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Claims  |
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I claim:
1. Medical investigative apparatus for deriving signals representative of
the impedance of a zone of an animal body, comprising
means for passing first currents between electrodes of a first group
suitable for location on at least one surface of an animal body, the
electrodes being, in operation, positioned to pass the first currents
through the zone whose irpedance is to be measured,
means for deriving the potential difference across the zone due to the
first currents and substantially in the direction of the said first
currents,
means for passing second currents through the body between electrodes of a
second group also suitable for location on at least one surface of the
body to establish virtual barriers generally coinciding with boundaries of
the zone within the body, a said virtual barrier being formed along a said
boundary when there is no potential gradient in the body perpendicular to
the boundary and maximum potential gradient along the boundary,
means for deriving from potentials in the body control signals
representative of the positions of the said virtual barriers, and
means for controlling the second currents in accordance with the control
signals to control the positions of the said virtual barriers,
the potential difference across the zone being representative of the
impedance of the zone.
2. Apparatus according to claim 1 wherein the means for deriving potential
difference comprises a plurality of potential sensing electrodes connected
to respective high input-impedance amplification means, the potential
sensing electrodes being, in operation, positioned in pairs, one on each
side of a said virtual barrier, and the means for deriving control signals
derives signals representative of the potential differences between
electrodes of each pair.
3. Apparatus according to claim 2 wherein each electrode in the first and
second groups is connected to a constant alternating-current generator,
the generators connected to electrodes in the first group generating a
constant amplitude output current, and the generators connected to
electrodes in the second group gdnerating output currents with amplitudes
which vary according to respective control signals.
4. Apparatus according to claim 2 for use in deriving the impedance of a
generally rectangular zone comprising six sensing electrodes positioned on
one surface of the body with one sensing electrode at each of opposite
ends of the zone between two sensitive electrodes outside the zone but in
line with each said end, the first group of electrodes comprising two
current electrodes remote from the zone, the second group of electrodes
corprising two pairs of current control electrodes, each pair being
positioned to pass current outside the zone substantially parallel to the
sides thereof and adjacent to two sensing electrodes in line with the
opposite ends of the zone.
5. Apparatus according to claim 2 wherein the electrodes are positioned in
two arrays each comprising electrodes in the first and second group and
sensing electrodes, the arrays being adapted to be positioned on different
surfaces of the body, and currents pass through the body from one surface
to another.
6. Apparatus according to claim 5 for use in deriving the impedance of a
zone which is generally rectangular in cross-section normal to current
flow wherein each array comprises a central current electrode which forms
a part of the first group of electrodes, eight current control electrodes
forming part of the second group of electrodes, the current electrodes
forming a rectangular three-by-three array centered on one end of the
zone, and four pairs of sensing electrodes, each pair being located in a
row or column of the three-by-three array adjacent to the central current
electrode, the means for deriving control signals deriving first
difference signals from each pair of sensing electrodes to control the
current supplied by that one of the said control current electrodes which
is external to that pair in the array and in the same row or column, and
deriving sum signals from each pair of first difference signals derived
from adjacent pairs of sensing electrodes to control the current supplied
by current control electrodes at that corner of the three-by-three array
which is adjacent to those pairs of sensing electrodes.
7. Apparatus according to claim 5 for use in deriving the impedances of a
plurality of zones which are generally rectangular in cross-section normal
to current flow, wherein each array is rectangular and comprises a central
current electrode which forms part of the first group of electrodes, rows
and columns of control current electrodes which form part of the second
group of electrodes, and sensing electrodes between the current electrodes
in the rows and columns, the seans for deriving control signals deriving a
respective control signal to control the current supplied by each control
current electrode from sensing electrodes in a region adjacent to, and
inward of, that control current electrode in the array, and the current
supplied by each current electrode and a potential derived from at least
one of the sensing electrodes adjacent to that current electrode being the
current and potential representative of the impedance of a zone centred on
that current electrode.
8. Apparatus according to claim 7 wherein the means for deriving control
signals derives control signals representative of the value:
##EQU5##
where P.sub.n is the potential at the n.sup.th sensing electrode from the
center of a row or column and S.sub.n is the distance through the body
from the n.sup.th electrode in one array to the corresponding electrode in
the other array.
