Aerodynamic characteristics of a dry powder inhaler at low inhalation flows using a mixing inlet with an Andersen Cascade Impactor

The aerodynamic characteristics of the dose emitted from a dry powder inhaler (DPI) are inhalation flow dependent but have not been determined for low flows. We have designed novel methodology to measure these at <28.3 l min−1. The original Andersen Cascade Impactor (ACI) designed for use at 60 l min−1 was adapted to include a mixing inlet (MIXINLET) which allows inhalation flows through the DPI from 5 to 60 l min−1. The mean fine particle dose (FPD) from a formoterol Turbuhaler using the MIXINLET method at 10, 20, 28.3, 40 and 60 l min−1 was 0.55, 1.39, 1.80, 2.88 and 5.86 μg and the mass median aerodynamic diameter (MMAD) was 6.6, 6.0, 5.4, 5.1 and 2.8 μm. Similarly, the FPD using the ACI method was 0.13, 0.69, 1.50, 2.48 and 5.42 μg and MMADs were 12.2, 7.4, 5.5, 4.8 and 2.7 μm. The accuracy of the original ACI <28.3 l min−1 is unknown. The ACI with the mixing inlet allows the determination of the in vitro dose emission properties of DPIs at flows <28.3 l min−1 whilst maintaining a constant flow through the ACI. This methodology, therefore, can help to focus attention to the lowest inhalation flow required for a DPI.