Particle deposition and intrinsic resistance of dry powder inhalers
In contrast to pressurized metered dose inhalers (pMDI) patients using a dry powder inhaler (DPI) must achieve a certain inspiratory flow rate to generate a breathable aerosol from the drug reservoir. Since people with obstructive lung diseases have to overcome a higher airway resistance during inspiration it could be assumed that devices with low intrinsic resistance may be easier to use and therefore be more effective for these patients.
However it has been shown that a higher intrinsic resistance of a DPI reduces oropharyngeal deposition of the particles. When velocity of airflow decreases due to an inhaler with high resistance impaction of particles in larger airways is diminished.
Intrinsic resistance varies considerably between different inhalers. In devices with high intrinsic resistance – e.g. Easyhaler® or Turbuhaler® – the pressure drop occurring within the device during inhalation rises extensively with increasing airflow rates. Since aerosol generation in DPI depends on the inspiratory flow rate this may result in dosing inconsistency. In devices with medium or low intrinsic resistance – e.g. Novolizer® or Diskus® – only a smooth increase of pressure drop occurs at higher flow rates.
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