Biomedical Engineering Reference
In-Depth Information
Tissue damping
G
r
(kPa/l) with FO2,
p<
3
e
−
5
(
left
) and with FO4,
p<
10
e
−
8
Fig. 7.3
(
right
);
1
: Healthy subjects and
2
:COPDpatients
Fig. 7.4
Tissue elastance
H
r
(kPa/l) with FO2,
p<
0
.
0012 (
left
) and with FO4,
p<
0
.
0004
(
right
);
1
: Healthy subjects and
2
:COPDpatients
in real part values of the impedance with frequency, whereas some patients may
present an increase.
Figures
7.3
,
7.4
and
7.5
depict the boxplots for the FO2 and FO4 for the tissue
damping
G
r
, tissue elastance
H
r
and hysteresivity
η
r
. Due to the fact that FO2 has
higher errors in fitting the impedance values, the results are no further discussed.
Although a similarity exists between the values given by the two models, the dis-
cussion will be focused on the results obtained using FO4 in the 4-48 Hz frequency
range.
The damping factor is a material parameter reflecting the capacity for energy ab-
sorption. In materials similar to polymers, as lung tissue properties are very much
alike polymers, damping is mostly caused by viscoelasticity, i.e. the strain response
lagging behind the applied stresses [
142
,
143
,
145
]. In both FO models, the exponent
β
r
governs the degree of the frequency dependence of tissue resistance and tissue
elastance. The increased lung elastance 1
/C
r
(stiffness) in COPD results in higher
values of tissue damping and tissue elastance, as observed in Figs.
7.3
and
7.4
.The
loss of lung parenchyma (empty spaced lung), consisting of collagen and elastin,
both of which are responsible for characterizing lung elasticity, is the leading cause
