Computational analyses of a pressurized metered dose inhaler and a new drug-aerosol targeting methodology

The popular pressurized metered dose inhaler (pMDI), especially for asthma treatment, has undergone various changes in terms of propellant use and valve design. Most significant are the choice of hydrofluoroalkane-134a (HFA-134a) as a new propellant (rather than chlorofluorocarbon, CFC), a smaller exit nozzle diameter and attachment of a spacer in order to reduce ultimately droplet size and spray inhalation speed, both contributing to higher deposition efficiencies and hence better asthma therapy. Although asthma medicine is rather inexpensive, the specter of systemic side effects triggered by inefficient pMDI performance and the increasing use of such devices as well as new targeted drug-aerosol delivery for various lung and other diseases make detailed performance analyses imperative. For the first time, experimentally validated computational fluid-particle dynamics technique has been applied to simulate airflow, droplet spray transport and aerosol deposition in a pMDI attached to a human upper airway model, considering different device propellants, nozzle diameters, and spacer use. The results indicate that the use of HFA (replacing CFC), smaller valve orifices (0.25 mm instead of 0.5 mm) and spacers (ID = 4.2 cm) leads to best performance mainly because of smaller droplets generated, which penetrate more readily into the bronchial airways. Experimentally validated computer simulations predict that 46.6% of the inhaled droplets may reach the lung for an HFA-pMDI and 23.2% for a CFC-pMDI, both with a nozzle-exit diameter of 0.25 mm. Commonly used inhalers are nondirectional, and at best only regional drug-aerosol deposition can be achieved. However, when inhaling expensive and aggressive medicine, or critical lung areas have to be reached, locally targeted drug-aerosol delivery is imperative. For that reason the underlying principle of a future line of "smart inhalers" is introduced. Specifically, by generating a controlled air-particle stream, most of the inhaled drug aerosols reach predetermined lung sites, which are associated with specific diseases and/or treatments. Using the same human upper airway model, experimentally confirmed computer predictions of controlled particle transport from mouth to generation 3 are provided.