Parameters influenced by the inspiratory flow rate
In order to overcome the intrinsic resistance of dry powder inhalers (DPIs) the patient has to generate a certain inspiratory flow. These flow rates vary depending on the kind of DPI used. About the same amount of respiratory work produces a considerably lower flow through a device with high intrinsic resistance compared to one with low resistance.
The dependency of DPI on inspiratory flow rates involves two aspects which are opposed to one another. A higher intrinsic resistance slows the inspiratory flow resulting in reduced oropharyngeal aerosol deposition. On the other hand pressure drop increases considerably with higher flow rates in devices with high resistance. This may lead to greater dose inconsistency.
Additionally the size of particles generated by a particular device may be also influenced by the inspiratory flow. In devices like the Turbuhaler® a certain value has to be achieved to produce particles small enough to reach the peripheral airways and alveoli. Respiratory work resulting in an inspiratory flow of about 30 l/min through the Turbuhaler® generates particles twice as large as particles from a flow of 40 l/min.
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Weuthen T, Roeder S, Brand P, Müllinger B, Scheuch G. In vitro testing of two formoterol dry powder inhalers at different flow rates. 2002. Journal of Aerosol Medicine; 15 (3): 297-303
PARTICLE DEPOSITION WITH TWO DIFFERENT DPIS IN RELATION TO INSPIRATORY FLOW RATE