Abstract
BackgroundCurrent pressurised metered dose asthma inhaler (pMDI) propellants are not inert pharmacologically as were previous chlorofluorocarbons, have smooth muscle relaxant‚ partial pressure effects in the lungs and inhaled hydrofluoroalkane 134a (norflurane) has anaesthetic effects. Volumes of propellant gas per actuation have never been measured.MethodsIn-vitro studies measured gas volumes produced by pMDIs on air oxygen (O2) levels in valved holding chambers (VHC) and the falls in O2% following actuation into lung ventilator delivery devices.ResultsVolumes of propellant gas hydrofluoroalkane (HFA) 134a and 227ea and redundant chlorofluorocarbons (CFC) varied from 7 ml per actuation from a small salbutamol HFA inhaler to 16 ml from the larger. Similar-sized CFC pMDI volumes were 15.6 and 20.4 ml. Each HFA salbutamol inhaler has 220 full volume discharges; total volume of gas from a small 134a pMDI was 1640 ml, and large 3885 ml. Sensing the presence of liquid propellant by shaking was felt at the 220th discharge in both large and small inhalers. Because of a partial pressure effect, VHC O2% in air was reduced to 11% in the smallest 127 ml volume VHC following 10 actuations of a large 134a salbutamol inhaler. The four ventilator delivery devices studied lowered 100% oxygen levels to a range of 93 to 81% after five actuations, depending on the device and type of pMDI used.ConclusionPressurised inhaler propellants require further study to assess smooth muscle relaxing properties.
Highlights
IntroductionCurrent pressurised metered dose asthma inhaler (pMDI) propellants are not inert pharmacologically as were previous chlorofluorocarbons, have smooth muscle relaxant‚ partial pressure effects in the lungs and inhaled hydrofluoroalkane 134a (norflurane) has anaesthetic effects
Current pressurised metered dose asthma inhaler propellants are not inert pharmacologically as were previous chlorofluorocarbons, have smooth muscle relaxant‚ partial pressure effects in the lungs and inhaled hydrofluoroalkane 134a has anaesthetic effects
This review looks at the performance of pressurised metered dose inhalers in vitro and discusses toxicology studies performed to assess propellant safety and publications comparing CFC and HFA inhalers
Summary
Current pressurised metered dose asthma inhaler (pMDI) propellants are not inert pharmacologically as were previous chlorofluorocarbons, have smooth muscle relaxant‚ partial pressure effects in the lungs and inhaled hydrofluoroalkane 134a (norflurane) has anaesthetic effects. Inhaled therapy delivery devices for asthma and chronic lung disease such as oxygen driven jet nebulisers, pressurised metered dose inhalers (pMDI), and valved holding chambers (VHC) have little evidence or research to support their efficacy [1, 2]; historical use determines current practice. The physical and pharmacological properties of the propellant and drugs in pMDIs gives a scientific explanation and reason for the increasing use of pMDIs and VHCs to manage acute severe asthma in place of evidence-baseless jet nebulisation of beta2-agonists in saline. Upper respiratory tract and oral absorption play an unknown-percentage part in the delivery of inhaled drugs to target organs.
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