Metered Dose Inhalers (MDI)
A metered dose inhaler (MDI) is most often a drug solution or suspension formulation and propellant. Designed to deliver a precise measured dose of medication under pressure, the devices have a long reliable history. The required test procedure measures aerodynamic particle size distribution of the drug leaving the device and can define the manner in which aerosols deposit in the respiratory tract during inhalation. Aerosol characterisation is often used in judging metered dose inhaler performance and is particularly relevant in the development of inhalation formulations during research, production, quality assurance and equivalency testing. The results of characterisations using multi-stage cascade impaction techniques are used for fine particle fraction or fine particle dose determination which may be correlated to the lung penetrating dose by the drug during inhalation. Other instruments collect and measure doses per container of a metered dose inhaler. Specified test applications described meet both the pharmaceutical industry and its legislators.