9. Apparatus according to claim 1 wherein the first and second groups of
electrodes include an electrode common to both groups and except for the
cormon electrode, each electrode in the first and second groups is
connected to a constant alternating-current generator, the generators
connected to electrodes in the first group generating a constant amplitude
output current, and the generators connected to electrodes in the second
group generating output currents with amplitudes which vary according to
respective control signals.
10. Apparatus according to claim 9 for deriving current and potential
signals representative of the impedances of zones of the human body
containing significant portions of the lungs, comprising two common
electrodes one located over the sternum and one located over the spine, a
plurality of current electrodes spaced apart along the sides of the body
in the region of the chest, with sensing electrodes between the current
electrodes, one current electrode on each side of the body being in the
first group of electrodes, the other current electrodes being in the
second group, and the current in each electrode in the second group being
controlled by the potential difference between sensing electrodes adjacent
thereto but on that side thereof adjacent to the nearest electrode in the
first group.
11. Apparatus for deriving signals representative of the impedance of a
zone of a cross-section of a closed body, comprising
an assembly including a supporting member, a plurality of electrode holders
mounted thereon, the supporting member being arranged to encircle, in
operation, a portion of the human body, and electrodes held by the said
holders, the said holders being adapted to hold the said electrodes in
contact with the surface of the said body,
means for passing first currents between a plurality of the electrodes in a
first group positioned to pass the first currents through the zone whose
impedance is to be measured,
means for deriving the potential difference across the zone due to the
first currents and substantially in the direction of the said first
currents,
means for passing currents through the body between a plurality of the
electrodes in a second group positioned in relation to electrodes of the
first group to establish virtual barriers generally coinciding with
boundaries of the zone within the body, a said virtual barrier being
formed along a said boundary when there is no potential gradient in the
body perpendicular to the boundary and maximum potential gradient along
the boundary,
means for deriving from potentials in the body control signals
representative of the positions of the potential barriers, and
means for controlling the second currents in accordance with the control
signals to control the positions of the potential barriers, the potential
difference across the zone being representative of the impedance of the
zone.
12. Apparatus according to claim 11 wherein the means for deriving
potential difference comprises a plurality of potential sensing electrodes
connected to respective high input-impedance amplification means, the
potential sensing electrodes being, in operation, positioned in pairs, one
on each side of a said virtual barrier, and the means for deriving control
signals derives signals representative of the potential differences
between electrodes of each pair.
13. Apparatus according to claim 12 wherein each electrode in the first and
second groups is connected to a constant alternating-current generator,
generators connected to electrodes in the first group generating a
constant amplitude output current, and generators connected to electrodes
in the second group generating output currents with amplitudes which vary
according to respective control signals.
14. Apparatus according to Claim 2 comprising means for applying forces to
the holders automatically to cause the holders to press on to the body
with equal contact pressure.
15. Apparatus according to claim 11 including means for generating
respective signals representative of the displacements of each electrode
from a circular datum.
16. Apparatus according to claim 11 including means for generating
respective signals representative of the inclinations of the holders from
the perpendicular to the plane of encirclement of the electrode holders.
17. Apparatus according to claim 11 wherein the said zone passes through
the center of a cross-section of the body, the apparatus including means
for supplying one electrode in each holder with a current according to the
expression:
##EQU6##
where I.sub.o is the current passed between electrodes at the ends of said
zone,
n is the number of electrode holders with n=0 for the electrode holders of
the central zone,
r.sub.n =R-d.sub.n
R is the radius of the ring formed when all the electrode holders have zero
displacement towards the center of the body,
d.sub.n is the displacement of the n.sup.th holder towards the center of
the body,
r.sub.(-1) is the value of r.sub.n for zone on the opposite side of the
central zone from the zone with the value r.sub.1, and
.theta..sub.n is the angle between the radii passing through the electrodes
corresponding to n=0 and n.
18. Apparatus according to claim 11, wherein the means for deriving control
signals derives control signals representative of the value:
##EQU7##
where n is the number of electrode holders with n=0 for the electrode
holders of the central zone,
R is the radius of the ring formed when all the electrode holders have zero
displacement towards the center of the body,
P.sub.n and P.sub.n+1 are the potentials of sensing electrodes in the
n.sup.th and (n+1).sup.th holders, respectively, and
.theta..sub.n is the angle between the radii passing through the electrodes
corresponding to n=0 and n.
