Dry powder inhalers and the right things to remember: a concept review

Dry powder inhalers (DPIs) are widely and increasingly used in clinical practice because they represent a substantial advancement in inhalation technology. The effectiveness of a powdered drug to inhale depends on the inspiratory flow rate generated by the patient and on the turbulence produced by the intrinsic resistance of the DPI. While the inspiratory flow is variable with the patient's ability and conditions, the turbulence is differently sized within each device because depending of its technical design. There are higher - medium-, and low-resistance devices. With low-resistance DPIs, the disaggregation and the microdispersion of the drug highly depend on the patient's inhalation airflow rate, because the role of the resistance-induced turbulence is obviously negligible in these cases. This flow-rate dependency is minimized in the presence of a sufficient regimen of turbulence as in the case of medium-resistance DPIs. Both the disaggregation and the micro-dispersion of the powdered drug are optimized in these circumstances even in the absence of a maximal inspiratory flow rate. The low resistance DPIs should not be regarded as the best performer DPIs because their intrinsic low-resistance regimen requires a higher inspiratory airflow rate and effort, which frequently cannot be achieved by subjects suffering from a disease-induced airflow limitation. Only when the ratio between the inhalation flow rate and the DPI intrinsic resistance is balanced, the speed of the particulate, the distribution of the drug within the lung, and the variability of the effective inhaled dose are optimized.