Biomedical Engineering Reference
In-Depth Information
This index expresses a relative ratio of the contributions at the non-excited fre-
quency points, with respect to the contributions at the excited frequency points.
Furthermore, it gives a relative measure of the gain between contributions in the
input and in the output of the system. Since this is a non-linear system whose output
depends on the input, the choice for this relative measure is technically sound.
9.4 Non-linear Effects Originated with Pathology
The non-linear distortions introduced in the input signal due to the device itself are
corrected in the measured pressure before calculating the BLA or the respiratory
impedance of the volunteers. This is done using the BLA of the device itself and
(
9.21
). For the signal processing part, we used
m
=
6 realizations,
p
=
3 intervals
and
n
=
5000 samples.
Figure
9.8
shows the results obtained for a patient diagnosed with cystic fibrosis,
a patient diagnosed with asthma and a healthy volunteer. One can observe that the
non-excited even harmonics are relatively high in asthma. In order to make a statisti-
cal analysis, one can characterize the non-linear distortions present in the calculated
even and odd contributions using the index
T
from (
9.26
). The results are depicted
in Fig.
9.9
by means of boxplots and a significant difference has been found between
the two groups (
p<
0
.
0107). No significant difference has been found between the
asthma and cystic fibrosis groups, perhaps due to small dataset.
One may conclude that the non-linear distortions tend to be significantly in-
creased in patients diagnosed with respiratory disease than in healthy subjects. From
clinical insight, this is indeed a valid conclusion (recall here structural changes
discussed in Chap.
2
). The respiratory system affected by cystic fibrosis is filled
with viscous secretions which will change the heterogeneous appearance of the tis-
sues and introduce non-linear effects originated with turbulent flow, viscoelasticity,
excessive inflammation, and clogged airways. The respiratory system affected by
asthma is subject to airway hyperresponsiveness leading to airway chronic inflam-
mation. This affects the airway remodeling, changing air-flow dynamics and hence
introducing non-linear effects from turbulent flow, airway obstruction, airway mus-
cle fibrosis etc. In both cases, changes in structure and morphology will change
the non-linear behavior of the respiratory system, hence the values of the proposed
index will change as well.
As a preliminary study, the information gathered in this work is crucial in the
further development and implementation of measuring devices and algorithms to
measure low-frequency respiratory impedance in a non-invasive and simple man-
ner, without requiring breathing maneuvers or complex respiratory tests. Although
this preliminary evaluation was performed on a limited number of volunteers, it sug-
gests that measuring non-linear contributions is beneficial to gather insight into the
evolution of respiratory diseases. The fact that respiratory mechanics at low frequen-
cies have inherent information on the viscoelastic properties of airways and tissue
is motivating the development of signal processing algorithms which can cancel the
