Biomedical Engineering Reference
In-Depth Information
Fig. 7.9
Tissue damping
G
r
(
left
) and tissue elastance
H
r
(
right
)in
1
: Healthy subjects and
2
:COPDpatients
Fig. 7.10
Tissue hysteresivity
η
r
(
left
) and real part of impedance
R
6 evaluated at 6 Hz (
right
)in
1
: Healthy subjects and
2
:COPDpatients
the fractional order of compliance, which generally expresses the capability of a
medium to propagate mechanical properties [
143
]. The damping factor is a material
parameter reflecting the capacity for energy absorption. In materials similar to poly-
mers, 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
]. In the FO model, the exponent
β
r
governs the degree of the frequency
dependence of tissue resistance and tissue elastance. The increased lung elastance
1
/C
r
(elasticity) in COPD results in higher values of tissue damping and tissue elas-
tance, as observed in Fig.
7.9
. 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 of increased elastance in COPD. Given the re-
sults observed in Fig.
7.10
, it is possible to distinguish between tissue changes from
healthy to COPD case from the variations in the hysteresivity index
η
r
(
p
0
.
01).
Since pathology of COPD involves significant variations between inspiratory and
expiratory air flow, an increase in the hysteresivity coefficient
η
r
reflects increased
inhomogeneities and structural changes in the lungs.
