U.S. patent application number 12/867224 was filed with the patent office on 2011-03-10 for analysing impedance measurements.
Invention is credited to Richelle Leanne Gaw.
Application Number | 20110060239 12/867224 |
Document ID | / |
Family ID | 40566413 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110060239 |
Kind Code |
A1 |
Gaw; Richelle Leanne |
March 10, 2011 |
ANALYSING IMPEDANCE MEASUREMENTS
Abstract
A method for use in analysing impedance measurements performed
on a subject, the method including, in a processing system
determining at least one impedance value indicative of the
impedance of at least one leg segment of the subject; and
determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least one leg segment and being used in the assessment of venous
insufficiency.
Inventors: |
Gaw; Richelle Leanne;
(Greenslopes, AU) |
Family ID: |
40566413 |
Appl. No.: |
12/867224 |
Filed: |
January 9, 2009 |
PCT Filed: |
January 9, 2009 |
PCT NO: |
PCT/US09/00163 |
371 Date: |
November 5, 2010 |
Current U.S.
Class: |
600/547 |
Current CPC
Class: |
H04W 72/0486 20130101;
H04W 72/048 20130101; H04W 28/08 20130101 |
Class at
Publication: |
600/547 |
International
Class: |
A61B 5/053 20060101
A61B005/053 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 22, 2008 |
US |
12010150 |
Claims
1) A method for use in analysing impedance measurements performed
on a subject, the method including, in a processing system: a)
determining at least one impedance value indicative of the
impedance of at least one leg segment of the subject; and, b)
determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least one leg segment and being used in the assessment of venous
insufficiency.
2) A method according to claim 1, wherein the method includes, in a
processing system: a) comparing the indicator to a reference; and,
b) providing an indication of the results of the comparison to
allow determination of a presence, absence or degree of venous
insufficiency.
3) A method according to claim 1, wherein the method includes, in a
processing system: a) determining a first indicator value with the
subject in a first orientation; b) determining a second indicator
value with the subject in a second orientation; and, c) determining
an indicator change based on a difference between the first and
second fluid indicator values, the indicator change being used in
the assessment of venous insufficiency.
4) A method according to claim 3, wherein the method includes, in
the processing system: a) comparing the indicator change to a
reference; and, b) providing an indication of the results of the
comparison to allow determination of a presence, absence or degree
of venous insufficiency.
5) A method according to claim 1, wherein the method includes, in a
processing system, determining an index using the at least one
impedance value, the index being indicative of a ratio of
extracellular to intracellular fluid levels in the at least leg
segment, the index being used in the assessment of venous
insufficiency.
6) A method according to claim 5, wherein the method includes, in
the processing system: a) comparing the index to a reference; and,
b) providing an indication of the results of the comparison to
allow determination of a presence, absence or degree of venous
insufficiency.
7) A method according to claim 6, wherein the method includes,
diagnosing the presence of venous insufficiency if: a) an indicator
is less than first reference; and, b) an index is greater than a
second reference.
8) A method according to claim 5, wherein the method includes, in a
processing system: a) determining a first index value with the
subject in a first orientation; b) determining a second index value
with the subject in a second orientation; and, c) determining an
index change based on a difference between the first and second
fluid index values, the index change being used in the assessment
of venous insufficiency.
9) A method according to claim 1, wherein the method includes, in
the processing system: a) determining a first indicator value with
the subject in a first orientation; b) after positioning the
subject in a second orientation for a predetermined time period,
determining a second indicator value with the subject in the first
orientation; and, c) determining a difference between the first and
second fluid indicator values, the difference being used in the
assessment of venous insufficiency.
10) A method according to claim 9, wherein the method includes, in
the processing system: a) monitoring the difference; b) determining
the time taken for the difference fall below a reference; and, c)
providing an indication of the time taken to allow determination of
a presence, absence or degree of venous insufficiency.
11) A method according to claim 1, wherein the method includes, in
a processing system: a) determining a pre-treatment indicator value
prior to treatment of the subject; b) determining a post-treatment
indicator value following treatment of the subject for venous
insufficiency; and, c) determining an indicator change based on a
difference between the pre-treatment and post-treatment indicator
values, the indicator change being used in the assessment of venous
insufficiency.
12) A method according to claim 11, wherein the method includes,
diagnosing the presence of venous insufficiency if the indicator
change is greater than a reference.
13) A method according to claim 1, wherein the method includes, in
the processing system, using a reference that is at least one of:
a) an indicator or index determined for another limb of the
subject; b) a reference determined from a sample population; and,
c) a previous indicator or index determined for the subject.