19. A method of deriving the impedance of a zone of a body using electrodes
placed in contact with the body wherein the dimensions of the zone are
comparable with the maximum distances between the electrodes, comprising
positioning and energizing electrodes of a first group on the surface of
the body to pass first currents through the zone,
deriving the potential difference across the zone due to the first currents
and substantially in the direction of the said first currents,
positioning and energizing electrodes of a second group on the surface of
the body to establish virtual barriers generally coinciding with
boundaries of the zone within the body by passing control currents between
the electrodes of the second group, a said virtual barrier being formed
along a said boundary when there is no potential gradient in the body
perpendicular to the boundary and maximum potential gradient along the
boundary,
adjusting the control currents in accordance with potentials representative
of the positions of the virtual barriers to control the said positions,
and
deriving an indication of the impedance of the zone from the said potential
difference across the zone. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to methods and apparatus for deriving
currents and potentials representative of the impedances of zones of a
body, and therefore representative of the internal structure of many types
of body. The term body is used generally here to mean any finite object
but the invention is particularly useful in connection with the animal,
especially human, body. Variations in time of the impedance of the zone
can also be derived. The invention has particular application in
diagnostic cardiology and other branches of investigative medicine.
BACKGROUND OF THE INVENTION
Methods and apparatus for the determination of the impedance of zones of
the thorax, head and limbs are known in which a current is passed through
the body and potentials in the body are measured. Impedance changes may be
recorded which are correlated with the cardiac, respiratory or other
functions of the body due to variations in blood, air or other contents of
regions of the body. For example by using such apparatus an evaluation of
the stroke-volume of the heart, and respiration and perfusion of the lungs
may be made.
A fundamental problem which occurs in such known methods is that a current
passed into the body tends to diverge from the entry electrode until
limited by an external boundary of the body, converging again at an exit
electrode. With bodies having an irregular boundary suitable geometrical
factors cannot be computed to allow for this boundary and changes in the
boundary may be interpreted as changes in the contents of a zone measured.
Further, when inhomogeneities are encountered in the body the current
tends to converge towards regions of high conductivity and diverge from
regions of low conductivity. Current does not therefore flow naturally in
straight or even easily described curved paths in an inhomogeneous medium.
Changes in the contents of a zone under investigation cause local changes
in impedance and therefore in current paths so that any impedance measured
changes not only because the contents of the zone change but also because
the current flow patterns change.
U.S. Pat. Nos. 2,712,627 to 2,712,630 describe measuring the resistivity of
earth formations by lowering electrodes into a borehole. In this method a
current sheet perpendicular to the borehole is generated by injecting
flanking currents but the sheet diverges so that the resistivities of
volumes of material remote from the measuring point and which are
irrelevant to the required value become involved in the measurement.
Variants of this method are described in "The Microlaterolog" by H. G.
Doll in Petroleum Transactions, AIME, Vol. 198, 1953, where the flanking
current is injected by a circular electrode concentric with a primary
electrode and by Jackson, Marine Geotechnology Vol. 1, no. 2, page 91 at
seq. (1975) where the electrodes are grouped into two concentric sets of
electrodes of opposite sign which are located on the same insulating pad.
With the Microlaterolog the path of the current tube and its cross-section
are thought to be modified by variations in conductivity as the electrodes
are lowered below the surface; that is as the electrodes are lowered down
the borehole and volume of material whose resistivity is measured varies
because current patterns vary.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided
medical diagnostic or investigative apparatus for deriving signals
representative of the impedance of a zone of an animal body, comprising
means for passing first currents between electrodes of a first group
suitable for location on at least one surface of an animal body, the
electrodes being, in operation, positioned to pass the first currents
through a zone whose impedance is to be measured,
means for deriving the potential across the zone due to the first currents
and in the general direction thereof,
means for passing second currents through the body between electrodes of a
second group also suitable for location on at least one surface of the
body to establish virtual barriers as hereinafter defined generally
coinciding with boundaries of the zone within the body,
means for deriving from potentials in the body control signals
representative of the positions of the said virtual barriers, and
means for controlling the second currents in accordance with the control
signals to control the positions of the said virtual barriers,
the potential across the zone being representative of the impedance of the
zone.
The first currents may also be used in providing an indication of the
impedance of the zone.
In this specification and claims a "virtual barrier" is a barrier to
electrical current formed when there is no potential gradient in a body
perpendicular to the barrier and maximum potential gradient along the
barrier. The currents on either side of the said virtual barrier originate
from and/or proceed to, separate electrodes.
