Biomedical Engineering Reference
In-Depth Information
devices which deliver aerosols of very fine particles to the deep regions of the
lungs (i.e. peripheral respiratory airways). In principle, particles smaller than 5
or 6 µm are considered necessary for possible deposition in the peripheral
airways (i.e. respiratory bronchioles and alveolar region) and particles less
than 2 µm are preferable for alveolar deposition (Stahlhofen et al., 1980).
Aerosol particles below 5 or 6 µm are commonly described as ―fine particle
fraction‖ or to be ―respirable‖.
The Mudge inhaler was invented in 1778 by the English physician John
Mudge for inhalation of opium to treat cough (Anderson, 2005). This is a
perforated tankard with a mouthpiece so that when patient inhales, air is drawn
through the holes and passes through the warm liquid infusion prior to
inhalation (Anderson, 2005). Nowadays, medical inhalation is achieved using
four types of device, which are pressurised metered dose inhalers (pMDIs),
dry powder inhalers (DPIs), nebulizers, and soft mist inhalers (SMIs).
3.1. Pressurized Metered-Dose Inhalers (pMDIs)
pMDIs (Figure 3) introduced in the 1950s, are still the most commonly
used inhalation devices. pMDI canisters are made of an inert, robust and light-
resistant material (e.g. aluminium). The canister is encased within a plastic
holder for easy and safe handling by patient. Formulations comprise solutions
or micronized suspensions of drug in liquefied high vapour pressure propellant
such as the non-ozone depleting hydrofluoroalkanes (HFAs). A blend of
propellant, drug and excipients (e.g. surfactants, cosolvents, etc.) are filled
under pressure into the inhalation canister and, on actuation and coordinated
inhalation, the propellant and drug are rapidly released from an actuator
orifice. The propellant rapidly evaporates due to its very high vapour pressure,
leaving an accurate metered dose of aerosolised drug to be inhaled by patient.
Synchronisation of inhalation with device actuation is essential although not
the only requirement for peripheral airways deposition of aerosols generated
from pMDIs (Crompton, 1982; Newman, 2005).
The high velocity of aerosols emitted from the pMDIs can result in high
drug deposition in the upper respiratory tract, particularly the oropharyngeal
region. This may lead to adverse side effects such as the oral candidiasis
caused by high deposition of antiasthma steroid in the mouth cavity.
Acessories such as spacers may be used with pMDIs to reduce the aerosol
deposition in the oropharayngeal region. Inhaler spacers are hollow chambers
