Aerosols in the Health Care Field

:: Section 3

How does it work?

1. When the canister is depressed into the actuator, the drug-propellant mixture in the metering valve is released under pressure.
2. The liquid propellant rapidly expands and vaporizes, or “flashes,” as it ejects from the pressurized valve into ambient pressure. This expansion and vaporization shatters the liquid stream into an aerosol. 
3. The initial vaporization of propellant causes cooling of the liquid-gas aerosol suspension, which can be felt if discharged onto the skin. This also can cause users to stop inhaling as the cold aerosol hits the oropharynx.
4. Upon release, the metering valve refills with the mixture of drug and propellant from the bulk of the canister and is ready for the next discharge.

Aerosol Delivery Characteristics

Although MDIs produce aerosols in the respirable range (MMAD 3-6 µm), the initial velocity and dispersion of the aerosol plume cause approximately 80% of the dose leaving the actuator to impact and become deposited in the oropharynx. Significant oropharyngeal deposition may be a factor in systemic absorption of some drugs. Pulmonary deposition ranges between 10% and 20%. It is difficult to predict the exact amount of drug delivered to an individual patient because of high variability between patients and because MDI drug administration is technique dependent.