One of the most important advantages of the invention is that obtained when
the impedance of a zone of a human body is derived using external
electrodes placed on the surface of a body. Even when the contents of the
zone vary or the external boundaries of the body change, the virtual
barriers localise the zone to allow its impedance to be derived.
Preferably the means for deriving control signals comprise pairs of
potential sensing electrodes straddling the virtual barriers. By means of
difference amplifiers connected to the potential sensing electrodes, the
second currents are controlled so that in each pair the sensing electrodes
are at equal potentials and the potential minima defining the barriers are
located between the sensing electrodes.
The invention can be applied to measuring the impedance of a plurality of
zones simultaneously. By positioning the electrodes round a portion of the
body and switching the currents so that they pass in different directions
through a segment of the portion, a tomographic image may be formed.
According to a second aspect of the invention, therefore, there is provided
apparatus for deriving signals representative of the impedance of a zone
of a cross-section of a closed body, comprising
an assembly including a supporting member, a plurality of electrode holders
mounted thereon, the supporting member being arranged to encircle, in
operation, a portion of the body with electrodes held by the holders in
contact with the surface thereof,
means for passing first currents between a plurality of the electrodes in a
first group positioned to pass the first currents through a zone whose
impedance is to be measured,
means for deriving the potential across the zone due to the first currents
and in the general direction thereof,
means for passing currents through the body between a plurality of the
electrodes in a second group positioned in relation to electrodes of the
first group to establish virtual barriers as hereinbefore defined
generally coinciding with boundaries of the zone within the body,
means for deriving from potentials in the body control signals
representative of the positions of the potential barriers, and
means for controlling the second currents in accordance with the control
signals to control the positions of the potential barriers,
the potential across the zone being representative of the impedance of the
zone.
The body may be that of an inanimate object such as a pipe or the pillar of
a building, or a plant such as a tree trunk, or of an animal particularly
a human.
For this purpose a fairly large number of electrodes has to be applied to
the skin at the same time and therefore respective piston and cylinder
arrangements may be provided for the holders so arranged to cause relative
movement between the pistons and cylinders which in one direction press
the electrodes on to the skin, when fluid is introduced into the pistons.
According to a third aspect of the present invention there is provided a
method of deriving the impedance of a zone of a body using electrodes
placed in contact with the body wherein the dimensions of the zone are
comparable with the maximum distance between the electrodes, comprising
positioning and energising electrodes of a first group on the surface of
the body to pass first currents through the zone,
deriving the potential across the zone due to the first currents an in the
general direction thereof,
positioning and energising electrodes of a second group in two regions of
the body to establish virtual barriers, as hereinbefore defined, generally
coinciding with boundaries of the zone within the body,
adjusting the control currents in accordance with potentials representative
of the positions of the virtual barriers to control the said positions,
and
deriving the impedance of the zone from the said potential across the zone.
Another important feature of the invention is the realisation that it can
be used for bodies having zones under investigation with dimensions
comparable with the maximum distance between electrodes for example
approximately equal.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain embodiments of the invention will now be described by way of
example with reference to the accompanying drawings, in which:
FIG. 1 shows the layout of electrodes for use in determining the impedance
and stroke-volume of the heart,
FIG. 2 is a block diagram of a circuit to be connected to the electrodes of
FIG. 1,
FIG. 3 shows the layout of electrodes for use in determining the impedance
of the heart,
FIG. 4 is a block diagram of a circuit to be connected to the electrodes of
FIG. 3,
FIGS. 5 and 6 show arrays of electrodes for application to the human body
with FIG. 5 showing a cross-section on the line a--a through front and
rear arrays and the body (excluding the arms),
FIG. 7 shows an array of electrode holders for use in tomographic imaging
of a body section,
FIGS. 8 to 11 give details of the electrode holders and electrodes for use
with the arrangement of FIG. 7,
FIG. 12 is a schematic view of current patterns set up by the electrodes of
FIG. 3,
FIG. 13 shows an arrangement of electrodes for measuring the impedance of
zones of the lungs,
FIG. 14 is a block diagram of part of a circuit for supplying currents to
the electrodes of the holders of FIGS. 7 to 11, and
FIGS. 15 and 16 show details of the circuit of FIG. 14.
