U.S. patent application number 14/484796 was filed with the patent office on 2015-01-01 for impedance determination.
The applicant listed for this patent is IMPEDIMED LIMITED. Invention is credited to Ian John Bruinsma, Scott Chetham, Christopher Newton Daly.
Application Number | 20150002177 14/484796 |
Document ID | / |
Family ID | 40625275 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150002177 |
Kind Code |
A1 |
Chetham; Scott ; et
al. |
January 1, 2015 |
IMPEDANCE DETERMINATION
Abstract
Apparatus for use in performing impedance measurements on a
subject. The apparatus includes a processing system for causing a
first signal to be applied to the subject, determining an
indication of a second signal measured across the subject, using
the indication of the second signal to determine any imbalance and
if an imbalance exists, determining a modified first signal in
accordance with the imbalance and causing the modified first signal
to be applied to the subject to thereby allow at least one
impedance measurement to be performed.
Inventors: |
Chetham; Scott; (Del Mar,
CA) ; Daly; Christopher Newton; (Newport, AU)
; Bruinsma; Ian John; (Kings Langley, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMPEDIMED LIMITED |
Pinkenba |
|
AU |
|
|
Family ID: |
40625275 |
Appl. No.: |
14/484796 |
Filed: |
September 12, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12741368 |
Oct 21, 2010 |
8836345 |
|
|
PCT/AU2008/001521 |
Oct 15, 2008 |
|
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14484796 |
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Current U.S.
Class: |
324/692 |
Current CPC
Class: |
G01R 1/06 20130101; G01R
1/18 20130101; A61B 5/6885 20130101; A61B 5/053 20130101; A61B
5/7203 20130101; G01R 27/02 20130101; G01R 1/30 20130101 |
Class at
Publication: |
324/692 |
International
Class: |
G01R 1/18 20060101
G01R001/18; A61B 5/053 20060101 A61B005/053; G01R 1/06 20060101
G01R001/06; G01R 27/02 20060101 G01R027/02; G01R 1/30 20060101
G01R001/30 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2007 |
AU |
2007906049 |
Claims
1) An apparatus for use in performing impedance measurements on a
subject, wherein the apparatus includes a number of electrode
systems, and wherein each electrode system includes: a first
substrate having a signal generator and sensor mounted thereon, the
signal generator being for applying a first signal to the subject
and the sensor for sensing a second signal across the subject; and
a second substrate having at least two conductive pads mounted
thereon, the conductive pads forming first and second electrodes
for coupling the signal generator and the sensor to a subject in
use.
2) The apparatus of claim 1, wherein the electrode system includes
a capacitive cancelling circuit for cancelling capacitive coupling
between the drive and sense electrodes.
3) The apparatus of claim 2, wherein the capacitive cancelling
circuit includes an inverting amplifier for coupling a signal
generator output to a sensor input.
4) The apparatus of claim 3, wherein the inverting amplifier
applies a capacitance cancelling signal to the sensor input to
thereby cancel any effective capacitance between the drive
electrode and the sense electrode.
5) The apparatus of claim 3, wherein an inverting amplifier output
is coupled to the sensor input via at least one of: a resistor; a
capacitor; and an inductor.
6) The apparatus of claim 5, wherein at least one of a resistor and
capacitor are adjustable, thereby allowing a capacitance cancelling
signal applied to the sensor input to be controlled.
7) The apparatus of claim 1, wherein the electrode system includes
an input capacitance cancelling circuit for cancelling an effective
input capacitance at a sensor input.
8) The apparatus of claim 7, wherein the electrode system includes
a feedback loop for connecting a sensor output to the sensor
input.
9) The apparatus of claim 8, wherein the feedback loop includes at
least one of: a resistor; a capacitor; and an inductor.
10) The apparatus of claim 9, wherein at least one of a resistor
and capacitor are adjustable, thereby allowing a current flow from
the sensor output to the sensor input to be controlled.
11) The apparatus of claim 9, wherein the feedback loop applies an
input capacitance cancelling signal to the sensor input to thereby
cancel any effective capacitance at the sensor input.
12) An apparatus for use in performing impedance measurements on a
subject, wherein the apparatus includes: a number of electrode
systems, and wherein each electrode system includes a signal
generator and sensor, the signal generator being for applying a
first signal to the subject and the sensor being for sensing a
second signal across the subject; and at number of leads for
connecting the measuring device to the electrode systems, each lead
including: at least two connections for connecting the measuring
device and the signal generator, and the measuring device and the
sensor; and a shield for each of the at least two connections, the
shields being electrically connected, and connected to a reference
voltage in each of the measuring device and the electrode
system.
13) The apparatus of claim 12, wherein the apparatus includes: at
least two electrode systems; a measuring device for controlling the
electrode systems to allow impedance measurements to be performed;
and at least two leads for connecting the measuring device to the
electrode systems.
14) The apparatus of claim 13, wherein the leads are arranged in
use to at least one of: extend from the measuring device in
different directions to thereby reduce inductive coupling
therebetween; and minimize the lead length.
15) An apparatus for use in performing impedance measurements on a
subject, wherein the apparatus includes: at least two electrode
systems, and wherein each electrode system includes a signal
generator and sensor, the signal generator being for applying a
first signal to the subject and the sensor being for sensing a
second signal across the subject; and a measuring device for
controlling the electrode systems to allow impedance measurements
to be performed; and at least two leads for connecting the
measuring device to the electrode systems, the leads being arranged
to at least one of: extend from the measuring device in different
directions to thereby reduce inductive coupling therebetween; and
minimize the lead length.
16) The apparatus of claim 15, wherein the apparatus includes: four
electrode systems; and four leads extending from the measuring
device in four different directions.
17) The apparatus of claim 15, wherein each lead includes: a first
cable for coupling the measuring device to the signal generator to
thereby allow the measuring device to control the signal generator
to apply a first signal to the subject; a second cable for coupling
the measuring device to the signal generator to thereby allow the
measuring device to determine a parameter relating to the first
signal applied to the subject; and a third cable for coupling the
measuring device to the sensor generator to thereby allow the
measuring device to determine a voltage measured at the
subject.
18) The apparatus of claim 15, wherein the electrode system
includes: a first substrate having the signal generator and sensor
mounted thereon; and, a second substrate having at least two
conductive pads mounted thereon, the conductive pads being for
coupling the signal generator and the sensor to a subject in use.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
use in performing impedance measurements on a subject.
DESCRIPTION OF THE PRIOR ART
[0002] The reference in this specification to any prior publication
(or information derived from it), or to any matter which is known,
is not, and should not be taken as an acknowledgment or admission
or any form of suggestion that the prior publication (or
information derived from it) or known matter forms part of the
common general knowledge in the field of endeavour to which this
specification relates.
[0003] One existing technique for determining biological indicators
relating to a subject, such as cardiac function, body composition,
and other health status indicators, such as the presence of oedema,
involves the use of bioelectrical impedance. This process typically
involves using a measuring device to measure the electrical
impedance of a subject's body using a series of electrodes placed
on the skin surface. Changes in electrical impedance measured at
the body's surface are used to determine parameters, such as
changes in fluid levels, associated with the cardiac cycle, oedema,
or the like.
[0004] Impedance measuring apparatus is sometimes sensitive to
external factors, including stray capacitances between the subject
and the local environment and the measurement apparatus, variations
in electrode/tissue interface impedances, also known as electrode
impedances, as well as stray capacitances and inductive coupling
between the leads used to connect the measuring device to the
electrodes.
SUMMARY OF THE PRESENT INVENTION
[0005] The present invention seeks to substantially overcome, or at
least ameliorate, one or more disadvantages of existing
arrangements.
[0006] In a first broad form the present invention seeks to
provides apparatus for use in performing impedance measurements on
a subject, wherein the apparatus includes a processing system for:
[0007] a) causing a first signal to be applied to the subject;
[0008] b) determining an indication of a second signal measured
across the subject; [0009] c) using the indication of the second
signal to determine if an unacceptable imbalance exists; and,
[0010] d) if an unacceptable imbalance exists: [0011] i)
determining a modified first signal in accordance with the
imbalance; and, [0012] ii) causing the modified first signal to be
applied to the subject to thereby allow at least one impedance
measurement to be performed.
[0013] Typically the processing system is for: [0014] a) comparing
the second signal to a threshold; and, [0015] b) determining if an
unacceptable imbalance exists depending on the results of the
comparison.
[0016] Typically the second signal includes voltages sensed at
respective second electrodes, and wherein the processing system is
for: [0017] a) determining the voltage sensed at each of the second
electrodes; [0018] b) determining an additive voltage; and, [0019]
c) determining the imbalance using the additive voltage.
[0020] Typically the additive voltage is a common mode signal.
[0021] Typically the processing system is for determining the
modified first signal so as to reduce the imbalance.
[0022] Typically first signals are applied to the subject via at
least two first electrodes, and wherein the processing system is
for modifying the first signal by modifying at least one of a phase
and a magnitude of at least one first signal applied to at least
one of the first electrodes.
[0023] Typically: [0024] a) the first signal is applied via first
electrodes coupled to first and second limbs of the subject; and,
[0025] b) the second signal is sensed via second electrodes coupled
to third and fourth limbs of the subject, the third and fourth
limbs being different to the first and second limbs.
[0026] Typically the processing system is for: [0027] a) causing
the first signal to be applied via first electrodes; [0028] b)
determining indications of second signals sensed at each of a
number of second electrodes; [0029] c) selecting second signals
sensed at selected ones of the second electrodes; and, [0030] d)
determining any imbalance using the selected second signals.
[0031] Typically the first signal includes voltages applied to the
subject using first electrodes and the second signal includes
voltages sensed at respective second electrodes.
[0032] Typically the processing system is for performing an
impedance measurement by: [0033] a) determining a sensed current
caused by applying the first signal to the subject; [0034] b)
determining a sensed voltage across the subject; and, [0035] c)
determining an impedance parameter using the sensed current and
voltage.
[0036] Typically the processing system is for: [0037] a)
determining a sensed current caused by applying the first signal to
the subject; [0038] b) comparing the sensed current to a threshold;
and, [0039] c) selectively halting the impedance measurement
process depending on the results of the comparison.
[0040] Typically the processing system is for: [0041] a)
determining a sensed current caused by applying the first signal to
the subject; and, [0042] b) using the sensed current in determining
the modified first signal.
[0043] Typically the processing system is for: [0044] a) causing a
first signal to be applied to the subject at a first frequency;
[0045] b) determining an indication of a second signal measured
across the subject; [0046] c) using the indication of the second
signal to determine any imbalance; [0047] d) if no unacceptable
imbalance exists, using at least the indication of the second
signal to determine at least one impedance value; [0048] e) if an
unacceptable imbalance exists: [0049] i) determining a modified
first signal in accordance with the imbalance; [0050] ii) causing
the modified first signal to be applied to the subject; [0051] iii)
determining an indication of a modified second signal measured
across the subject; and [0052] iv) repeating steps c) to e) for the
indication of the modified second signal; [0053] f) repeating steps
a) to e) for at least one second frequency.
[0054] Typically the processing system is for: [0055] a) causing
voltage drive signals to be applied to the subject via first
electrodes; [0056] b) determining sensed current signals caused by
the voltage drive signals; [0057] c) determining sensed voltages
measured via respective second electrodes; [0058] d) determining a
body centre voltage from the sensed voltages; [0059] e) determining
upper and lower impedances for the subject using the sensed current
signals, the voltage drive signals and the body centre voltage;
and, [0060] f) determining modified voltage drive signals using the
upper and lower impedances and an ideal current signal
indication.
