Biomedical Engineering Reference
In-Depth Information
Fig. 2.1
Particle velocity-airway generation profiles in the HRT for three levels of adult physical
activity; sedentary is defined as
V
t
= 500 mL, 14 breaths/min, and PIFR = 14 L/min; light activity is
V
t
= 1,291 mL, 15.5 breaths/min, and PIFR = 40 L/min; heavy activity is
V
t
= 2,449 mL, 40 breaths/
min, and PIFR = 120 L/min (
adapted from
[
3
])
Moving from the stagnant environment in which Stokes's law is defined to
consider particle motion in the flow through the respiratory tract, an interesting
feature is the steady decrease in velocity with increasing airway generation number
beyond generation 3 (Fig.
2.1
) [
3
]. In contrast, CIs size particles by increasing the
velocity of air from stage to stage, as the aerosol passes through the apparatus.
Furthermore, their diameters may decrease slightly during passage, due to the
continuous evaporation of any volatile liquid species present, whereas in the HRT,
some drug particles may grow due to the presence of near-saturation relative humid-
ity conditions in their immediate surroundings. Hence, although the physical prin-
ciples of impaction apply equally to the CI as to the lung, these aerosol measurement
devices do not in any sense “simulate” the lung, as will be discussed further below.
Aerodynamic diameter (
d
ae
) represents the size scale that takes into account the
effect of both particle density and shape on mobility in any flow field, such as in the
respiratory tract, and is related to
d
p
, which is the diameter of a spherical particle
(dynamic shape factor,
c
, of unity and density of 10
3
kg/m
3
) through the expression
12
/
=
r
rc
(2.3)
p
d
d C
ae
pc
0
where
r
p
is the particle density (kg/m
3
).
C
c
is the Cunningham slip correction factor,
which reflects reduction in
F
d
due to “slip” of the particle by adjacent gas molecules.
For particles with
d
ae
< 1.0 μm,
C
c
can be described in terms of the particle Knudsen
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