DETAILED DESCRIPTION OF THE INVENTION
In most applications of the present invention it is required to pass
current through a human body. In order to carry out such a procedure it is
necessary to restrict the operating currents and frequencies within known
safety limits. Consequently it must not be attempted by persons not having
a comprehensive knowledge of the consequent effects. As a general guide,
but not one which should be followed by persons without the above
knowledge, British Standard BS 5724 Part 1, Paragraph 19.3 should be
complied with. This allows for a "Patient Auxiliary Current" of up to 0.1
mA (rms) below 1 kHz and 0.1 mA.times.frequency/10.sup.3 up to a maximum
of 10 mA (rms) for higher frequencies. Impedance plethysmography is noted
as a specific example in the definition of "Patient Auxiliary Currents"
(Paragraph 25.4). Typically the circuits described below operate with a
nominal 100 .mu.A peak to peak current in each zone (e.g. for FIG. 1 a
total of 300 .mu.A peak to peak at 10 kHz and 900 .mu.A peak to peak for
FIG. 3) which is well within the allowed limits. In those circuits with a
plurality of zones, the total current is subdivided to the additional
zones rather than increased in proportion to their number. Of course these
considerations do not apply when the body under investigation is
inanimate.
In FIG. 1 the front of the body is represented in outline at 1, with the
heart shown by dashed lines at 2. Three current input electrodes 4, 5, 6
are shown as bands applied around the arms and the neck. Additionally
three current output electrodes 7, 8, 9 are shown as short strips applied
at the waistline. Between the electrodes 4 and 7, 5 and 8, 6 and 9 lie
electrode pairs 10 and 11, 12 and 13, and 14 and 15, respectively, for the
determination of potential and potential differences. The electrodes 12
and 13 are placed on the left sternal margin just above and below the
limits of the heart outline. The electrodes 10 and 14 are equally spaced
to the right and the left of electrode 12, and the electrodes 11 and 15
equally to the right and left of the electrode 13. The spacings are such
that the line joining the mid-point of the line joining the electrodes 10
and 12 with the mid-point of the line joining the electrodes 11 and 13
passes just to the right of the heart outline and a similar line with
respect to the electrodes 12 and 14, 13 and 15 passes just on the left of
the heart outline. The heart is thus mainly contained within a rectangular
outline 16 hereinafter referred to as the measurement zone.
A master oscillator 17 (shown in FIG. 2) drives six isolated constant
current generators 18 to 23 from antiphase outputs such that the outputs
of the generators 18, 19 and 20 connected to the electrodes 4, 5 and 6,
respectively, are opposite in sign to those from the generators 21, 22 and
23 which are connected to the electrodes 7, 8 and 9, respectively.
Six high impedance buffer amplifiers 24 to 29 amplify the potentials at
electrodes 10 to 15, which are then combined by means of difference
amplifiers 30 to 34 which each incorporate rectification and filtering so
that each amplifier has a d.c. output signal of magnitude and sign
representing the difference between its input signals. Thus the potentials
of the electrodes 10 and 12 are passed to buffer amplifiers 24 and 26
respectively and the difference between these potentials is obtained in
the difference amplifier 30. The output from the amplifier 30 is used to
control the output of the current generator 18 to the electrode 4 so that
the potential difference of the electrode 10 is adjusted by changing the
current through the impedance between the electrodes 4 and 10 to reduce
the potential difference between the electrodes 10 and 12. Any difference
between the potentials at the electrodes 11 and 13 is reduced to
substantially zero by current from the generator 21 which is under the
control of the difference amplifier 31 which in turn receives inputs from
the buffers 25 and 27. Similarly any potential differences between the
electrodes 12 and 14 and 13 and 15 are reduced to substantially zero by
controlling the generators 20 and 23, respectively, by way of respective
difference amplifiers 32 and 33 receiving inputs from the pairs of buffers
26, 28 and 27, 29. The output currents of the generators 19 and 22 passing
by way of the electrodes 5 and 8 are held constant and in order to provide
an indication of the impedance of the zone 16 containing the heart 2 the
voltage between the electrodes 12 and 13 is measured. If an absolute
measurement of impedance is required the voltage so obtained may be
divided by a current passing between these electrodes. The required
voltage is taken at the output from a difference amplifier 34 while a
series milliametere between the generator 19 and the electrode 5 can be
used to measure the current passing through the zone 16.