[0061] Typically the voltage drive signals include first and second
voltage drive signals applied to the subject via respective first
electrodes, the first voltage drive signal having a first magnitude
and first phase, and the second voltage drive signal having a
second magnitude and second phase and wherein the processing system
is for determining the modified voltage drive signals by modifying
at least one of: [0062] a) the first phase; [0063] b) the first
magnitude; [0064] c) the second phase; and, [0065] d) the second
magnitude.
[0066] Typically the processing system is for: [0067] a) causing
the modified voltage drive signals to be applied to the subject;
[0068] b) determining sensed voltages measured via respective
second electrodes; [0069] c) determining if an unacceptable
imbalance exists using the sensed voltages; and, [0070] d) if an
unacceptable imbalance exists: [0071] i) determining further
modified voltage drive signals; and, [0072] ii) repeating steps (a)
to (d) until any imbalance is acceptable.
[0073] Typically the method includes performing impedance
measurements at multiple frequencies, in turn.
[0074] Typically the method includes: [0075] a) for a first
frequency: [0076] i) determining a modified first signal that
results in an acceptable imbalance; and, [0077] ii) causing an
impedance measurement to be performed using the modified first
signal; and, [0078] b) for a second frequency: [0079] i) causing a
first signal to be applied to the subject, the first signal being
based on the modified first signal determined for the first
frequency; and, [0080] ii) determining if an unacceptable imbalance
exists.
[0081] Typically the method includes: [0082] a) for a first
frequency: [0083] i) causing first and second voltage drive signals
to be applied to the subject via respective first electrodes;
[0084] ii) determining modified first and second voltage drive
signals that result in an acceptable imbalance, the first voltage
drive signal having a first magnitude and first phase, and the
second voltage drive signal having a second magnitude and second
phase; and, [0085] b) for a second frequency: [0086] i) causing
first and second voltage drive signals to be applied to the
subject, the first voltage drive signal having the first magnitude
and the first phase, and the second voltage drive signal having the
second magnitude and the second phase; and, [0087] ii) determining
if an unacceptable imbalance exists.
[0088] Typically the processing system is for: [0089] a) generating
control signals; [0090] b) transferring the control signals to at
least one signal generator thereby causing the first signal to be
applied to the subject; [0091] c) receiving an indication of the
one or more signals applied to the subject from the at least one
signal generator; [0092] d) receiving an indication of one or more
second signals measured across the subject from at least one
sensor; and, [0093] e) performing at least preliminary processing
of the indications to thereby allow impedance values to be
determined.
[0094] Typically the apparatus includes a differential amplifier
for amplifying second signals measured at each of two second
electrodes.
[0095] Typically the differential amplifier generates at least one
of: [0096] a) a differential voltage indicative of the voltage
measured at the second electrodes; and, [0097] b) a common mode
signal indicative of any imbalance.
[0098] Typically the apparatus includes at least one signal
generator for applying the first signal to the subject via a first
electrode.
[0099] Typically each signal generator is for: [0100] a) receiving
one or more control signals from the processing system; and, [0101]
b) amplifying the control signals to thereby generate the first
signal.
[0102] Typically each signal generator is for: [0103] a)
determining a sensed current caused by applying the first signal to
the subject; and, [0104] b) providing an indication of the sensed
current to the processing system.
[0105] Typically the apparatus includes at least two signal
generators, each signal generator being for connection to a
respective first electrode.
[0106] Typically the apparatus includes at least one sensor for
measuring the second signal via second electrodes.
[0107] Typically the apparatus includes at least two sensors, each
sensor being for connection to a respective second electrode.
[0108] Typically the apparatus includes a number of electrode
systems, and wherein each electrode system includes: [0109] a) a
sensor; and, [0110] b) a signal generator.
[0111] Typically electrode system includes: [0112] a) a first
substrate having the signal generator and sensor mounted thereon;
and, [0113] b) a second substrate having at least two conductive
pads mounted thereon, the conductive pads forming a first and a
second electrode for coupling the signal generator and the sensor
to a subject in use.
[0114] Typically the electrode system includes a capacitive
cancelling circuit for cancelling capacitive coupling between the
first and second electrodes.
[0115] Typically the capacitive cancelling circuit includes an
inverting amplifier for coupling a signal generator output to a
sensor input.
[0116] Typically the inverting amplifier applies a capacitive
cancelling signal to the sensor input to thereby cancel any
effective capacitance between the first electrode and the second
electrode.
[0117] Typically an inverting amplifier output is coupled to the
sensor input via at least one of: [0118] a) a resistor; [0119] b) a
capacitor; and, [0120] c) an inductor.
[0121] Typically at least one of a resistor and capacitor are
adjustable, thereby allowing a capacitive cancelling signal applied
to the sensor input to be controlled.
[0122] Typically the electrode system includes an input capacitance
cancelling circuit for cancelling an effective input capacitance at
a sensor input.
[0123] Typically the electrode system includes a feedback loop for
connecting a sensor output to the sensor input.
[0124] Typically the feedback loop includes at least one of: [0125]
a) a resistor; [0126] b) a capacitor; and, [0127] c) an
inductor.
[0128] Typically at least one of a resistor and capacitor are
adjustable, thereby allowing a current flow from the sensor output
to the sensor input to be controlled.
[0129] Typically the feedback loop applies an input capacitance
cancelling signal to the sensor input to thereby cancel any
effective capacitance at the sensor input.
[0130] Typically the apparatus includes: [0131] a) a number of
electrode systems, and wherein each electrode system includes a
signal generator and sensor; and, [0132] b) at number of leads for
connecting the measuring device to the electrode systems, each lead
including: [0133] i) at least two connections for connecting the
measuring device and the signal generator, and the measuring device
and the sensor; and, [0134] ii) a shield for each of the at least
two connections, the shields being electrically connected, and
connected to a reference voltage in each of the measuring device
and the electrode system.
[0135] Typically the apparatus includes: [0136] a) at least two
electrode systems, each electrode system including: [0137] i) a
signal generator for applying a first signal to the subject; [0138]
ii) a sensor for sensing a second signal across the subject; [0139]
iii) a first electrode for coupling the signal generator to the
subject; and, [0140] iv) a second electrode for coupling the sensor
to the subject; and, [0141] b) a measuring device for controlling
the electrode systems to allow impedance measurements to be
performed; and, [0142] c) at least two leads for connecting the
measuring device to the electrode systems.
[0143] Typically the leads are arranged in use to at least one of:
[0144] i) extend from the measuring device in different directions
to thereby reduce inductive coupling therebetween; and, [0145] ii)
minimise the lead length.
[0146] Typically the apparatus includes an interface for coupling
the processing system to a computer system, the processing system
being for: [0147] a) generating control signals in accordance with
commands received from the computer system; and, [0148] b)
providing data indicative of measured impedance values to the
computer system to allow impedance values to be determined.
[0149] Typically the first signal is includes two first signals
applied to the subject via at least two first electrodes, and the
second signal includes two second signals sensed at two second
electrodes.
[0150] In a second broad form the present invention seeks to
provides apparatus for use in performing impedance measurements on
a subject, wherein the apparatus includes a number of electrode
systems, and wherein each electrode system includes: [0151] a) a
first substrate having a signal generator and sensor mounted
thereon, the signal generator being for applying a first signal to
the subject and the sensor for sensing a second signal across the
subject; and, [0152] b) a second substrate having at least two
conductive pads mounted thereon, the conductive pads forming first
and second electrodes for coupling the signal generator and the
sensor to a subject in use.
[0153] Typically the electrode system includes a capacitive
cancelling circuit for cancelling capacitive coupling between the
drive and sense electrodes.
[0154] Typically the capacitive cancelling circuit includes an
inverting amplifier for coupling a signal generator output to a
sensor input.
[0155] Typically the inverting amplifier applies a capacitance
cancelling signal to the sensor input to thereby cancel any
effective capacitance between the drive electrode and the sense
electrode.
[0156] Typically an inverting amplifier output is coupled to the
sensor input via at least one of: [0157] a) a resistor; [0158] b) a
capacitor; and, [0159] c) an inductor.
[0160] Typically at least one of a resistor and capacitor are
adjustable, thereby allowing a capacitance cancelling signal
applied to the sensor input to be controlled.
[0161] Typically the electrode system includes an input capacitance
cancelling circuit for cancelling an effective input capacitance at
a sensor input.
[0162] Typically the electrode system includes a feedback loop for
connecting a sensor output to the sensor input.
[0163] Typically the feedback loop includes at least one of: [0164]
a) a resistor; [0165] b) a capacitor; and, [0166] c) an
inductor.
[0167] Typically at least one of a resistor and capacitor are
adjustable, thereby allowing a current flow from the sensor output
to the sensor input to be controlled.
[0168] Typically the feedback loop applies an input capacitance
cancelling signal to the sensor input to thereby cancel any
effective capacitance at the sensor input.
[0169] In a third broad form the present invention seeks to
provides apparatus for use in performing impedance measurements on
a subject, wherein the apparatus includes: [0170] a) a number of
electrode systems, and wherein each electrode system includes a
signal generator and sensor, the signal generator being for
applying a first signal to the subject and the sensor being for
sensing a second signal across the subject; and, [0171] b) at
number of leads for connecting the measuring device to the
electrode systems, each lead including: [0172] i) at least two
connections for connecting the measuring device and the signal
generator, and the measuring device and the sensor; and, [0173] ii)
a shield for each of the at least two connections, the shields
being electrically connected, and connected to a reference voltage
in each of the measuring device and the electrode system.
[0174] Typically the apparatus includes: [0175] a) at least two
electrode systems; [0176] b) a measuring device for controlling the
electrode systems to allow impedance measurements to be performed;
and, [0177] c) at least two leads for connecting the measuring
device to the electrode systems.
[0178] Typically the leads are arranged in use to at least one of:
[0179] a) extend from the measuring device in different directions
to thereby reduce inductive coupling therebetween; and, [0180] b)
minimise the lead length.
[0181] In a fourth broad form the present invention seeks to
provides apparatus for use in performing impedance measurements on
a subject, wherein the apparatus includes: [0182] a) at least two
electrode systems, and wherein each electrode system includes a
signal generator and sensor, the signal generator being for
applying a first signal to the subject and the sensor being for
sensing a second signal across the subject; and, [0183] b) a
measuring device for controlling the electrode systems to allow
impedance measurements to be performed; and, [0184] c) at least two
leads for connecting the measuring device to the electrode systems,
the leads being arranged to at least one of: [0185] i) extend from
the measuring device in different directions to thereby reduce
inductive coupling therebetween; and, [0186] ii) minimise the lead
length.
[0187] Typically the apparatus includes: [0188] a) four electrode
systems; and, [0189] b) four leads extending from the measuring
device in four different directions.
[0190] Typically each lead includes: [0191] a) a first cable for
coupling the measuring device to the signal generator to thereby
allow the measuring device to control the signal generator to apply
a first signal to the subject; [0192] b) a second cable for
coupling the measuring device to the signal generator to thereby
allow the measuring device to determine a parameter relating to the
first signal applied to the subject; and, [0193] c) a third cable
for coupling the measuring device to the sensor generator to
thereby allow the measuring device to determine a voltage measured
at the subject.
[0194] Typically the electrode system includes: [0195] a) a first
substrate having the signal generator and sensor mounted thereon;
and, [0196] b) a second substrate having at least two conductive
pads mounted thereon, the conductive pads being for coupling the
signal generator and the sensor to a subject in use.
