There are two main types of hand held devices for delivering doses of aerosol medicament to a patient. Both are a propellant-driven metered dose inhaler (MDI) and a dry powder inhaler (DPI).
In a metered dose inhaler, the manufacturers either suspend or dissolve the medicament in a propellant. They then "contain" the propellant in a pressurized canister with a metered valve. Upon activation, the propellant produces a single dose of the medicament in the form of a gas stream. The device may include a tapered discharge nozzle baffle or a venturi to accelerate particles through a discharge nozzle, and to remove oversized particles. The propellants usually are low-boiling-point-halocarbons such as hydrofluorocarbons, hydrofluorochlorocarbons and fluorochlorocarbons. Are you familiar with the name- "Freon"?
However, only about 8% percentage of the medicament is delivered to a patient's lung, because of incomplete evaporation of the propellant and sticking of medications to the back of the throat. This sticking of medications may cause localized problems in the impact area, other unwanted systemic side effects if patients are swallowed. Furthermore, metered dose inhalers require coordination between activation and inhalation, which may exclude Infants, small children and the elderly from the use of the medications.
To overcome this problem, manufacturer often adds a space in metered dose inhalers to provide an additional volume for the propellant to evaporate. However, fine particle fractions deposit within the spacer, and lower the absorption of the medications. However, this may avoid the deposition of the drug to the throat and relieve some systemic side effects.
In a dry powder inhaler, manufacturers use no propellant but the device relies upon a burst of inspired air drawn through the unit by the patient. However, the force of inspiration varies considerably from person to person. Some patients with lung problems are unable to generate sufficient air in-flow to activate the device. Dry powder inhalers may have incomplete particle dispersion and also the impact of particle at the back of the throat.
In an attempt to overcome the problem of particle aggregation, the manufacturers usually formulate the medicament with a carrier in a particular way to aid de-agglomeration. Thus dry powder inhalers are unsuitable for high dose medications. Other common problems may lead to low absorption include high adhesiveness between the drug and the carrier and high moisture affinity of predominantly amorphous structure, e.g blend of dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG) (DPPC : PG 7:3).