Biomedical Engineering Reference
In-Depth Information
We also introduce two dimensionless indices, namely the quality factor at 6 Hz
( QF 6), denoted by the ratio of the reactive power to the real power:
Im 6
Re 6 =
QF 6
=
tan φ z
(7.8)
where Re 6 and Im 6 denote the real and imaginary parts of the complex impedance
evaluated at 6 Hz and φ z denotes the phase angle at 6 Hz. From ( 7.8 ), one can
calculate the corresponding power factor PF 6:
1
QF 6 2
Re 6
( Re 6 +
Re 6
Re 6 +
PF 6
=
1 =
Im 6 ) 2 =
Im 6 =
cos φ z
(7.9)
+
In engineering, the quality factor QF compares the time constant for decay of
an oscillating physical system's amplitude with respect to its oscillation period. In
other words, it compares the frequency at which a system oscillates to the rate at
which it dissipates its energy, also known as the damping factor. For a second order
linear time invariant system, a system is said to be over-damped if QF < 0 . 5, under-
damped for QF > 0 . 5 and critically damped for QF
0 . 5. In other words, a low
QF denotes a high energy loss, while a high QF denotes a low energy loss. For the
power factor PF , we find that for PF
=
0 the energy flow is entirely reactive (hence
the stored energy in the load returns to the source with each cycle), and if PF
=
=
1,
all the energy supplied by the source is consumed by the load.
Because FO are natural solutions in dielectric materials [ 80 , 131 ], it is interesting
to look at the permittivity property of respiratory tissues. In electric engineering, it
is common to relate permittivity to a material's ability to transmit (or permit )an
electric field. By electrical analogy, changes in trans-respiratory pressure relate to
voltage difference, and changes in air flow relate to electrical current flows. When
analyzing the permittivity index, one may refer to an increased permittivity when
the same amount of air-displacement is achieved with smaller pressure difference.
The complex permittivity has a real part, related to the stored energy within the
medium and an imaginary part related to the dissipation (or loss) of energy within
the medium. The imaginary part of permittivity corresponds to
ε r = L r sin π
2 α r
(7.10)
7.2.2 Subjects
The first group evaluated here consists of male volunteers without a history of respi-
ratory disease, whose lung function tests were performed in the laboratory of Ghent
University, Department of Electrical energy, Systems and Automation. Table 7.1
presents their biometric parameters, whereas a detailed analysis on their respiratory
impedance parameters will be discussed later in this chapter.
A second group consists of former coal miners from the Petrosani area, tested
periodically for their lung function at the 'Leon Danielo' Hospital in Cluj-Napoca,
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