[0197] In a fifth broad form the present invention seeks to
provides a method for use in performing impedance measurements on a
subject, wherein the method includes, in a processing system:
[0198] a) causing a first signal to be applied to the subject;
[0199] b) determining an indication of a second signal measured
across the subject; [0200] c) using the indication of the second
signal to determine any imbalance; and, [0201] d) if an imbalance
exists: [0202] i) determining a modified first signal in accordance
with the imbalance; and, [0203] ii) causing the modified first
signal to be applied to the subject to thereby allow at least one
impedance measurement to be performed.
[0204] In a sixth broad form the present invention seeks to
provides a method for use in performing impedance measurements on a
subject, wherein the method includes: [0205] a) providing a pair of
first and second electrodes on at least one wrist and at least one
ankle of the subject; [0206] b) coupling each pair of electrodes to
an electrode system, the electrode system including a signal
generator and sensor, the signal generator being for applying a
first signal to the subject via the first electrode and the sensor
being for sensing a second signal via the second electrode; [0207]
c) positioning a measuring device near the subject's knees, the
measuring device being for controlling the electrode systems to
allow impedance measurements to be performed; and, [0208] d)
coupling the measuring device to the electrode systems via
respective leads such that the leads extend from the measuring
device in different directions.
[0209] It will be appreciated that the broad forms of the invention
may be used individually or in combination, and may be used for
diagnosis of the presence, absence or degree of a range of
conditions and illnesses, including, but not limited to oedema,
pulmonary oedema, lymphodema, body composition, cardiac function,
and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0210] An example of the present invention will now be described
with reference to the accompanying drawings, in which:
[0211] FIG. 1 is a schematic diagram of an example of an impedance
measuring device;
[0212] FIG. 2 is a flowchart of an example of a process for
performing impedance measuring;
[0213] FIG. 3 is a schematic diagram of a second example of an
impedance measuring device;
[0214] FIG. 4 is a schematic diagram of an example of a computer
system;
[0215] FIG. 5 is a schematic of an example of the functionality of
the processing system of FIG. 3;
[0216] FIGS. 6A to 6C are a flowchart of a second example of a
process for performing impedance measurements;
[0217] FIG. 7A is a schematic diagram of an example of an electrode
system incorporating a signal generator and a sensor;
[0218] FIG. 7B is a schematic diagram illustrating cross electrode
capacitive coupling;
[0219] FIG. 7C is a schematic diagram of an example of a cross
electrode capacitance cancelling circuit;
[0220] FIG. 7D is a schematic diagram of an example of an input
capacitance cancelling circuit;
[0221] FIG. 8 is a schematic diagram of an example of lead
connections between the measuring device and the electrode system
of FIG. 7A;
[0222] FIG. 9 is a schematic diagram of an example of a lead
arrangement;
[0223] FIGS. 10A and 10B are schematic diagrams of examples of
electrode configurations used during balancing;
[0224] FIG. 10C is a schematic diagram of effective electrical
models for the electrode arrangements of FIGS. 10A and 10B;
and,
[0225] FIG. 11 is a flow chart of a further example of an impedance
measurement process.
[0226] FIG. 12A is a schematic diagram of an effective electrical
model of the body;
[0227] FIG. 12B is a schematic diagram of the complex voltages for
the electrical model of FIG. 12A when the voltage is balanced based
on the voltage magnitude only;
[0228] FIG. 12C is a schematic diagram of the complex voltages for
the electrical model of FIG. 12A when the voltage is balanced based
on the voltage magnitude and phase;
[0229] FIG. 12D is a schematic diagram of an effective electrical
model of the body;
[0230] FIG. 12E is a schematic diagram of the complex voltages for
the electrical model of FIG. 12D when the voltage is balanced based
on the voltage magnitude only; and,
[0231] FIG. 12F is a schematic diagram of the complex voltages for
the electrical model of FIG. 12D when the voltage is balanced based
on the voltage magnitude and phase.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0232] An example of apparatus suitable for performing an analysis
of a subject's bioelectric impedance will now be described with
reference to FIG. 1.
[0233] As shown the apparatus includes a measuring device 100
including a processing system 102, connected to one or more signal
generators 117A, 117B, via respective first leads 123A, 123B, and
to one or more sensors 118A, 118B, via respective second leads
125A, 125B. The connection may be via a switching device, such as a
multiplexer, although this is not essential.
[0234] In use, the signal generators 117A, 117B are coupled to two
first electrodes 113A, 113B, which therefore act as drive
electrodes to allow signals to be applied to the subject S, whilst
the one or more sensors 118A, 118B are coupled to the second
electrodes 115A, 115B, which act as sense electrodes, allowing
signals across the subject S to be sensed.
[0235] The signal generators 117A, 117B and the sensors 118A, 118B
may be provided at any position between the processing system 102
and the electrodes 113A, 113B, 115A, 115B, and may be integrated
into the measuring device 100. However, in one example, the signal
generators 117A, 117B and the sensors 118A, 118B are integrated
into an electrode system, or another unit provided near the subject
S, with the leads 123A, 123B, 125A, 125B connecting the signal
generators 117A, 117B and the sensors 118A, 118B to the processing
system 102.
[0236] It will be appreciated that the above described system is a
two channel device, used to perform a classical four-terminal
impedance measurement, with each channel being designated by the
suffixes A, B respectively. The use of a two channel device is for
the purpose of example only, as will be described in more detail
below.
[0237] An optional external interface 103 can be used to couple the
measuring device 100, via wired, wireless or network connections,
to one or more peripheral devices 104, such as an external database
or computer system, barcode scanner, or the like. The processing
system 102 will also typically include an I/O device 105, which may
be of any suitable form such as a touch screen, a keypad and
display, or the like.
[0238] In use, the processing system 102 is adapted to generate
control signals, which cause the signal generators 117A, 117B to
generate one or more alternating signals, such as voltage or
current signals of an appropriate waveform, which can be applied to
a subject S, via the first electrodes 113A, 113B. The sensors 118A,
118B then determine the voltage across or current through the
subject S, using the second electrodes 115A, 115B and transfer
appropriate signals to the processing system 102.
[0239] Accordingly, it will be appreciated that the processing
system 102 may be any form of processing system which is suitable
for generating appropriate control signals and at least partially
interpreting the measured signals to thereby determine the
subject's bioelectrical impedance, and optionally determine other
information such as relative fluid levels, or the presence, absence
or degree of conditions, such as oedema, lymphoedema, measures of
body composition, cardiac function, or the like.
[0240] The processing system 102 may therefore be a suitably
programmed computer system, such as a laptop, desktop, PDA, smart
phone or the like. Alternatively the processing system 102 may be
formed from specialised hardware, such as an FPGA (field
programmable gate array), or a combination of a programmed computer
system and specialised hardware, or the like, as will be described
in more detail below.
[0241] In use, the first electrodes 113A, 113B are positioned on
the subject to allow one or more signals to be injected into the
subject S. The location of the first electrodes will depend on the
segment of the subject S under study. Thus, for example, the first
electrodes 113A, 113B can be placed on the thoracic and neck region
of the subject S to allow the impedance of the chest cavity to be
determined for use in cardiac function analysis. Alternatively,
positioning electrodes on the wrist and ankles of a subject allows
the impedance of limbs and/or the entire body to be determined, for
use in oedema analysis, or the like.
[0242] Once the electrodes are positioned, one or more alternating
signals are applied to the subject S, via the first leads 123A,
123B and the first electrodes 113A, 113B. The nature of the
alternating signal will vary depending on the nature of the
measuring device and the subsequent analysis being performed.
[0243] For example, the system can use Bioimpedance Analysis (BIA)
in which a single low frequency signal (typically <50 kHz) is
injected into the subject S, with the measured impedance being used
directly in the assessment of relative intracellular and
extracellular fluid levels. In contrast Bioimpedance Spectroscopy
(BIS) devices utilise frequencies ranging from very low frequencies
(4 kHz) to higher frequencies (1000 kHz), and can use as many as
256 or more different frequencies within this range, to allow
multiple impedance measurements to be made within this range.
[0244] Thus, the measuring device 100 may either apply an
alternating signal at a single frequency, at a plurality of
frequencies simultaneously, or a number of alternating signals at
different frequencies sequentially, depending on the preferred
implementation. The frequency or frequency range of the applied
signals may also depend on the analysis being performed.
[0245] In one example, the applied signal is generated by a voltage
generator, which applies an alternating voltage to the subject S,
although alternatively current signals may be applied. In one
example, the voltage source is typically symmetrically arranged,
with each of the signal generators 117A, 117B being independently
controllable, to allow the signal voltage across the subject to be
varied.
[0246] A voltage difference and/or current is measured between the
second electrodes 115A, 115B. In one example, the voltage is
measured differentially, meaning that each sensor 118A, 118B is
used to measure the voltage at each second electrode 115A, 115B and
therefore need only measure half of the voltage as compared to a
single ended system.
[0247] The acquired signal and the measured signal will be a
superposition of voltages generated by the human body, such as the
ECG (electrocardiogram), voltages generated by the applied signal,
and other signals caused by environmental electromagnetic
interference. Accordingly, filtering or other suitable analysis may
be employed to remove unwanted components.
[0248] The acquired signal is typically demodulated to obtain the
impedance of the system at the applied frequencies. One suitable
method for demodulation of superposed frequencies is to use a Fast
Fourier Transform (FFT) algorithm to transform the time domain data
to the frequency domain. This is typically used when the applied
current signal is a superposition of applied frequencies. Another
technique not requiring windowing of the measured signal is a
sliding window FFT.
[0249] In the event that the applied current signals are formed
from a sweep of different frequencies, then it is more typical to
use a signal processing technique such as multiplying the measured
signal with a reference sine wave and cosine wave derived from the
signal generator, or with measured sine and cosine waves, and
integrating over a whole number of cycles. This process, known
variously as quadrature demodulation or synchronous detection,
rejects all uncorrelated or asynchronous signals and significantly
reduces random noise.
[0250] Other suitable digital and analogue demodulation techniques
will be known to persons skilled in the field.
[0251] In the case of BIS, impedance or admittance measurements are
determined from the signals at each frequency by comparing the
recorded voltage and the current through the subject. The
demodulation algorithm can then produce amplitude and phase signals
at each frequency.
[0252] As part of the above described process, the distance between
the second electrodes 115A, 115B may be measured and recorded.
Similarly, other parameters relating to the subject may be
recorded, such as the height, weight, age, sex, health status, any
interventions and the date and time on which they occurred. Other
information, such as current medication, may also be recorded. This
can then be used in performing further analysis of the impedance
measurements, so as to allow determination of the presence, absence
or degree of oedema, to assess body composition, or the like.
[0253] The accuracy of the measurement of impedance can be subject
to a number of external factors. These can include, for example,
the effect of capacitive coupling between the subject and the
surrounding environment, the leads and the subject, the electrodes,
or the like, which will vary based on factors such as lead
construction, lead configuration, subject position, or the like.
Additionally, there are typically variations in the impedance of
the electrical connection between the electrode surface and the
skin (known as the "electrode impedance"), which can depend on
factors such as skin moisture levels, melatonin levels, or the
like. A further source of error is the presence of inductive
coupling between different electrical conductors within the leads,
or between the leads themselves.
[0254] Such external factors can lead to inaccuracies in the
measurement process and subsequent analysis and accordingly, it is
desirable to be able to reduce the impact of external factors on
the measurement process.
[0255] One form of inaccuracy that can arise is caused by the
voltages across the subject being unsymmetrical, a situation
referred to as an "imbalance". Such a situation results in a
significant signal voltage at the subject's body centre, which in
turn results in stray currents arising from parasitic capacitances
between the subject's torso and the support surface on which the
subject is provided.
[0256] The presence of an imbalance, where the voltage across the
subject is not symmetrical with respect to the effective centre of
the subject, leads to a "common mode" signal, which is effectively
a measure of the signal at the subject S that is unrelated to the
subject's impedance.
[0257] To help reduce this effect, it is therefore desirable for
signals to be applied to the subject S that they result in a
symmetrical voltage about the subject's body centre. As a result, a
reference voltage within the subject S, which is equal to a
reference voltage of the measurement apparatus, will be close to
the effective body centre of the subject, as considered relative to
the electrode placement. As the measuring device reference voltage
is typically ground, this results in the body centre of the subject
S being as close to ground as possible, which minimises the overall
signal magnitude across the subject's torso, thereby minimising
stray currents.
[0258] In one example, a symmetrical voltage about the sensing
electrodes can be achieved by using a symmetrical voltage source,
such as a differential bidirectional voltage drive scheme, which
applies a symmetrical voltage to each of the drive electrodes 113A,
113B. However, this is not always effective if the contact
impedances for the two drive electrodes 113A, 113B are unmatched,
or if the impedance of the subject S varies along the length of the
subject S, which is typical in a practical environment.
[0259] In one example, the apparatus overcomes this by adjusting
the differential voltage drive signals applied to each of the drive
electrodes 113A, 113B, to compensate for the different electrode
impedances, and thereby restore the desired symmetry of the
voltages across the subject S. This process is referred to herein
as balancing and in one example, helps reduce the magnitude of the
common mode signal, and hence reduce current losses caused by
parasitic capacitances associated with the subject.
[0260] The degree of imbalance, and hence the amount of balancing
required, can be determined by monitoring the signals at the sense
electrodes 115A, 115B, and then using these signals to control the
signal applied to the subject via the drive electrodes 113A, 113B.
In particular, the degree of imbalance can be calculated by
determining an additive voltage from the voltages detected at the
sense electrodes 115A, 115B.
[0261] In one example process, the voltages sensed at each of the
sense electrodes 115A, 115B are used to calculate a first voltage,
which is achieved by combining or adding the measured voltages.
Thus, the first voltage can be an additive voltage (commonly
referred to as a common mode voltage or signal) which can be
determined using a differential amplifier.
[0262] In this regard, a differential amplifier is typically used
to combine two sensed voltage signals V.sub.a, V.sub.b, to
determine a second voltage, which in one example is a voltage
differential V.sub.a-V.sub.b across the points of interest on the
subject S. The voltage differential is used in conjunction with a
measurement of the current flow through the subject to derive
impedance values. However, differential amplifiers typically also
provide a "common mode" signal (V.sub.a+V.sub.b)/2, which is a
measure of the common mode signal.
[0263] Whilst differential amplifiers include a common mode
rejection capability, this is generally of only finite effect and
typically reduces in effectiveness at higher frequencies, so a
large common mode signal will produce an error signal superimposed
on the differential signal.
[0264] The error caused by common mode signals can be minimised by
calibration of each sensing channel. In the ideal case where both
inputs of a differential amplifier are perfectly matched in gain
and phase characteristics and behave linearly with signal
amplitude, the common mode error will be zero. In one example, the
two sensing channels of the differential amplifier are digitised
before differential processing. It is therefore straightforward to
apply calibration factors independently to each channel to allow
the characteristics to be matched to a high degree of accuracy,
thereby achieving a low common mode error.
[0265] Accordingly, by determining the common mode signal, the
applied voltage signals can be adjusted, for example by adjusting
the relative magnitude and/or phase of the applied signals, to
thereby minimise the common mode signal and substantially eliminate
any imbalance.
[0266] An example of the operation of the apparatus of FIG. 1 to
perform this will now be described with reference to FIG. 2.
[0267] At step 200, a first signal is applied to the subject S,
with a second signal measured across the subject S being determined
at step 210. This will typically be achieved using the techniques
outlined above. Accordingly, the processing system 102 will cause
the signal generators 117A, 117B to generate the first signal,
which is typically applied to the subject S via the first
electrodes 113A, 113B. Similarly the second signal will be sensed
by the sensors 118A, 118B, via the second electrodes 115A, 115B,
with an indication of the second signal being provided to the
processing system 102.
[0268] At step 220, an imbalance is determined by the processing
system 102 using the second signal sensed at the second electrodes
115A, 115B, which in one example represents a common mode
signal.
[0269] At step 230, the measuring device optionally adjusts the
first signal applied to the subject S, so as to reduce the
imbalance and hence the magnitude of the common mode signal. Thus,
the magnitude of the signal applied at either one of the first
electrodes 113A, 113B can be adjusted, for example by increasing or
decreasing the relative signal magnitudes and/or altering the
relative signal phases, so as to balance the signal within the
subject and centralise the position of the reference voltage within
the subject relative to the electrode positioning.
[0270] At step 240, the measuring device can then determine the
signal applied to the subject and the voltages measured at the
electrodes 113A, 113B, thereby allowing an impedance to be
determined at step 250.
[0271] As the position of the reference voltage within the subject
S is impedance dependent, the imbalance will typically vary
depending on the frequency of the applied signal. Accordingly, in
one example, it is typical to determine the imbalance and adjust
the applied signal at each applied frequency. However, this may
depend on the preferred implementation.
[0272] A specific example of the apparatus will now be described in
more detail with respect to FIG. 3.
[0273] In this example, the measuring system 300 includes a
computer system 310 and a separate measuring device 320. The
measuring device 320 includes a processing system 330 coupled to an
interface 321 for allowing wired or wireless communication with the
computer system 310. The processing system 330 may also be
optionally coupled to one or more stores, such as different types
of memory, as shown at 322, 323, 324, 325, 326.
[0274] In one example, the interface is a Bluetooth stack, although
any suitable interface may be used. The memories can include a boot
memory 322, for storing information required by a boot-up process,
and a programmable serial number memory 323, that allows a device
serial number to be programmed. The memory may also include a ROM
(Read Only Memory) 324, flash memory 325 and EPROM (Electronically
Programmable ROM) 326, for use during operation. These may be used
for example to store software instructions and to store data during
processing, as will be appreciated by persons skilled in the
art.
[0275] A number of analogue to digital converters (ADCs) 327A,
327B, 328A, 328B and digital to analogue converters (DACs) 329A,
329B are provided for coupling the processing system 330 to the
sensors 118A, 118B and the signal generators 117A, 117B, as will be
described in more detail below.
[0276] A controller (not shown), such as a microprocessor,
microcontroller or programmable logic device, may also be provided
to control activation of the processing system 330, although more
typically this is performed by software commands executed by the
processing system 330.
[0277] An example of the computer system 310 is shown in FIG. 4. In
this example, the computer system 310 includes a processor 400, a
memory 401, an input/output device 402 such as a keyboard and
display, and an external interface 403 coupled together via a bus
404, as shown. The external interface 403 can be used to allow the
computer system to communicate with the measuring device 320, via
wired or wireless connections, as required, and accordingly, this
may be in the form of a network interface card, Bluetooth stack, or
the like.
[0278] In use, the computer system 310 can be used to control the
operation of the measuring device 320, although this may
alternatively be achieved by a separate interface provided on the
measuring device 300. Additionally, the computer system can be used
to allow at least part of the analysis of the impedance
measurements to be performed.
[0279] Accordingly, the computer system 310 may be formed from any
suitable processing system, such as a suitably programmed PC,
Internet terminal, lap-top, hand-held PC, smart phone, PDA, server,
or the like, implementing appropriate applications software to
allow required tasks to be performed.
[0280] In contrast, the processing system 330 typically performs
specific processing tasks, to thereby reduce processing
requirements on the computer system 310. Thus, the processing
system typically executes instructions to allow control signals to
be generated for controlling the signal generators 117A, 117B, as
well as the processing to determine instantaneous impedance
values.
[0281] In one example, the processing system 330 is formed from
custom hardware, or the like, such as a Field Programmable Gate
Array (FPGA), although any suitable processing module, such as a
magnetologic module, may be used.
[0282] In one example, the processing system 330 includes
programmable hardware, the operation of which is controlled using
instructions in the form of embedded software instructions. The use
of programmable hardware allows different signals to be applied to
the subject S, and allows different analysis to be performed by the
measuring device 320. Thus, for example, different embedded
software would be utilised if the signal is to be used to analyse
the impedance at a number of frequencies simultaneously as compared
to the use of signals applied at different frequencies
sequentially.
[0283] The embedded software instructions used can be downloaded
from the computer system 310. Alternatively, the instructions can
be stored in memory such as the flash memory 325 allowing the
instructions used to be selected using either an input device
provided on the measuring device 320, or by using the computer
system 310. As a result, the computer system 310 can be used to
control the instructions, such as the embedded software,
implemented by the processing system 330, which in turn alters the
operation of the processing system 330.
[0284] Additionally, the computer system 310 can operate to analyse
impedance determined by the processing system 330, to allow
biological parameters to be determined.
[0285] Whilst an alternative arrangement with a single processing
system may be used, the division of processing between the computer
system 310 and the processing system 330 can provide some
benefits.
[0286] Firstly, the use of the processing system 330 allows the
custom hardware configuration to be adapted through the use of
appropriate embedded software. This in turn allows a single
measuring device to be used to perform a range of different types
of analysis.
[0287] Secondly, this vastly reduces the processing requirements on
the computer system 310. This in turn allows the computer system
310 to be implemented using relatively straightforward hardware,
whilst still allowing the measuring device to perform sufficient
analysis to provide interpretation of the impedance. This can
include for example displaying information such as relative fluid
levels, body composition parameters, a "Wessel" plot, or other
indicators, as well as using the impedance values to determine
parameters relating to cardiac function, the presence, absence or
degree of lymphoedema, oedema, or the like.
[0288] Thirdly, this allows the measuring device 320 to be updated.
Thus for example, if an improved analysis algorithm is created, or
an improved current sequence determined for a specific impedance
measurement type, the measuring device can be updated by
downloading new embedded software via flash memory 325 or the
external interface 321.
[0289] In use, the processing system 330 generates digital control
signals, indicative of the voltage drive signals V.sub.DA, V.sub.DB
to be applied via the drive electrodes 113A, 113B, which are
converted to analogue control signals by the DACs 329. The analogue
control signals are transferred to the signal generators 117,
allowing voltage drive signals V.sub.DA, V.sub.DB to be generated
by each of the signal generators 117A, 117B.
[0290] Analogue signals representing sensed current signals
I.sub.SA, I.sub.SB, induced by the voltage drive signals V.sub.DA,
V.sub.DB are received from the signal generators 117A, 117B and
digitised by the ADCs 328A, 328B. Similarly, analogue signals
representing sensed voltages V.sub.SA, V.sub.SB measured at the
second electrodes 115A, 115B are received from the sensors 118A,
118B and digitised by the ADCs 327A, 327B. The digital signals can
then be returned to the processing system 330 for preliminary
analysis.
