Biomedical Engineering Reference
In-Depth Information
In the commercial device, the oscillation pressure is generated by a loudspeaker
(LS) connected to a chamber [
112
,
116
,
141
,
158
].TheLSisdrivenbyapower
amplifier fed with the oscillating signal generated by a computer (
U
g
). The move-
ment of the LS cone generates a pressure oscillation inside the chamber, which is
applied to the patient's respiratory system by means of a tube connecting the LS
chamber and the bacterial filter (bf). A side opening of the main tubing (BT) allows
the patient to have fresh air circulation. This pipeline has high impedance at the
frequencies above 5 Hz to avoid the loss of power from the LS pressure chamber.
It has been shown that in some patients, using a nose clip causes the first anti-
resonance peak in respiratory impedance to be distorted, resulting also in splitting of
the anti-resonance peak into two smaller peaks [
154
,
155
]. This is mainly because
the use of a nose clip frequently results in velum opening and biased anti-resonance
parameters. During the measurements performed for this study, the patient wears a
nose clip and keeps the cheeks firmly supported to counteract this influence. Before
starting the measurements, the frequency response of the transducers (PT) and of
the pneumotachograph (PN) are calibrated. The measurements of air pressure,
P
,
and air flow,
Q
dV/dt
(with
V
the air volume), during the FOT lung function test
is done at the mouth of the patient. The FOT lung function tests were performed ac-
cording to the recommendations described in [
116
,
159
]. The FOT excitation signal
was kept within a range of a peak-to-peak size of 0
.
1-0
.
3 kPa, in order to ensure
optimality, patient comfort, and linearity. Averaged measurements from three tech-
nically acceptable tests (i.e. no artifacts and coherence values above 0.8) were taken
into consideration for each subject, with typical time records depicted in Fig.
3.1
-C.
The time records were sampled at a sampling time of 1 ms. All patients were tested
in the sitting position, with cheeks firmly supported and elbows resting on the ta-
ble. The posture is important in estimating values for respiratory impedance and,
therefore a straight back was as much as possible applied (some patients who were
too tall for the adjusted maximum height of the device, were excluded from the
database). Each and every group of patients and volunteers has been tested in its
unique location, using the same FOT device, and under the supervision of the same
FOT team.
=
3.2 Frequency Response of the Respiratory Tissue and Airways
The global experimental setup from Fig.
3.1
-A can be modeled by the electrical
analogy from Fig.
3.1
-B, where
U
g
denotes the generator test signal (known);
U
r
denotes the effect of spontaneous breathing (unknown);
Z
r
denotes the total respi-
ratory impedance (to be estimated);
Z
1
denotes the impedance (unknown) describ-
ing the transformation of driving voltage (
U
g
) to chamber pressure;
Z
2
denotes the
impedance (unknown) of both bias tubes and loudspeaker chamber;
Z
3
denotes the
impedance (unknown) of tube segment between bias tube and mouth piece (effect
of pneumotachograph essentially).
Using the basic laws for analyzing electrical networks, the following relation-
ships can be derived:
