-state flow of 30 l/min, and bolus injection into unsteady flow accelerating through 30 l/min at either 2 or 4l/s2. In addition, numerical simulation of the effects of FIR on mouth–throat deposition of aerosol boluses was performed.Our results show that unsteady flow resulted in enhanced mouth–throat deposition. This difference can be explained by the higher velocity reached by the particles at their primary deposition site within the mouth–throat due to the accelerating flow rate (which reaches approximately 40 l/min by the time the particles deposit in the area of the larynx). This hypothesis is supported by data with bolus injection into a steady flow rate of 40 l/min, which yields mouth–throat deposition that does not differ significantly from the unsteady case.