[0291] In this example, a respective set of ADCs 327, 328, and DACs
329 are used for each of two channels, as designated by the
reference numeral suffixes A, B respectively. This allows each of
the signal generators 117A, 117B to be controlled independently and
for the sensors 118A, 118B to be used to detect signals from the
electrodes 115A, 115B separately. This therefore represents a two
channel device, each channel being designated by the reference
numerals A, B. It will be appreciated that similarly, voltage drive
signals V.sub.D, sensed current signals I.sub.S, and sensed voltage
signals V.sub.S can also similarly be identified by a suffix A, B,
representing the respective channel.
[0292] In practice, any number of suitable channels may be used,
depending on the preferred implementation. Thus, for example, it
may be desirable to use a four channel arrangement, in which four
drive and four sense electrodes are provided, with a respective
sense electrode and drive electrode pair 113, 115 being coupled to
each limb. In this instance, it will be appreciated that an
arrangement of eight ADCs 327, 328, and four DACs 329 could be
used, so each channel has respective ADCs 327, 328, and DACs 329.
Alternatively, other arrangements may be used, such as through the
inclusion of a multiplexing system for selectively coupling a
two-channel arrangement of ADCs 327, 328, and DACs 329 to a four
channel electrode arrangement, as will be appreciated by persons
skilled in the art.
[0293] Additional channels may also be provided for performing
additional measurements at other locations on the subject, such as
to allow direct measurement of voltages at the shoulder, the hip or
a variety of abdominal locations.
[0294] An example of the functionality implemented by the
processing system 330 will now be described with reference to FIG.
5. In this example the processing system 330 implements the
functionality using appropriate software control, although any
suitable mechanism may be used.
[0295] In this example the processing system 330 includes a timing
and control module 500, a messaging module 501, an analysis module
502, sine wave look up tables (LUTs) 503, 504, a current module
505, and a voltage module 506.
[0296] In use, the processing system 330 receives information
representing the frequency and amplitude of signals to be applied
to the subject S from the computer system 310, via the external
interface 321. The timing and control module 500 uses this
information to access the LUTs 503, 504, which in turn cause a
digital sine wave signal to be produced based on the specified
frequency and amplitude. The digital control signals are
transferred to the DAC's 329A, 329B, to thereby allow analogue
control signals indicative of the voltage drive signals V.sub.DA,
V.sub.DB to be produced.
[0297] Measured analogue voltage and current signals V.sub.SA,
V.sub.SB, I.sub.SA, I.sub.SB are digitised by the ADC's 327, 328
and provided to the current and voltage modules 505, 506. This
allows the processing system 330 to determine the current flow by
having the current module 505 determine the total current flow
through the subject using the two current signals I.sub.SA,
I.sub.SB, with an indication of this being provided to the analysis
module 502. The voltage module 506, which is typically in the form
of a differential voltage amplifier, or the like, operates to
determine a differential voltage, which is also transferred to the
analysis module 502, allowing the analysis module to determine
impedance values using the current and differential voltage
signals.
[0298] In addition to this, the voltage module 506 determines a
common mode signal, which is returned to the timing and control
module 500. This allows the timing and control module 500 to
determine any imbalance in the voltage sensed at the subject S,
which as mentioned above is indicative of the reference voltage not
being positioned centrally within the subject S, with respect to
the electrodes.
[0299] If the degree of imbalance is unacceptable the timing and
control module 500 can adjust the relative amplitude and/or phase
of the sine waves representing the voltage drive signals V.sub.DA,
V.sub.DB as will be described below, allowing a new differential
voltage, hence indication of any imbalance, to be determined.
[0300] Once the imbalance is determined to be acceptable the timing
and control module 500 can provide an indication of this to the
analysis module 502, allowing this to use appropriate analysis,
such as phase quadrature extraction, to determine a ratio and phase
difference for the measured impedance, based on the current flow
through the subject and the differential voltage signals. The ratio
and phase can then be transferred to the messaging module 510
allowing an indication of measured impedance to be provided to the
computer system 310 via the interface 321.
[0301] The processing system 330 may also implement a signal level
fault detection module 508. This monitors the magnitude of signals
applied to the subject to determine if these are within acceptable
threshold levels. If not, the fault detection module 508 can cause
a message to be transferred to the computer system 310 to allow the
process to be halted or to allow an alert to be generated.
[0302] During this process, any measurements made, including raw
current and voltage signals, may be stored in a suitable one of the
memories 322, 323, 324, 325, 326, or otherwise output, allowing
this to be used to monitor device operation. This can be used in
performing diagnostics, as well as calibration of the device.
[0303] An example of the process for performing impedance
measurements will now be described with reference to FIG. 6A to
6C.
[0304] At step 600 the computer system 310 is used to select an
impedance measurement type, with this triggering the computer
system 310 to cause desired instructions, such as embedded
software, to be implemented by the processing system 330. It will
be appreciated that this may be achieved in a number of manners,
such as by downloading required embedded software from the computer
system 310 to the processing system 330 or alternatively by having
the processing system 330 retrieve relevant embedded software from
internal memory or the like.
[0305] At step 610 the computer system 310 or the processing system
330 selects a next measurement frequency f.sub.i, allowing the
processing system 330 to generate a sequence of digital voltage
control signals at step 615, as described above. The digital
control signals are converted to analogue control signals
indicative of the voltage drive signals V.sub.DA, V.sub.DB using
the DACs 329A, 329B at step 620. This allows the analogue control
signals to be provided to each of the signal generators 117A, 117B
at step 625, causing each signal generator 117A, 117B to generate
respective voltage drive signals V.sub.DA, V.sub.DB and apply these
to the subject S at step 630, via the respective drive electrodes
113A, 113B.
[0306] At step 635 the voltage induced across the subject is
determined by having the sensors 118A, 118B sense voltages
V.sub.SA, V.sub.SB at the sense electrodes, 115A, 115B, with the
sensed voltage signals V.sub.SA, V.sub.SB being digitised by the
corresponding ADC 327A, 327B at step 640. At step 645 current
signals I.sub.SA, I.sub.SB, caused by application of the voltage
drive signals V.sub.DA, V.sub.DB, are determined using the signal
generators 117A, 117B. An indication of the current signals
I.sub.SA, I.sub.SB are transferred to the ADCs 328A, 328B for
digitisation at step 650.
[0307] At step 655 the digitised current and voltage signals
I.sub.SA, I.sub.SB, V.sub.SA, V.sub.SB are received by the
processing system 330 allowing the processing system 330 to
determine the magnitude of the applied current I.sub.S at step 660.
This may be performed using the current addition module 505 in the
above described functional example of FIG. 5, allowing the fault
detection module 508 to compare the total current flow I.sub.S
through the subject to a threshold at step 665. If it is determined
that the threshold has been exceeded at step 670 then the process
may terminate with an alert being generated at step 675.
[0308] This situation may arise, for example, if the device is
functioning incorrectly, or there is a problem with connections of
electrodes to the subject, such as if one is not in correct
electrical contact with the subject's skin. Accordingly, the alert
can be used to trigger a device operator to check the electrode
connections and/or device operation to allow any problems to be
overcome. It will be appreciated, that any suitable form of
corrective action may be taken such as attempting to restart the
measurement process, reconnecting the electrodes to the subject S,
reducing the magnitude of the current through the subject, or the
like.
[0309] At step 680 the processing system 330 operates to determine
a common mode voltage based on the amplitude of the sensed voltages
V.sub.SA, V.sub.SB sensed at each of the electrodes 115A, 115B, and
this is typically achieved using the voltage processing module 506
in the above functional example. The common mode voltage or common
mode signal is then used to determine any imbalance at step
685.
[0310] At step 690 an assessment is made as to whether the
imbalance is acceptable. This may be achieved in any one of a
number of ways, such as by comparing the amplitude of the common
mode signal to a threshold, or the like. The threshold will
generally be previously determined and stored in one of the
memories 324, 325, 326, for example during device manufacture or
calibration.
[0311] In the event that the imbalance is deemed to not be
acceptable, then at step 695 the processing system 330 modifies the
digital control signals representing the voltage drive signals
V.sub.DA, V.sub.DB to reduce the imbalance. This is typically
achieved by having the processing system 330 implement an algorithm
that adjusts the applied voltage drive signals V.sub.DA, V.sub.DB
to maintain the common mode voltage at the centre of the body as
close to the device reference voltage as possible. This is
generally achieved by adjusting the amplitude and/or phase of the
voltage drive signals V.sub.DA, V.sub.DB applied to the subject,
using the algorithm. The nature of this adjustment will depend on
the nature of the imbalance, and an example algorithm will be
described in more detail below.
[0312] The process can then return to step 620 to allow the
modified digital control signals to be converted to analogue
signals using DACs 324, with modified voltage drive signals
V.sub.DA, V.sub.DB being applied to the drive electrodes 113A,
113B. This process is repeated until an acceptable balance is
achieved.
[0313] Once an acceptable balance is achieved, the processing
system 330 operates to determine the differential voltage sensed
across the subject at step 700. In the functional example described
above with respect to FIG. 5, this can be achieved using the
differential voltage module 506.
[0314] At step 705 the processing module 330 operates to determine
ratio and phase signals, representing the impedance of the subject
S, at the applied frequency f.sub.i using the current and
differential voltage signals. In the above functional example, this
can be performed using the analysis module, and some form of signal
analysis, such as phase quadrature analysis, depending on the
preferred implementation. At step 710, an indication of the ratio
and phase signals are sent to the computer system 310 for further
processing.
[0315] Once this is completed the process may return to step 610 to
allow the process to be repeated at a next measurement frequency
f.sub.i otherwise if all required frequencies are complete, the
measurement process can terminate, allowing the computer system 310
to analyse the impedance measurements, and determine required
information, such as any biological indicators, impedance
parameters, or the like. The manner in which this is achieved will
depend on the type of analysis being performed.
[0316] Accordingly, it will be appreciated that by repeating the
above described process this allows a number of impedance
measurements to be performed over a range of different frequencies.
Furthermore, prior to at least one, and more typically, to each
measurement, a check can be performed to ensure that the common
mode of the subject and the device are approximately matched,
thereby reducing inaccuracies in the measurement procedure.
[0317] FIG. 7A is an example of an electrode system for a single
one of the channels, which incorporates both a drive electrode 113
and sense electrode 115.
[0318] The electrode system incorporates a first substrate 750,
such as a printed circuit board (PCB), or the like, having the
respective signal generator 117 and sensor 118 mounted thereon. The
general functionality of the signal generator 117 and sensor 118
are represented by the components shown. In practice a greater
number of components may be used in a suitable arrangement, as
would be appreciated by persons skilled in the art, and the
components shown are merely intended to indicate the functionality
of the signal generator and the sensor 117, 118.
[0319] The substrate 750 and associated components may be provided
in a suitable housing to protect them during use, as will be
appreciated by persons skilled in the art.
[0320] The signal generator 117 and the sensor 118 are coupled via
respective cables 761, 762 to conductive pads 763, 765, which may
be mounted on a second substrate 760, and which form the first and
second electrodes 113, 115, respectively. It will be appreciated
that in use, the cables 761, 762 may include clips or the like, to
allow the conductive pads to be easily replaced after use.
[0321] As will be appreciated, the conductive pads are typically
formed from a silver pad, having a conductive gel, such as
silver/silver chloride gel, thereon. This ensures good electrical
contact with the subject S.
[0322] The conductive pads may be mounted on the substrate 760, so
as to ensure that the conductive pads 763, 765 are positioned a set
distance apart in use, which can help ensure measurement
consistency. Alternatively the conductive pads 763, 765 can be
provided as separate disposable conductive pads, coupled to the
first substrate 750 by cables 761, 762. Other suitable arrangements
may also be used.
