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Conventional pMDI
As seen in Figure 15, the pMDI consists of a canister, the medication, the
propellant/excipient, a metering valve, the mouthpiece, and actuator. 42 The medication represents
only 1–2% of the mixture emitted from the pMDI and is either suspended or dissolved
in the propellant/excipient mixture. The propellant of the pMDI makes up 80% of
the mixture. A surface-active agent, such as surfactant, is occasionally used in order to maintain
suitable particle sizes produced in the aerosol plume by chlorofluorocarbon (CFC)
pMDIs. These agents prevent aggregation of the drug particles and lubricate the metering
valve. They also ensure that the drug is well suspended in the canister. The metering valve
acts to prepare a pre-measured dose of medication along with the propellant. The volume of
the metering valve changes from 25–100 µL and provides 50 µg to 5 mg of drug per actuation,
depending on the drug formulation.
Figure 15. Standard components of pMDI (Modified with permission from Reference 42)
The conventional pMDI has a press-and-breathe design. Depressing the canister into the
actuator releases the drug-propellant mixture, which then expands and vaporizes to convert
the liquid medication into an aerosol. The initial vaporization of the propellant cools the
aerosol suspension. The canister aligns the hole in the metering valve with the metering
chamber when it is pressed down. Then, the high propellant vapor pressure forces a premeasured
dose of medication out of this hole and through the actuator nozzle. Last, releasing
the metering valve refills the chambers with another dose of the drug-propellant mixture.
The two types of propellants used with pMDIs are CFCs and HFAs. Because of CFCs’
detrimental effect on the earth’s ozone layer and contributions to global warming, CFCs
were banned worldwide. As a replacement, HFAs were developed and incorporated into
pMDIs. HFAs are pharmacologically inert and have similar characteristics to CFCs.
However, there are some important differences between CFC and HFA propellants. For
example, CFCs use surfactant for dispersion; HFAs do not contain surfactant and use alcohol
for this purpose. Figure 16 shows the spray differences between HFA pMDIs and CFC
pMDIs. HFA pMDIs (Figure 16, left) have a softer spray than CFC pMDIs (Figure 16,
right). Also, HFA pMDIs have a much warmer spray temperature than CFC pMDIs. Due to
the cold mist from a pMDI, inhalation may be interrupted by patient sensitivity. Although
the more widespread use of pMDI HFAs will overcome the issues associated with the use
of pMDI CFCs, including priming, temperature effects, tail-off, and plume geometry, clini-
24 American Association for Respiratory Care A Guide to Aerosol Delivery Devices for Respiratory Therapists, 3rd Edition © 2013