Biomedical Engineering Reference
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Fig. 10.65
Modified NGI by Svensson and Berg showing “O”-cup option (
a
and
b
) and internal
filter option (
c
) for abbreviated testing of OIPs (
From
[
59
]—
courtesy of M. Svensson
)
section of the apparatus was directed through an externally mounted filter and then
redirected into the NGI again after passage through the filter.
Both configurations can also be seen in Fig.
10.65
; the modified outlet stage cups
(necessary for the external filter approach) are shown in (a), the external and inter-
nal filter options are depicted in (b) and (c), respectively.
The main aim of their study was to judge whether these so-called filter methods
can be regarded equivalent to the standard impactor method in which all stages are
individually analyzed. Therefore, parallel experiments using full NGI set-up were
performed and used for equivalence testing versus the proposed filter methods.
Three different DPIs with three different formulations and two different pMDI
products (CFC and HFA propellant) were used in this study. The comparison of the
two methods was performed by comparing a total of 91 mean values from the full
NGI and the corresponding filter method (Fig.
10.66
).
Every mean value comprised data from either two or three impactor tests (full
NGI) and 5-10 filter samples for the filter method, so that in total, around 650 indi-
vidual tests were conducted. Except for the pMDI, each DPI were tested at two dif-
ferent flows corresponding to 2 and 4 kPa differential pressure over the device,
spanning a flow range from 40 to 77 L/min. A strong linear relationship was obtained
between the filter measure and the full NGI measures of
FPM
obtained at the same
selected size limits. The correlation coefficients (
r
2
) were >0.95 for nozzle filter and
>0.98 for the outlet cup method. The slopes for the fitted lines were close to the line
of identity, varying from 0.89 to 1.03. However, the authors noted that the linear
relationship, determined from the slope and associated
r
2
value, was weaker for
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