[0323] In one example, the substrate 760 is formed from a material
that has a low coefficient of friction and/or is resilient, and/or
has curved edges to thereby reduce the chances of injury when the
electrodes are coupled to the subject. The substrate 760 is also
typically arranged to facilitate electrical contact between the
conductive pads 763, 765 and the subject's skin at the typical
measurement sites, such as the wrist and ankle. This can be
achieved by providing a substrate 760 that adapts to, or is shaped
to conform with the irregular shapes and angles of the anatomy.
[0324] In this example, the signal generator 117 includes an
amplifier A.sub.1 having an input coupled to a cable 751. The input
is also coupled to a reference voltage, such as ground, via a
resistor R.sub.1. An output of the amplifier A.sub.1 is connected
via a resistor R.sub.2, to a switch SW, which is typically a CMOS
(complementary metal-oxide semiconductor) switch or a relay that is
used to enable the voltage source. The switch SW is controlled via
enabling signals EN received from the processing system 330 via a
cable 752.
[0325] The switch SW is in turn coupled via two resistors R.sub.3,
R.sub.4, arranged in series, and then, via the cable 761, to the
conductive pad 763. A second amplifier A.sub.2 is provided with
inputs in parallel with the first of the two series resistor
R.sub.3 and with an output coupled via a resistor R.sub.5, to a
cable 753.
[0326] It will be appreciated from the above that the cables 751,
752, 753 therefore forms the lead 123 of FIG. 1. A range of
different resistor values may be used, but in one example, the
resistors have values of R.sub.1=R.sub.2=R.sub.5=50.OMEGA., and
R.sub.3=R.sub.4=100.OMEGA..
[0327] The sensor 118 generally includes an amplifier A.sub.3
having an input connected via a resistor R.sub.6, to the cable 762.
The input is also coupled via a resistor R.sub.7, to a reference
voltage such as a ground. An output of the amplifier A.sub.3 is
coupled to a cable 754, via a resistor R.sub.7.
[0328] It will be appreciated from the above that the cable 754
therefore forms the lead 125 of FIG. 1. A range of different
resistor values may be used, but in one example, the resistors have
values of R.sub.6=100.OMEGA., R.sub.7=10M.OMEGA. and,
R.sub.8=50.OMEGA..
[0329] Optional power cables 755 can be provided for supplying
power signals +Ve, -Ve, for powering the signal generator 117 and
the sensor 118, although alternatively an on board power source
such as a battery, may be used. Additionally, a cable 756 may be
provided to allow an LED 757 to be provided on the substrate 750.
This can be controlled by the processing system 330, allowing the
operating status of the electrode system to be indicated.
[0330] Operation of the signal generator 117 and the sensor 118
will now be described in more detail. For the purpose of this
explanation, the voltage drive signal, current signal and sensed
voltage will be generally indicated as V.sub.D, I.sub.S, V.sub.S,
and in practice, these would be equivalent to respective ones of
the voltage drive signals, current signals and sensed voltages
V.sub.DA, V.sub.DB, I.sub.SA, I.sub.SB, V.sub.SA, V.sub.SB in the
example above.
[0331] In use, the amplifier A.sub.1 operates to amplify the
analogue voltage signal received from the DAC 329 and apply this to
the subject S via the cable 761, so that the applied voltage drive
signal V.sub.D drives a current signal I.sub.S through the subject
S. The voltage drive signal V.sub.D, will only be applied if the
switch SW is in a closed position and the switch SW can therefore
be placed in an open position to isolate the voltage source from
the subject S. This may be used if a pair of drive and sense
electrodes 113, 115 are being used to sense voltages only, and are
not being used to apply a voltage drive signal V.sub.D to the
subject S. Isolating the signal generator 117 from the drive
electrode 113 removes the unintended return current path(s) that
would otherwise be present due to the low output impedance of the
amplifier A.sub.1, thereby constraining current to flow only
between the two selected drive electrodes 113. Other techniques may
be used to achieve a similar effect, such as using an amplifier
incorporating a high impedance output-disable state.
[0332] The current signal I.sub.S being applied to the subject S is
detected and amplified using the amplifier A.sub.2, with the
amplified current signal I.sub.S being returned to the processing
system 330, along the cable 753 and via the ADC 328.
[0333] Similarly, the sensor 118 operates by having the amplifier
A.sub.3 amplify the voltage detected at the second electrode 115,
returning the amplified analogue sensed voltage signal V.sub.S
along the cable 754, to the ADC 327.
[0334] The cables 751, 752, 753, 754, 755, 756 may be provided in a
number of different configurations depending on the preferred
implementation. In one example, each of the cables 751, 752, 753,
754, 755, 756 are provided in a single lead L, although this is not
essential, and the cables could be provided in multiple leads, as
will be described in more detail below.
[0335] Another potential source of error is caused by cross
electrode capacitive coupling. As shown in FIG. 7B, the relative
proximity of the electrodes 113, 115 and the corresponding
connections 761, 762, results in an effective capacitance C.sub.DS,
between the output of the drive amplifier A.sub.1 and the input of
the sense amplifier A.sub.3. Accordingly, this will cause a
parasitic current flow between the amplifiers electrodes A.sub.1,
A.sub.3, which can in turn result in inaccuracies in the
measurements, particularly at higher frequencies.
[0336] To cancel the cross electrode capacitive coupling a cross
electrode capacitance cancelling circuit is provided, as shown in
FIG. 7C, which shows an equivalent circuit modelling the electrical
responsiveness of the electrodes 113, 115 in use.
[0337] In this example, the impedances of each electrode 113, 115
and the subject S are represented by respective impedances
Z.sub.113, Z.sub.115, Z.sub.S, formed by respective resistor and
capacitor arrangements. The cross electrode capacitance cancelling
circuit 770 is coupled to the output of the drive amplifier A.sub.1
and the input of the sense amplifier A.sub.3, and includes an
inverting amplifier A.sub.4, having an input coupled to the output
of the drive amplifier A.sub.1. The output of the inverting
amplifier is connected in series via a resistor R.sub.10 and a
capacitor C.sub.10, to the input of the sense amplifier
A.sub.3.
[0338] In this arrangement any signal output from the drive
amplifier A.sub.1 will be inverted and then applied to the input of
the sense amplifier A.sub.3. By selecting appropriate values for
the resistor R.sub.10 and a capacitor C.sub.10, this allows the
inverted signal to have a magnitude equal to the magnitude of any
signal resulting from the effective cross electrode capacitance
C.sub.DS.
[0339] In one example, the resistance and/or capacitance of the
resistor R.sub.10 and capacitor C.sub.10 respectively, can be
adjusted, through the use of suitable adjustable components, such
as a variable resistor or capacitor. This allows the magnitude
and/or phase of the inverted signal to be controlled so that it
effectively cancels the signal resulting from the effective cross
electrode capacitance C.sub.DS. It will be appreciated that
adjustment of the components may be performed during a calibration
process, which will typically include the complete electrode unit
together with its associated electrodes attached so that all
parasitic capacitances are accurately represented.
[0340] Accordingly, the cross electrode capacitance cancelling
circuit 770 provides an effective negative capacitance between the
drive electrode 113 and corresponding sense electrode 115, so that
a negative current flow occurs, thereby cancelling the parasitic
current. This therefore negates the effect of any capacitive
coupling between the drive and sense electrodes 113, 115.
[0341] The electrode system may also include an input capacitance
cancelling circuit, an example of which is shown in FIG. 7D.
[0342] In use, the sense electrodes 115 can capacitively couple to
the environment, which results in an effective input capacitance
C.sub.EI at the input of the sense amplifier A.sub.3. The effective
capacitance allows signal leakage from the input of the sense
amplifier to ground, thereby reducing the signal available at the
amplifier input.
[0343] Accordingly, in this example, an input capacitance
cancelling circuit 780 is provided which connects the positive
amplifier input of the sense amplifier A.sub.3 to the output of the
sense amplifier, via a resistor R.sub.11 and a capacitor C.sub.11.
This acts as a positive feedback loop, allowing a proportion of the
amplified signal to be returned to the amplifier input. This acts
to cancel the reduction in signal at the amplifier input that is
caused by the effective input capacitance C.sub.EI, and therefore
provides an effective negative capacitance that cancels the effect
of the effective input capacitance C.sub.EI at the amplifier input.
Again, the input capacitance cancelling circuit requires tuning,
which can be achieved during calibration by suitable adjustment of
the values of the resistor R.sub.11 and/or the capacitor
C.sub.11.
[0344] As briefly mentioned above, when separate leads 123, 125,
are used for the voltage signal V.sub.S and the current signal
I.sub.S, then inductive coupling between the leads 123, 125 can
result in EMFs being induced within the leads 123, 125. The
magnitude of the EMF is dependent on the degree of coupling between
the leads 123, 125 and hence their physical separation, and also
increases in proportion to the frequency and amplitude of the
current signal I.sub.S.
[0345] The EMF induced within the leads 123, 125 results in an
effective EMF across the input of the sensor 118. As a result, a
component of the sensed voltage signal V.sub.S is due to the
induced EMF, which in turn leads to inaccuracies in the determined
voltage signal V.sub.S and the current signal I.sub.S.
[0346] The effect of inductive coupling varies depending on the
physical separation of the leads 123, 125. Accordingly, in one
example, the effect of inductive coupling between leads can be
reduced by physically separating the leads as much as possible.
Thus, in one example, the cables 751, 752, 753, 754, 755, 756 are
provided in separate physically separated leads. However, a problem
with this arrangement is that the amount of inductive coupling will
vary depending on the physical lead geometry, which can therefore
vary between measurements. As a result, the magnitude of any
inductive coupling can vary, making this difficult to account for
when analysing the impedance measurements.
[0347] An alternative to using physically separate leads for each
of the cables 751, 752, 753, 754, 755, 756 is to use a single
combined lead L. The lead is formed so that the cables 751, 752,
753, 754, 755, 756 are held in a substantially constant relative
physical configuration. In one example, the leads L are formed so
as to provide a constant geometric arrangement by twisting each of
the respective cables together. However, alternative fabrication
techniques could be used such as making the leads from separate
un-insulated shielded cables that are over moulded to maintain
close contact.
[0348] As a result of the constant physical geometry, any EMF
induced along the leads 123, 125 is substantially constant,
allowing this to be accounted for during a calibration process.
[0349] Accordingly, when the measuring device 320 is initially
configured, and in particular, when the algorithms are generated
for analysing the voltage and current signals V.sub.S, I.sub.S, to
determine impedance measurements, these can include calibration
factors that take into account the induced EMF. In particular,
during the configuration process, a measuring device 320 can be
used to take measurements from reference impedances, with the
resulting calculations being used to determine the effect of the
induced EMF, allowing this to be subtracted from future
measurements.
[0350] A further issue with the lead arrangement is that of
capacitive coupling between the respective cables, as will now be
described with respect to FIG. 8. For the purpose of this example,
only cables 751, 753, 754 are shown for clarity.
[0351] In this example, the measuring device 320 is connected to
the PCB's 750A, 750B to provide connections for each of the
electrodes 113A, 113B, 115A, 115B. As also shown, each of the
cables 751, 753, 754 have respective shielding 851, 853, 854
provided thereon. The shielding is used to help prevent coupling
between the respective cables 751, 753, 754. It will therefore be
appreciated that the cables 751, 753, 754 are generally formed from
a shielded wire core. In practice, the shielded cables may be
50.OMEGA. transmission lines, which minimize signal transmission
distortion at high frequencies, thereby minimizing errors. In
addition to this, the shields 851, 853, 854 are typically
interconnected at each end, to a reference voltage such as a
ground, via respective connections 855, 856.