By keeping the electrodes 10 and 12 at substantially the same potential a
portion of a virtual barrier as hereinbefore defined occurs at a point
approximately halfway between the electrodes and a similar portion of a
virtual barrier occurs between the electrodes 11 and 13 for the same
reason. In effect a virtual barrier is set up along the left-hand boundary
of the zone 16 as seen in FIG. 1. Thus current passing into the zone 16
through the top of the zone will tend to stay inside the zone. In addition
when the impedance of the heart changes, for example when a comparatively
large quantity of blood flows from the heart into the lungs, the boundary
defined by the virtual barrier between the electrodes 10 and 12, and 11
and 13 is controlled to remain in the same position by adjustment of the
currents flowing at the electrodes 4 and 7. In a similar way the
right-hand boundary of the zone 16 is controlled by keeping the virtual
barrier between the electrodes 12 and 14 and that between the electrodes
13 and 15 in a stable position by adjusting the currents at the electrodes
6 and 9. Thus the volume of the zone 16 will remain approximately constant
and a measurement of its impedance variation with time and therefore with
blood content can be measured. Measurement of the variations in the
impedance of the zone 16 and therefore the heart 2 can be used to derive
the stroke-volume of the heart and the cardiac output waveform.
The current generators 18 to 23 may each be integrated circuits type LM
13600 (manufactured by National Semiconductors). These circuits are
transconductance amplifiers which each receive one input from the master
oscillator 17 which can be regarded as setting the amplitude of a current
provided by the circuit. The generators 18, 20, 21 and 23 also receive
control inputs which in a transconductance amplifier form a product with
the other inputs and thus vary the amplitude of the output current.
In an alternative control circuit the pairs of electrodes 10, 11; 12, 13
and 14, 15 are connected by way of respective buffer amplifiers as inputs
to first, second and third difference amplifiers, respectively. The
outputs of the first and second difference amplifiers are connected as
inputs to a further difference amplifier with output connected to control
the current through the electrodes 4 and 7, while the outputs of the
second and third difference amplifier are connected as inputs to another
further difference amplifier to control the current through the electrodes
6 and 9.
By keeping the potential difference between each outer pair of potential
sensing electrodes (for example 10 and 11) equal to that between the inner
electrodes 12 and 13 virtual barriers are set up along the vertical dotted
lines of FIG. 1.
The arrangement of FIG. 1 has advantages in that it requires a relatively
small number of electrodes to be applied to the body and these electrodes
can be applied reasonably conveniently. However, since control is carried
out from one surface of the body only, the results obtained cannot be
expected to be as accurate as those obtained with an arrangement partly
shown in FIGS. 3 and 4 where more electrodes are used and electrodes are
applied to the back and front of the body.
Nine current input electrodes 35, 36, 37, 38, 39, 40, 41, 42 and 43 (see
FIG. 3) of an array are shown applied to the front of the body. Between
the electrodes 36 and 39, lie electrodes 44 and 45 for the determination
of potential and potential differences. Other similar electrode pairs 46
and 47; 48 and 49; 50 and 51 lie on the lines from electrode 39 to the
electrodes 38, 42 and 40 respectively. Electrode 39 is centred over the
heart. A similar array is placed on the back of the body opposite the
first. Currents enter by the front array and leave by the electrodes of
the rear array. A zone 52 from front to back with approximately square
cross-section is formed containing most of the heart. The shape of this
zone may be varied by adjusting the positions of the corner electrodes 35,
37, 41 and 42.
A master oscillator 62 drives nine isolated constant current generators 53
to 61 in the same phase which are connected to the electrodes 35 to 43 of
the front array. The oscillator 62 also drives a similar set of nine
generators which are connected to electrodes of the rear array, the front
and rear arrays being driven in antiphase. Eight high impedance buffer
amplifiers 64 to 71 are coupled to the electrodes 44 to 51, respectively,
of the front array, and similar buffer amplifiers are coupled to
corresponding electrodes in the rear array. The potential difference
between the electrodes 44 and 45 is determined by a difference amplifier
72; similarly the potential differences between the electrodes 46 and 47;
48 and 49 and 50 and 51 are determined by difference amplifiers 73, 74 and
75, respectively. The output of the amplifier 72 controls the current
supplied by the generator 54 to electrode 36 in such a way that the
potential difference between the electrodes 44 and 45 is reduced. The
amplifiers 73, 74, 75 act similarly on generators 56, 60, 58 to control
currents supplied to the electrodes 38, 42, 40 respectively. The outputs
of amplifiers 72 and 73 are summed in a summing amplifier 76 which in turn
controls the current supplied by generator 53 to the corner electrode 35.
