Biomedical Engineering Reference
In-Depth Information
2.8
Add-On Devices Used with MDIs
The evaluation of add-on devices that are widely prescribed for use with MDIs
[ 59 - 61 ] poses some additional considerations beyond those that exist for the testing
of the inhaler alone. Spacers are open extensions to the MDI actuator mouthpiece
whose primary purpose is to place distance between the inhaler and the patient, such
that plume development and impaction of the ballistic fraction takes place within
the spacer itself (Fig. 2.20 ).
As such, they do not retain the medication should the patient fail to either inhale
at the time of MDI actuation or exhale into the add-on device. Their evaluation is
therefore probably best accomplished following the compendial methods for MDI
performance evaluation, without any attempt to simulate delayed inhalation.
The valved holding chamber (VHC), on the other hand, has a valve at its exit that
enables the aerosol to be retained if the patient is poorly coordinated. These devices
are equipped with either a mouthpiece (Fig. 2.21 ) or face mask (Fig. 2.22 ) as the
patient interface, as they are used by patients of all ages from infants to geriatrics.
As such, it follows that it is logical that these devices be evaluated simulating
delayed inhalation, rather than by the methods in the pharmacopeias that currently
require the tester to sample the aerosol immediately upon MDI actuation.
Fig. 2.20 Open-tube spacer
for use with MDIs
Fig. 2.21 VHC mouthpiece
as patient interface for use
with MDIs
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