DDU of Metered-Dose Inhalers (MDIs)

MDI aerosol characteristics are relatively insensitive to changes in air flow rate because the aerosolisation and dispersion mechanisms are dependent on the force generated by the propellant, rather than the patient’s inspiratory effort. Therefore, for MDIs, the test flow rate is fixed at an arbitrary value of 28.3 L/min. A vacuum pump is used to draw air through the assembled test set-up at this flow rate. However, these test conditions are not applied for DDU testing when the MDI is intended for use with an add-on device such as a spacer or valved holding chamber (VHC).