Biomedical Engineering Reference
In-Depth Information
Fig. 3.3
Structure of the “DuBois” model from [
32
].
R
aw
airway resistance;
L
aw
airway iner-
tance;
R
t
tissue resistance;
C
t
tissue compliance;
L
t
tissue inertance;
C
g
gas compression compli-
ance
tissue resistance;
C
ve
viscoelastic tissue compliance. They assessed the impedance
in seven anesthetized paralyzed patients with no respiratory disease. The advantage
of this scenario is that the influences from upper airway shunt and muscular activity
are not significant and therefore do not bias the estimates. The
R
aw
is hypothesized
to represent airways resistance plus a purely viscous component of tissue resistance,
presumably in the chest wall. The
C
s
is the static compliance of the respiratory sys-
tem. The
R
ve
and
C
ve
are related to viscoelastic properties of the tissue. However,
there is virtually no inertance (air mass) quantified in this model, one may expect
that this model will provide biased estimates at relative higher frequencies.
All patients exhibited a marked frequency dependence of effective respiratory re-
sistance (real part of impedance) at low frequencies. The resistance fell sharply from
6
.
2
0
.
6cmH
2
O/(l/s) at 2 Hz and decreased
moderately with frequency, such that its value at 32 Hz was 1
.
5
±
2
.
1cmH
2
O/(l/s)at0.25Hzto2
.
3
±
±
0
.
5cmH
2
O(l/s).
−
±
The imaginary part of the impedance was
5
.
9cmH
2
O/(l/s) at 0.25 Hz
and increased with frequency, crossing zero line around 14 Hz and reached 2
.
3
22
.
2
±
0
.
8cmH
2
O/(l/s) at 32 Hz. They observed that the inertance becomes important as
early as with 4 Hz, which rather contradicts DuBois [
32
,
93
]. The strong negative
dependence in the vicinity of spontaneous breathing frequencies in the real part
of impedance in anesthetized patients agreed with studies in awaken subjects. The
authors agree that this dependence at low frequencies can hardly be attributed to
regional inhomogeneities of tissues. They suggest that the mechanical behavior of
the respiratory system at spontaneous breathing frequencies is largely determined
by intrinsic features of tissues, such as plasto-elastic properties. They also report
an average value of
9cmH
2
O/(l/s) for total resistance, mainly influenced by tis-
sue properties at very low frequencies. The authors suggest that a nonlinear plastic
model should be considered to account for the mechanical behavior of the respira-
tory system.
A relatively good model structure, dividing the airway tissue and alveolar proper-
ties into different compartments, is the one proposed by DuBois and schematically
depicted in Fig.
3.3
[
32
]. This model has the following elements:
R
aw
, airway resis-
tance;
L
aw
, airway inertance;
R
t
, tissue resistance;
C
t
, tissue compliance;
L
t
, tissue
inertance;
C
g
, gas compression compliance.
≈