Summing amplifiers 77, 78, 79 control the generators 59, 61, 55 in a
similar way to supply the other three corner electrodes 41, 43, 57
respectively. An amplifier 80 sums the output of the four buffer
amplifiers 65, 67, 69, 71 to produce an output proportional to the mean
potential of the electrodes 45, 47, 49 and 51 surrounding the primary
current electrode 39. A similar circuit is connected to the electrodes of
the rear array. An amplifier 81 determines the potential difference
between the front and rear surfaces of the measurement zone 52 by
differencing the outputs of the amplifier 80 and of a similar amplifier
connected to the rear array. The output of amplifier 81 is proportional to
the impedance of the measurement zone since the current through the zone
is constant.
In order to illustrate the operation of the invention a three dimensional
view of the currents from the front array when this array is positive is
shown in FIG. 12. The front array is shown horizontal and only currents
flowing within the rectangle defined by the electrodes 35, 37, 41 and 43
appear. As a result of the layout of the electrodes of the front and rear
arrays a virtual barrier in the form of a plane with edge indicated by the
dashed line 63 is set up between the electrodes 36 and 39 and the
corresponding electrodes in the rear array. By keeping the potential
difference between the electrodes 44 and 45, and between the corresponding
electrodes substantially at zero, this virtual barrier is kept in the
position shown regardless of how the impedance of the zone 52 changes. The
pairs of electrodes 46 and 47, 48 and 59 and 50 and 51 have the same
effect on virtual boundaries forming other edges of the zone 52. Control
signals derived by differencing the potentials at the electrodes 44 and
45, and 46 and 47 control a current passed through the electrode 45 to
keep the top left corner of the zone 52 in approximately the position
indicated. Similar control signals for the currents passed through the
electrodes 37, 41 and 43 keep the other "corners" in constant positions.
The zone 52 need not, and in practice is not, of the perfect rectangular
shape shown but provided its outline remains reasonably constant impedance
measurements of the heart can be made. For this purpose the output voltage
of the amplifier 81 may be divided by a constant current passed through
the electrode 39 and the corresponding electrode of the rear array.
Front and rear arrays of electrodes (see FIGS. 5 and 6) may be used for
generating a trans-thoracic impedance map of the body
The body is represented in outline at 1, the heart at 2 and the ribs at 3.
A central row of electrodes forming part of an array 82 are shown in
section all touching the skin as are a row of electrodes in a second array
83 at the rear. In a practical arrangement each electrode is spring loaded
perpendicular to the plane of the array so that contact is established
with the irregularly shaped human body. The electrodes 84 and 86 at either
end of the row are control current electrodes, passing currents which are
controlled but not measured; whilst the group 85, 87 and 89 in the centre
are measurement current electrodes, passing currents which are measured
and, except for the electrode 85, controlled. Interposed between each pair
of current electrodes are pairs of potential sensing electrodes 88, 90, 91
and 92.
FIG. 6 shows the array 82 consisting of several similar rows of electrodes
lying on either side of the central row. Every third row except the
outermost rows is constituted as described above, although in this
instance only one such row on either side of the central row is shown. The
two intermediate rows provide potential sensing pairs in every third
column.
The outermost rows at the extreme ends of the array function as control
current electrodes, current being supplied from every third electrode, and
the intermediate electrodes being disconnected.
Current generators, buffer amplifiers and difference amplifiers are
provided in a circuit (not shown) according to the same principle of
interconnection illustrated in FIGS. 3 and 4. For example the electrode 85
is connected to a constant current generator which, during measurements,
supplies a constant current and the electrode pair 92 is connected by way
of respective buffers to a difference amplifier whose output controls a
constant current generator supplying current to the electrode 89.
Similarly the potential difference between the electrodes 90 is used to
control the current supplied by the electrode 86. Working out from the
central electrode 85 in each direction each electrode in the row "aa" is
connected according to the same principle as are the electrodes in the
column at right angles thereto. Current electrodes on diagonals through
the electrode 85 are each controlled by four current measuring electrodes;
for example pairs of potential sensing electrodes 110 and 111 are
connected by way of buffer amplifiers to respective difference amplifiers
and the outputs of these difference amplifiers are summed by a summing
amplifier whose output is used to control current passing by way of a
current electrode 112. Alternatively the potential differences between the
electrodes 92 and electrodes 92' may be used to control current through
the electrode 112 and this principle is used for the corner electrodes of
the array.
In order to measure th | | |
