Biomedical Engineering Reference
In-Depth Information
therefore constant and is put outside the integral. J
0
0
is
the lead field at
t ΒΌ
0 and conductivity s
0
, J
0
D
s
is the
reciprocal lead field dependent on
D
s. The choice of
which port is to be current carrying and which is to be
potential reading is arbitrary.
J
0
is current density with
unit current excitation [1/m
2
]. If
D
s is positive,
DZ
is
negative because of the minus sign in eq. (4.1.7). If
D
s is
zero,
DZ
is zero, this is not the case for eq. (25.1) in
Malmivuo and Plonsey (1995).
the FDA definition includes the word ''flow.'' On the
basis of previous data REG is actually a reflection of
volume rather than flow (Nyboer, 1960). REG and Ce-
rebral Blood Flow (CBF) correlation have been described
earlier (Hadjiev, 1968; Jacquy et al., 1974; Moskalenko,
1980; Jenkner, 1986). However, the correlation of global,
local CBF and carotid flow was not investigated. More
REG and related references can be found at www.
isebi.org/.
REG pulse amplitude change reflects arteriolar, cap-
illary and venular volume changes together rather than
absolute brain blood flow. Early CBF-REG studies did
not focus on this topic. It was previously described that
the involvement of a vessel in CBF autoregulation is size-
dependent: larger arteries are less involved than arteriola
(Kontos et al., 1978). Consequently, the arteriolar change
observed in brain by REG reflects arteriolar function
more than it reflects functions in larger arteries (e.g. ca-
rotid). The clinical importance of these findings is that
REG can be measured more conveniently and continu-
ously in humans than Doppler ultrasound. Therefore,
measurement of CBF autoregulation by REG has
potential for use as a life sign monitoring modality.
REG is a potential method for cerebrovascular di-
agnostics as well. In order to reach the potential of
widespread application of REG, there is a need for re-
search to clarify the physiological and pathophysiological
correlations and adequate data processing.
The physical basis of the REG measurement is based
on the fact that blood and cerebrospinal fluid (CSF) are
better conductors than the brain or other ''dry'' tissue.
The REG signal reflects the impedance change: during
blood inflow into the cranial cavity, electrical conduc-
tivity is increased (resistance decreased) represented by
increasing REG pulse amplitude. The same electrical
impedance change occurs generating pulse wave on pe-
ripheral site, as it was first described by Nyboer (1970) in
the parallel-column model. In the skull the input is the
volume of the arterial pulse, and the output is the venous
outflow and the CSF together. The resulting impedance
change - REG curve - is the result of the equation -
involving all mentioned factors but not detailed in-
dividually. The measured electrical impedance value
offers the basis of several volume calculations, detailed
by Jenkner (1986).
A typical REG change is known to occur as a conse-
quence of arteriosclerosis, expressed as elongation of
REG pulse amplitude peak time or decreased slope
of anacrotic part (Jenkner, 1986). The possible cause of
this alteration is the decreased elasticity of arteriolar
wall, which is the most sensitive indicator of disease
progression.
Animal studies (Bodo et al., 2004, 2005a,b, 2007)
show that REG can be measured more conveniently
and continuously in humans than Doppler ultrasound.
Sigman effect
Sigman et al. (1937) were the first to report that the
resistivity of blood is flow dependent. They found that
the resistivity fell about 7% when the blood velocity was
increased from 10 to 40 cm/s. This is an application area
for the Geselowitz (1971) equation, a change in mea-
sured conductance not related to volume and therefore
not plethysmographic. It is a source of error in volume
estimations, but not necessarily in flow estimations.
No Sigman effect is found in plasma or electrolytes
(Geddes and Baker, 1989). The Sigman effect is due to
the non-spherical bodies in the blood, in particular the
erythrocytes. At higher velocities but still linear flow the
erythrocytes reorient into the flow direction, and in
a tube they also clump together around the central axis.
The erythrocyte orientation means less hindrance to
electric current flow and lower resistivity if resistance is
measured in the axial direction. Kanai et al. (1976)
reported that resistivity changes occurred at double the
flow pulsation frequency, and that the magnitude became
very small
>
3 Hz. This means that the orientation and
clumping effects are rather slow.
4.1.2.4 Rheoencephalography
Rheoencephalography (REG) is a plethysmographic
bioimpedance method widely used in countries like
Russia and China, but not very well known in the USA
and Europe. The ambition is to assess cerebral blood flow,
(Geddes and Baker, 1989), however only a little part of
the REG signal is caused by changes in brain conductivity,
the rest relates to the pulsating blood flow of the scalp.
REGs use has been limited because the reading is so
highly contaminated by this scalp component. The ana-
tomical background of REG is not clearly understood,
and a multilayer spherical model of the head has been
used so that the REG information is split into the ex-
tracranial and intracerebral flow signals.
The United States Food and Drug Administration
definition states (Anonymous 1, 1997: (a) Identifica-
tion). A rheoencephalograph is a device used to estimate
a patient's cerebral circulation (blood flow in the brain)
by electrical impedance methods with direct electrical
connections to the scalp or neck area.'' In other words,
