Figure 4 Essential components of a metered-dose inhaler.
may be included to ensure that the drug is well suspended and to help lubricate the metering valve.
The metering valve is the key to measuring and presenting a consistent and accurate dose to the patient; it is made up of a number of precision-made plastic or metal components. The valve is crimped onto a canister, which is ordinarily made of aluminum. Finally, there is the actuator, which holds the canister and through which the patient inhales the dose.
Marketed MDIs usually contain either CFCs or, more recently, HFCs as propellants. CFC-containing MDIs contain CFC-12 and CFC-11 and sometimes CFC-114. HFCs 134a and 227 have been developed as replacements for CFCs and are used in newer MDIs. In addition, some preliminary work has been conducted with MDIs using hydrocarbons as propellants.
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