Biomedical Engineering Reference
In-Depth Information
Therefore, measurement of CBF autoregulation by REG
has potential for use as a life sign monitoring modality.
Studies on humans have shown that reproducibility
and sensitivity of the bioimpedance measurement -
including REG - were comparable to the sensitivities of
the pulse oximeter, laser Doppler and Doppler ultra-
sound. Results demonstrated that bioimpedance offers
potential for use as a multifunctional, continuous, non-
invasive life sign monitor for both military and civilian
purposes (Bodo et al., 2006).
In a comparative population screening study (546
volunteers) REG measurements revealed symptoms of
arteriosclerosis in 54% of the subjects; within the iden-
tical population the Doppler ultrasound measurements
showed 30% with arteriosclerosis (Sipos et al., 1994;
Bodo et al., 1995a).
REG may have potential for non-invasive continuous
life sign monitoring and detection of early cerebrovas-
cular changes. Since the early days of REG research,
advances in the development of electronics, computation
and signal processing techniques offer the possibility to
reconsider the feasibility of implementing a portable or
even wearable version of the REG monitoring technique
to evaluate the adequacy of CBF reactivity.
REG is also used in cardiac applications and evaluation
of edema in the legs.
In order to fulfill some of the expectations REG must
reach a much higher level of standardization of both
instrumentation and electrode geometry. The lack of
electrode sensitivity analysis of the REG techniques has
severely reduced the scientific
which is in accordance with eq. (4.1.4). The ICG tradi-
tion has since then been to use impedance Z instead of
resistance R , and to use the series model. Equation
( 4.1.8 ) is surely valid as the condition DZ Z 0 is fulfilled
in ICG, but the DZ / Z 2 term is not so directly evident as
the simple DG term in eq. (4.1.2) . Also an increased
volume corresponds to a conductance increase, but to an
impedance decrease. Therefore a minus sign is often in-
troduced as in eq. (4.1.4) (Geddes and Baker, 1989).
Typically values for Z 0 is 25 U and DZ 0.2 U . The Z
waveform is similar to the aorta blood pressure curve.
SV as developed by Kubicek et al. (1966) is still
compatible with a basic physical model:
SV ¼ dZ
dt
T r L
Z 0
2
½ m 2 Kubicek Þ
(4.1.9)
max
The first time derivative dZ/dt is called the impedance
cardiographic curve (ICG). T is the ventricular ejection
time. As the pick-up electrodes are positioned near the
heart they also pick-up the ECG signal, and this is used
for the time estimation.
In the original Nyboer model the changes in conduc-
tance was associated with cylinders of different and
changing cross sectional areas. The blood distribution
process is of course much more complicated. With chest
electrodes we have signals from the filling and emptying
of the heart, aorta, lungs, muscles of the chest; as well as
the Sigman effect.
Many electrode geometries have been used, in par-
ticular the old four-band technique and the newer spot
electrode technique with four or eight or more elec-
trodes. Some systems use two current carrying systems
with four excitation and four recording electrodes, eight
electrodes in total. Then four electrodes are connected
around the neck, the others on the lower thorax. With
two current sources the sensitivity field is complicated,
and many algorithms are possible when weighing the
results obtained in the two channels. Kauppinen et al.
(1998) compared four different electrode systems using
either band or spot electrodes. They used a 3D computer
model with data from the US Library of Medicines
Visible Human Project. They found that more than 55%
of the measured transfer impedance was due to the
skeletal muscle mass in the thorax and only about 15%
originated from blood, heart and lungs. The sensitivity
field for the four tested systems showed only small dif-
ferences in total sensitivity, but all the same each was
a complicated mixture of many factors.
Sramek (1981) and Bernstein (1986) further de-
veloped empirical equations also with biometrical data
such as patient height, actual weight, ideal weight, body
surface area, age and gender. Other transthoracic equa-
tions have also appeared (reviewed by Moshkovitz et al.,
2004), partly with proprietary modifications making
soundness of
the
method.
4.1.3 Impedance cardiography
Impedance Cardiography (ICG) is impedance plethys-
mography based on the measurement of thoracic elec-
trical bioimpedance (TEB). It also includes a component
from the resistivity dependence on blood flow (Sigman
effect). This is not a plethysmographic but a blood ve-
locity component. Usually a measuring frequency of 50-
100 kHz has been used. ATEB picks up both cardiac and
respiration signals. The ambition is that the SV (L) and
therefore CO (L/min) can be calculated with ICG, as
well as the total thoracic fluid volume,for example,
according to eq. (4.1.1) .
Nyboer (1950) used two band electrodes around the
neck, one band electrode corresponding to the apex of
the heart and a fourth further in caudal direction. Nyboer
regarded the thorax as a cylinder volume of length L and
used the expression:
D y ¼ DZ r L
Z 0
2
½ m 3 Nyboer Þ
(4.1.8)
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