Insight into pressure drop dependent efficiencies of dry powder inhalers

The purpose of this study was to assess the effectiveness of three commercial capsule-based dry powder passive inhalers [Rotahaler® (RH), Monodose Inhaler® (MI) and Handihaler® (HH)] in de-agglomerating salbutamol sulphate (SS) and micronized lactose (LH300) powders and their sensitivity to air flow rate changes and air flow resistance.

Aerosolisation was assessed in real-time using a laser diffraction method: this approach was possible as only single-component formulations were tested. Volume percent of the aerosolised particles with diameter less than 5.4 μm at air flow rates from 30 to 180 l min−1 was obtained with the RH, MI and HH and provided a parameter, relative de-agglomeration (RD), as a measure of de-agglomeration. The pressure drops across the device at various flow rates were obtained from a differential pressure meter.

The relationship between RD of SS and LH300 and air flow rate appeared substantially different between the devices. It was surprisingly found that in some cases RD dropped at the highest air flows: this indicates a device specific maxima in RD occurs, and this may in part be attributed to changes in capsule motion. It is proposed that this relationship between RD and pressure drop provides a patient focussed simple way to assess RD performance. This assessment indicated that MI was the most efficient relative de-agglomerator at lower pressure drops, while HH increases its effectiveness at higher pressure drops.

The approach of measuring RD as a function of pressure drop revealed instructive variations in the aerosolisation performances of different devices. This new approach helps compare device performances with different powders, and hence improve optimisation and consistency of performance.