[0352] The use of shielded and grounded cables in this fashion
helps reduce the effect of capacitive coupling, helping to further
reduce inaccuracies in obtained measurements.
[0353] A further potential issue is that of inductive coupling
between the different leads L, as well as capacitive coupling
between the subject and the subject and the bed. In this regard,
parasitic capacitances allow high frequency currents to bypass the
intended current path through the body, resulting in measurement
errors. To take this into account, in one example, the leads L for
each electrode system can be physically separated as much as
possible and/or provided in an arrangement that minimizes lead
length in use. An example of an arrangement for achieving this will
now be described with respect to FIG. 9.
[0354] For the purpose of this example, the measuring system
provides four measuring channels, designated by the suffixes A, B,
C, D. It will be appreciated that this can be achieved by using a
modified version of the measuring device 320 of FIG. 3, in which
further ADCs 327, 328 and DACs 329 are provided as briefly
described above.
[0355] In this example, the subject S is laying on a bed 900, with
arms 931, 932 positioned by the subject's side, and the legs 933,
934 resting on a support 940, which incorporates the measuring
device 320. The support may be any form of support, but is
typically formed from moulded foam, or the like, which arranges the
subject with the measuring device 320 positioned substantially
between the subject's knees. The measuring device 320 is typically
incorporated into the support both to ensure accurate location of
the subject relative to the measuring device 320, and also to
protect the subject S from damage caused by rubbing or other impact
with a housing of the measuring device 320.
[0356] By providing a four channel arrangement, this allows a
respective electrode system to be mounted to each of the subject's
limbs. Thus, as shown, each limb 931, 932, 933, 934 has a
respective substrate 760 mounted thereon, to thereby provide a
drive and sense electrode 113, 115 on each wrist and ankle. The
electrodes 113, 115, are coupled to respective signal generators
and sensors mounted on the substrates 750, which are in turn
coupled to the measuring device 320 via respective leads LA, LB,
LC, LD.
[0357] The leads are arranged so that each lead LA, LB, LC, LD
extends away from the measuring device 320 in different directions,
thereby maximizing the physical separation of the leads and hence
helping to reduce any inductive coupling therebetween.
[0358] Additionally, the leads LA, LB, LC, LD are preferably
adapted to extend perpendicularly from both the measuring device
320 and the subject S, to thereby further reduce the effects of
capacitive coupling.
[0359] Furthermore, by having the measuring device 320 positioned
near the subject's knee, this places the measuring device 320
approximately equi-distant between the subject's wrists and ankles.
Thus, by arranging the measuring device 320 towards the lower end
of the bed 900, this reduces the length of leads LA, LB, LC, LD
needed to place the electrodes on the wrist and ankle of the
subject S, whilst maintaining substantially equal lead lengths,
which helps further reduce both inductive and capacitive coupling
effects. In this regard, the EMF originating from any inductive
coupling effect is proportional to the relevant lead length,
thereby equalising any effect for the different leads. Similarly,
capacitive coupling between the leads (ground) and the subject S,
which can create current shunt paths, is also minimized.
[0360] The above described arrangement is for the purpose of
example only, and it will be appreciated that in practice, any
suitable mechanisms for positioning the measuring device 320 in the
vicinity of the subject's upper legs (approximately midway between
the wrists and ankles) can be used. Thus, for example, this could
involve simply resting the measuring device 320 on the subject's
legs, providing a custom built support, or the like.
[0361] It will be appreciated that in this arrangement, by having
four first electrodes and four second electrodes positioned on the
limbs, this allows a range of different limb and/or whole body
impedance measurements to be performed.
[0362] The electrode configuration shown in FIG. 9 can be used to
perform an alternative balancing process, as will now be described
with reference to FIGS. 10A and 10B.
[0363] For the purpose of these examples, the subject S has arms
1031 1032, legs 1033, 1034 and a torso 1035 and the measuring
device 300 (not shown for clarity) is provided in a multi-channel
configuration similar to that shown in FIG. 9, with respective
pairs of drive and sense electrodes 113A, 115A; 113B, 115B; 113C,
115C; 113D, 115D provided on the wrist and ankles of the subject.
In FIGS. 10A and 10B, active electrodes only are shown.
[0364] In each example, a drive electrode configuration is used
that applies a drive signal to the drive electrodes 113B, 113D, so
that the signal passes through the arm 1031, the torso 1035 and the
leg 1033, as shown by the dotted line 1040.
[0365] In the example of FIG. 10A sense electrodes 115B, 115D
provided on the arm 1031 and on the leg 1033 are used to perform
the balancing. In contrast, in the arrangement of FIG. 10B, the
sense electrodes 115A, 115C provided on the contra-lateral limbs
1032, 1034 are used to perform balancing. This leads to different
effective electrical models for the balancing process, as shown in
FIG. 10C. The effective electrical model represents impedances
encountered by the drive signal, including impedances Z.sub.113B,
Z.sub.113D, Z.sub.1031, Z.sub.1035, Z.sub.1033, representing the
impedances of the drive electrode impedances 113B, 113D, the arm
1031, the torso 1035 and the leg 1033, respectively.
[0366] In the electrode configuration of FIG. 10A, the sense
electrodes are provided on the arm 1031 and the leg 1033, so that
voltages induced within the subject are effectively sensed at the
points between the drive electrodes 113B, 113D and the respective
limb 1031, 1033. The sensed voltages measured at the electrodes
115B, 115D are shown at V.sub.SB and V.sub.SD, respectively, and
these effectively take into account current flow through the arm
1031, the torso 1035 and the leg 1033.
[0367] When performing balancing, the drive signal is controlled to
minimise the common mode voltage such that
V.sub.SB.apprxeq.-V.sub.SD. In this configuration, the effective
ground reference voltage V.sub.R is electrically centred between
the sensed voltages V.sub.SB, V.sub.SD, such that the differences
.DELTA.V.sub.B, .DELTA.V.sub.D between the reference voltage
V.sub.R and each sensed voltage V.sub.SB, V.sub.SD is approximately
equal .DELTA.V.sub.B.apprxeq..DELTA.V.sub.D. This therefore takes
into account differences in impedances for the drive electrodes
113B, 113D, which typically arise from different contact
impedances, so that if one of the electrodes has a significantly
higher impedance than the other electrode, the signal applied to
the body after the electrodes is still symmetrical with respect to
the sense electrodes 115B, 115D.
[0368] As the arm impedance of the arm Z.sub.1031 is generally
higher than the torso impedance Z.sub.1035 and leg impedance
Z.sub.1033, then generally the signal voltage difference across the
arm 1031 is approximately equal to that across the torso 1035 and
leg 1033 combined. Consequently, the location of the reference
voltage V.sub.R does not generally occur at the geometric centre of
the subject's body, but rather occurs somewhere near the shoulder
region of the subject S. As a result, the subject's body centre
voltage Vc is not necessarily minimised by balancing according to
the sensed voltages V.sub.SB, V.sub.SD and there can be a
significant residual signal voltage V at the centre of the
subject's torso 1035, which corresponds to the subject's body
centre. Thus, the body centre voltage V.sub.C=V.noteq.V.sub.R. The
residual signal voltage will result in current flow due to
capacitive coupling between the subject and the environment, such
as the bed on which the subject is positioned. This in turn impacts
on the accuracy of the impedance measurements.
[0369] By contrast, the arrangement shown in FIG. 10B senses the
voltages in the subject using the sense electrodes 115A, 115C
provided on the contralateral limbs 1032, 1034. As there is no
current flow through the contralateral limbs 1032, 1034, the
contralateral limbs 1032, 1034 are effectively at the same voltage
along their entire length (i.e. isopotential). Accordingly, the
sense electrodes 115A, 115C effectively measures the voltages at
the point where the torso 1035 joins the arm 1031 and the leg 1033
as also shown in FIG. 10C.
[0370] In this instance if the balancing is performed, the
reference voltage V.sub.R is electrically centred between the
sensed voltages V.sub.SA, V.sub.SC, such that the difference
.DELTA.V.sub.A, .DELTA.V.sub.C between the reference voltage
V.sub.R and each sensed voltages V.sub.SA, V.sub.SC is
approximately equal .DELTA.V.sub.A.apprxeq..DELTA.V.sub.C. As the
voltage induced by the overall drive signal V.sub.D is measured
across the torso only, and as the upper and lower torso have
similar impedances, the reference voltage V.sub.R is positioned
midway along the torso 1035. As the reference voltage is typically
set to 0V, this minimises the amplitude of the signal voltage on
the torso 1035, as induced by the drive signal, which in turn
reduces the effect of capacitive coupling between the subject and
the bed.
[0371] Accordingly, whilst it will be appreciated that balancing
can be performed using the configuration of FIG. 10A, this
typically only takes into account variations in electrode
impedances of the drive electrodes 113B, 113D. Whilst this will
also generally reduce the overall potential of the subject's torso,
and hence reduce the effect of parasitic capacitances, it still
does not necessarily result in the voltages in the body being
balanced symmetrically with respect to the torso. Accordingly, in
one example it is preferred to use the electrode configuration
shown in FIG. 10B.
[0372] Thus, balancing can be performed for a range of different
electrode configurations, including sensing voltages on the same
limbs to which the voltage drive signals are applied. However, in
one example, the balancing is performed by passing signals along a
first limb, the torso and a second limb with the voltage signals
being measured by different third and fourth limbs. By measuring
the voltages on different limbs, this ensures that balancing is
performed about the subject's torso which in turn results in
reduced effect of capacitive coupling between the subject and the
environment.
[0373] It will be appreciated that in practice, there will always
be some parasitic current flow from the torso even when the
centre-body voltage is balanced. This is due to the relatively
large physical size of the torso. However, the process of balancing
the centre-body voltage attempts to minimise this error and also
enables a repeatable reference point to be achieved.
[0374] A further example measurement sequence will now be described
in more detail with reference to FIG. 11.
[0375] For the purpose of this example, it is again assumed that
the device is provided in a multi-channel configuration similar to
that shown in FIG. 9, with respective pairs of drive and sense
electrodes 113A, 115A; 113B, 115B; 113C, 115C; 113D, 115D provided
on the wrist and ankles of the subject. In this example, when a
measurement process is being performed, a drive electrode
configuration is selected at step 1100. This may involve for
example selecting the drive electrodes 113B, 113D, although any
suitable combination of drive electrodes may be used, depending on
the type of impedance measurement to be performed.
[0376] At step 1105 a next measurement frequency is selected, with
voltage drive signals V.sub.DB, V.sub.DD being applied to the
subject at 1110. This allows voltages V.sub.SA], V.sub.SB,
V.sub.SC, V.sub.SD at each sense electrode 115A, 115B, 115C, 115D
to be measured by the respective sensors 118A, 118B, 118C, 118D,
and current signals I.sub.SA, I.sub.SB, I.sub.SC, I.sub.SD,
resulting from the voltage drive signals V.sub.DB, V.sub.DD to be
measured by the signal generators 117A, 117B, 117C, 117D, with a
indication of the sensed voltage signals V.sub.SA, V.sub.SB,
V.sub.SC, V.sub.SD and current signals I.sub.SB, I.sub.SD being
transferred to the measuring device 320.
