Biomedical Engineering Reference
In-Depth Information
cutoff diameter appropriate to each stage. This behavior has been described in detail
in Chap. 2 [ 46 ] and illustrates the importance of evaluating the effect of removing
stages on CI performance for the most accurate work with abbreviated systems.
12.4
Additional Refi nements for the Complete
AIM-pHRT System
Two further refi nements would improve realism in connection with clinically rele-
vant aerosol size-based metrics from OIPs. The fi rst is to operate the inhaler
mimicking the process of inhalation [ 47 ]. The second is the ability to interface the
inlet with anatomically appropriate representations of infant, small child, and adult
faces for the testing of OIPs and add-on devices with a facemask as the patient
interface [ 48 ]. Both improvements are equally applicable to full resolution as to
abbreviated CI measurements.
The testing of DPIs of necessity involves the simulation of an inhalation, but the
compendial methods are structured in such a way by developing a fi xed pressure
differential of 4 kPa and sampling a fi xed volume of 4 L [ 49 , 50 ], that the inhaler
may not perform in the way it would in the hands of patients, whose inspiratory fl ow
rate-time profi les, given age, obstructive lung disease severity, etc., could differ
markedly from these reference conditions [ 51 ]. Although interfacing a CI to some
form of breathing simulator is diffi cult, given the need to ensure constant fl ow rate
at all times through the impactor, work by Daniels and Hamilton, with their elec-
tronic e-Lung™ [ 52 ], may offer promise (Fig. 12.14 ). Their evaluation of a FSI
compared with a full-resolution NGI [ 53 ] was discussed in detail in Chap. 10 . They
demonstrated how a programmable bellows arrangement connected via a feedback
loop to the e-Lung™ may be used to vary the pressure drop-time profi le. They made
the important fi nding that by decoupling the “inhalation” process from the DPI from
the requirement of the CI to sample at a constant fl ow rate, bias caused by different
fl ow ramp-up profi les between abbreviated and full-resolution CIs could be avoided.
In the context of this chapter, it is important to note that they made use of a cast,
anatomically correct adult inlet to which they connected their DPI. Furthermore,
they were able to replicate actual profi les of patients with asthma and COPD [ 53 ].
This arrangement is shown schematically in Fig. 12.14 .
Another promising new development is a mixing inlet, originally developed by
Miller [ 54 ] and now commercially available (Copley Scientifi c Ltd, UK, and also
available through RDD Online, Virginia Commonwealth University, VA, USA),
that can be interposed between the inhaler and CI, so that the former can be evalu-
ated using breath simulation. For the purpose of discussion here, this apparatus is
referred to as the “Miller” mixing inlet. In the basic setup described in the sales lit-
erature, the continuously variable fl ow-time profi le from the inhaler (in this case
with a DPI) is controlled by a breathing simulator (Fig. 12.15 ) that in turn is coupled
to the sidearm of the mixer. A source of compressed air also provides a constant
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