Breathing Simulators

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).

The delivered dose testing of nebulisers is carried out to determine the total amount of drug a patient might be expected to receive during a treatment period, rather than through one inhalation. Given the mode of operation of nebulisers, well-defined tidal breathing profiles for specific patient types are specified for testing. The two tests specified in the Pharmacopoeias to characterise delivered dose, Active Substance Delivery Rate and Total Active Substance Delivered, are based on tidal flow conditions generated by a breathing simulator, as opposed to fixed flow rates.