[0377] The indication of each of the signals is then typically
stored at step 1115. This information can be recorded for a number
of purposes and in general, it is easiest to simply record
indication of each of the signals, rather selectively record
information based on a measurement protocol.
[0378] By recording all signals, including all four sensed current
and sensed voltage signals, this also allows a single measurement
collection protocol to be performed for a variety of different
purposes. The recorded data can then be subsequently analysed in a
variety of different manners, depending on the intended measurement
to be performed. Thus, for example, recorded data could be analysed
to provide information regarding body composition, the presence,
absence or degree of oedema, or the like.
[0379] At step 1120 the measuring device 320 determines if the
balance is acceptable. Thus, for example, if the voltage drive
signals V.sub.DB, V.sub.DD are being applied via the electrodes
113B, 113D, the measuring device 320 will select the sensed
voltages V.sub.SA, V.sub.SC, at the sense electrodes 115A, 115C
thereby allowing balancing to be assessed, in a manner similar to
that described above. In this instance, an additive voltage
V.sub.SA+V.sub.SC will be determined based on the sensed voltages
V.sub.SA, V.sub.SC. The additive voltage will be compared to a
threshold, and if this is below the threshold, this indicates that
the balancing is acceptable.
[0380] In the event that the balancing is not acceptable, then the
voltage drive signals V.sub.DB, V.sub.DD applied to the subject S
are modified at step 1125. The manner in which the signals are
adjusted can depend on the preferred implementation. In one
example, the adjustment is performed based on the results of the
measurements performed at step 1110.
[0381] Thus, for example, the sensed voltages V.sub.SA, V.sub.SC
can be used to determine a body centre voltage V.sub.C. The sensed
current signals I.sub.SB, I.sub.SD, and voltage drive signals
V.sub.DB, V.sub.DD, applied via each drive electrode 113B, 113D are
used together with the body centre voltage V.sub.C to determine
upper and lower impedances Z.sub.upper, Z.sub.lower, which
represent the impedance of the subject's body and the drive
electrodes 113B, 113D on either side of the body centre. The upper
and lower impedances Z.sub.upper, Z.sub.lower can then be used to
determine the modified signals, based on a preferred current flow
through the subject.
[0382] An example calculation is shown in more detail below. In
this example, the body centre voltage V.sub.C is based on:
V.sub.C=(V.sub.SA+V.sub.SC)/2 (1)
[0383] A current flow through the subject is then determined based
on:
I=(I.sub.SB-I.sub.SD)/2 (2) [0384] where: I.sub.SB=sensed current
flow caused by positive voltage drive signal V.sub.DB applied to
electrode 113B [0385] I.sub.SD=sensed current flow caused by
negative voltage drive signal V.sub.DD applied to electrode
113D
[0386] This allows an impedance to be determined for the upper and
lower portions of the subject, where:
Z.sub.upper=(V.sub.DB-V.sub.C)/I (3)
Z.sub.lower=(V.sub.DD-V.sub.C)/I (4) [0387] where:
Z.sub.upper=upper body and drive electrode 113B impedance [0388]
Z.sub.lower=lower body and drive electrode 113D impedance
[0389] Following this, an ideal current value I.sub.ideal
(typically set to 90 .mu.A RMS to ensure subject safety) is used to
determine predicted voltage drive signals that will result in a
balanced measurement arrangement, using the equation:
V.sub.DB predicted=I.sub.ideal.times.Z.sub.upper (5)
V.sub.DD predicted=I.sub.ideal.times.Z.sub.lower (6) [0390] where:
V.sub.DB predicted=predicted ideal voltage drive signal for
electrode 113B [0391] V.sub.DD predicted=predicted ideal voltage
drive signal for electrode 113D
[0392] Thus, it will be appreciated that in this example, the
modified voltage drive signals applied to the subject S are the
predicted ideal voltage V.sub.DB predicted, V.sub.DD predicted. The
above described example calculation is for the purpose of example
only, and alternative calculations may be used.
[0393] In one example, the calculations are performed on the basis
of the magnitude of the signals only. This is because the magnitude
of the voltage at the body centre will have the greatest impact on
leakage current between the subject and the environment.
[0394] However, balancing the magnitude only can lead to phase
differences between the drive signals, which in turn can lead to
the body centre voltage V.sub.C including an imaginary component.
Examples of this will now be described with reference to FIGS. 12A
to 12F.
[0395] In the example of FIG. 12B, the voltages are shown based on
the equivalent circuit of FIG. 12A, in which the subject is
represented by body impedances Z.sub.B1, Z.sub.B2, positioned
either side of the body centre. Electrode impedances are shown as
part of the body impedances, with drive voltages V.sub.DB, V.sub.DD
being applied directly to the body impedances Z.sub.B1, Z.sub.B2 as
shown.
[0396] As shown in FIG. 12B, if drive voltages V.sub.DB, V.sub.DD
including only real components are applied, then the complex nature
of the body impedances Z.sub.B1, Z.sub.B2, will result in a phase
shift in the voltages V.sub.ZB1, V.sub.ZB2 across the body
impedances Z.sub.B1, Z.sub.B2. As a result, there exists an
imaginary component to the body centre voltage. This residual
complex component to the body centre voltage can lead to a leakage
current from the body as well as extra common mode error in the
sensed voltage signals, thereby making it undesirable.
[0397] However, in the example of FIG. 12C, if drive voltages
V.sub.DB, V.sub.DD include imaginary components, representing a
respective phase difference between the applied signal, then this
ensures that the phase of the voltages at the body centre are
matched. This ensures that the magnitude of the body centre voltage
V.sub.C, is minimised both in respect of the real and imaginary
components.
[0398] An example of this scenario in which electrode impedances
Z.sub.113B, Z.sub.113D, for the drive electrodes 113B, 113D are
taken into account are shown in FIGS. 12D to 12F. Again, it can be
seen that introducing a suitable phase change in the drive voltage
signals V.sub.DB, V.sub.DD can result in a body centre voltage that
is balanced in respect of both real and imaginary components.
[0399] Accordingly, in another example, the balancing procedure can
be performed by representing the voltage signals as complex numbers
representing both the magnitude and phase of the voltage signals,
and by using a complex representation of the impedance. In this
instance, this ensures that both the magnitude and phase of the
voltage signals are balanced, thereby ensuring a minimal body
centre voltage.
[0400] In general, when modifying the phase of the applied voltage
drive signals, the half body impedances are assumed to have
symmetrical phase shift relative to the drive. Thus an impedance
vector difference of 20.degree. will be resolved as +10.degree. at
one drive and -10.degree. at the second drive. By keeping the
drives as symmetrical as possible, any leakage current induced by
the capacitance of each limb is equalised and thus halved. However,
this is not essential, and any method of modifying the phase may be
used.
[0401] Following determination of the modified voltage drive
signals, steps 1110 to 1120 are repeated using the modified voltage
drive signals, with further modified voltage drive signals being
calculated until an acceptable balance situation results. It will
be appreciated that the number of iterations required to reach an
acceptable balance will depend on how close to a balanced situation
the initial drive signals are.
[0402] Whilst, voltage drive signals V.sub.DB, V.sub.DD having
equal magnitudes and/or phase could initially be applied, so that
V.sub.DB=-V.sub.DD, this can lead to a relatively large number of
different modified signals being tried until a balance condition is
reached. As the frequency of the voltage drive signal changes, the
body impedance will also change. Accordingly, in one example, for a
given frequency f.sub.i+1 the initially applied drive signals
V.sub.DB(f.sub.+1), V.sub.DD(f.sub.i+1) are calculated based on the
signals V.sub.DB predicted(f.sub.i), V.sub.DD predicted(f.sub.i)
determined for a previous frequency f.sub.i. Thus, the signals
V.sub.DB predicted(f.sub.i), V.sub.DD predicted(f.sub.i) are used
to calculate Z.sub.upper(f.sub.i), Z.sub.lower(f.sub.i). The
complex representation of Z.sub.upper(f.sub.i),
Z.sub.lower(f.sub.i) are used to determine Z.sub.upper(f.sub.i+1),
Z.sub.lower(f.sub.i+1) which are in turn used together with the
ideal current to calculate initial values for V.sub.DB
predicted(f.sub.i+1), V.sub.DD predicted(f.sub.i+1). These values
are used as the initial signals applied to the subject at step 1110
for the next frequency f.sub.i+1.
[0403] By using the balance condition determined for a previous
frequency as the initial starting point for the balancing algorithm
at a next frequency, this significantly reduces the number of
iterations required to achieve a balance condition in which
V.sub.C.apprxeq.0. Typically, using this technique, the balance
condition can be determined to less than 0.1% error within three
iterations.
[0404] Thus, the first iteration with the voltage drive signals
V.sub.DB(f.sub.i+1), V.sub.DD(f.sub.i+1) based on the previously
determined modified signals V.sub.DB predicted(f.sub.i), V.sub.DD
predicted(f.sub.i) typically results in a body centre voltage
V.sub.C that is within 10% of that required. Thus, the common mode
signal voltage at body centre has a magnitude that is approximately
10% of the signal voltage sensed between V.sub.SA, V.sub.SC. For
the second iteration, the voltage drive signals V.sub.DB, V.sub.DD
can be set to achieve V.sub.C to within 1.0% and the third
iteration achieves 0.1% error.
[0405] This can therefore dramatically reduce the time required for
a complete frequency sweep. The measurement time can be further
optimised by taking into account the amplitude of noise on the
measurements. Measurement time is dependent on the number of
samples required to achieve the desired accuracy. Increased noise
requires more samples, which takes more time. Therefore, if the
number of samples is optimised according to measured noise level,
measurement times can be further reduced (from what would otherwise
need to be a default sample number).
[0406] Once a balance is achieved, the measurements recorded at
step 1115 can be used to calculate impedance values at step 1130.
It is then assessed whether all frequencies are complete and if not
the process returns to step 1105 to select a next measurement
frequency. Otherwise it is determined if all drive configurations
are complete and if not the process returns to step 1100 to allow
an alternative drive configuration to be selected.
[0407] Otherwise the process finishes at step 1145, allowing any
determined impedance values to be provided to the processing system
310 for subsequent analysis.
[0408] Persons skilled in the art will appreciate that numerous
variations and modifications will become apparent. All such
variations and modifications which become apparent to persons
skilled in the art, should be considered to fall within the spirit
and scope that the invention broadly appearing before
described.
[0409] For example, two different approaches to balancing are
described above. In the first example, the balancing is performed
using sense electrodes attached to the same limbs as the drive
electrodes, whereas in the second example, the sense electrodes
used for balancing are attached to contralateral limbs. In one
example, sense and drive electrode are provided on all limbs,
allowing balancing to be performed in a similar manner using any
suitable combination of drive and sense electrodes. The electrode
combinations used may depend on the impedance measurement being
performed.
[0410] Additionally features from different examples above may be
used interchangeably or in conjunction, where appropriate. Thus,
for example, a range of different techniques are described for
minimising errors and these can be used independently of each
other, or in conjunction, depending on the particular
implementation.
[0411] Furthermore, whilst the above examples have focussed on a
subject S such as a human, it will be appreciated that the
measuring device and techniques described above can be used with
any animal, including but not limited to, primates, livestock,
performance animals, such as race horses, or the like.
[0412] The above described processes can be used for diagnosing the
presence, absence or degree of a range of conditions and illnesses,
including, but not limited to oedema, lymphoedema, body
composition, or the like.
* * * * *