INTRINSIC RESISTANCE

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.

Literature:

Kohler D. Novolizer: the new technology for the management of asthma therapy. 2003. Current Opinion in Pulmonary Medicine; 9 (Suppl. 1): S11-S16

Svartengren K, Lindestad PA, Svartengren M, Philipson K, Bylin G, Canner P. Added external resistance reduces oropharyngeal deposition and increases lung deposition of aerosol particles in asthmatics. 1995. Am J Respir Crit Care Med; 152: 32-37

Voshaar T. Therapie mit Aerosolen. 2005. Uni-Med: Bremen

INCREASED BRONCHOALVEOLAR DEPOSITION BY DEVICES WITH HIGHER RESISTANCE

3213_intrinsic_resistance