Dry powder inhalers have been developed to generate a dry aerosol without using propellants. Most common classification of these devices refers to the kind of drug storage used. Some systems employ single drug capsules, which require refilling after every use, some devices contain several solely blistered single doses and others enclose a collective drug reservoir, from which single doses are metered before inhalation. Some of theses devices may be refilled others not.
Systems can also be characterised as low, medium or high resistance devices. Due to the unfavourable flow characteristics of very small breathable particles the powder in DPIs needs a coarse grained adjuvant – often lactose. During inhalation agglomerated particles must be dispersed, which is dependent on the inspiratory flow generated by the patient. Inspiratory flow rates also influence the amount of dose released with a higher variability at low rates.
For adequately high flow rates patients therefore have to inhale deeply and forcefully beginning at the start of inspiration. However it has been demonstrated that most patients with obstructive lung diseases including children are able to generate sufficient flow rates through dry powder inhalers. In general a flow rate of 60 l/min or more leads to adequate bronchoalveolar particle deposition in most DPIs.
Besides abdication of propellants in dry powder inhalers they generally are breath-actuated making their correct use easier for the patient. Numerous studies have shown that patients using breath-actuated devices make significantly less errors regarding correct inhalation manoeuvre.
Vogelberg C, Kremer HJ, Ellers-Lenz B, Engel M, Maus J, Conrad F, Hermann R. Clinical evaluation of the peak inspiratory flow generated by asthmatic children through the Novolizer. 2004. Respiratory Medicine; 98: 924-931 Voshaar T. Therapie mit Aerosolen. 2005. Uni-Med: Breme
Course of particles using a dry powder inhaler (DPI)
-video to be updated-
This animated model shows the aerosol flow generated by a dry powder inhaler into the upper respiratory tract.