14) A method according to claim 1, wherein the method includes, in
the processing system, displaying at least one of: a) an indicator;
b) an index ratio; c) an index; d) an indicator change; e) an index
change; f) one or more impedance parameter values; and, g) results
of a comparison.
15) A method according to claim 1, wherein the at least one
impedance value is measured at a measurement frequency of at least
one of: a) less than 100 kHz; b) less than 50 kHz; and, c) less
than 10 kHz.
16) A method according to claim 16, wherein the method includes, in
the processing system, using the at least one impedance measurement
as an estimate of a resistance of the subject at a zero measurement
frequency.
17) A method according to claim 1, wherein the method includes
measuring at least one second impedance value, the at least one
second impedance value being measured at a measurement frequency of
at least one of: a) greater than 200 kHz; b) greater than 500 kHz;
and, c) greater than 1000 kHz.
18) A method according to claim 16, wherein the method includes, in
the processing system, using the at least one second impedance
measurement as an estimate of a resistance of the subject at an
infinite measurement frequency.
19) A method according to claim 1, wherein the method includes, in
the processing system: a) determining a plurality of impedance
values; and, b) determining at least one impedance parameter value
from the plurality of impedance values.
20) A method according to claim 19, wherein the impedance parameter
values include at least one of: R.sub.0 which is the resistance at
zero frequency; R.sub..infin. which is the resistance at infinite
frequency; and, Z.sub.c which is the resistance at a characteristic
frequency.
21) A method according to claim 20, wherein the method includes, in
the processing system: a) determining values for impedance
parameters R.sub.0 and R.sub..infin. from the measured impedance
values; and, b) calculating the index (I) using the equation: I = R
.infin. R 0 - R .infin. ##EQU00009##
22) A method according to claim 20, wherein the method includes, in
the processing system, determining the parameter values using the
equation: Z = R .infin. + R 0 - R .infin. 1 + ( j.omega. .tau. ) (
1 - .alpha. ) ##EQU00010## where: Z is the measured impedance at
angular frequency .omega., .tau. is a time constant, and .alpha.
has a value between 0 and 1.
23) A method according to claim 1, wherein the method includes, in
the computer system, causing the impedance measurements to be
performed.
24) A method according to claim 18, wherein the method includes, in
the computer system: a) causing one or more electrical signals to
be applied to the subject using a first set of electrodes; b)
measuring electrical signals across a second set of electrodes
applied to the subject in response to the applied one or more
signals; and, c) determining from the applied signals and the
measured signals at least one measured impedance value.
25) Apparatus for use in analysing impedance measurements performed
on a subject, the apparatus including a processing system for: a)
determining at least one impedance value, indicative of the
impedance of at least leg segment of the subject; b) determining an
indicator using the at least one impedance value, the indicator
being indicative of extracellular fluid levels in the at least leg
segment and being used in the assessment of venous
insufficiency.
26) Apparatus according to claim 25, wherein the apparatus includes
a processing system for: a) causing one or more electrical signals
to be applied to the subject using a first set of electrodes; b)
measuring electrical signals across a second set of electrodes
applied to the subject in response to the applied one or more
signals; and, c) determining from the applied signals and the
measured signals at least one measured impedance value.
27) Apparatus according to claim 26, wherein the apparatus
includes: a) a signal generator for generating electrical signals;
and, b) a sensor for sensing electrical signals.
28) A method for use in assessing the presence, absence or degree
of venous insufficiency, the method including, in a processing
system: a) determining at least one impedance value, indicative of
the impedance of at least one leg segment of the subject; and, b)
determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least leg segment and being used in the assessment of venous
insufficiency.
29) A method for use in analysing impedance measurements performed
on a subject, the method including, in a processing system: a)
determining at least one impedance value indicative of the
impedance of at least one body segment of the subject; b)
determining an indicator using at least one impedance value, the
indicator being indicative of extracellular fluid levels in the at
least one body segment; c) determining an index using at least one
impedance value, the index being indicative of a ratio of
extracellular to intracellular fluid levels in the at least one
body segment; d) comparing the indicator to a first reference; e)
comparing the index to a second reference; and, f) providing an
indication of the results of the comparisons.
30) A method according to claim 29, wherein the method includes, in
the processing system, determining the indicator using an impedance
measurement performed at a single low frequency.
31) A method according to claim 29, wherein the at least one
impedance measurement is measured at a measurement frequency of at
least one of: a) less than 100 kHz; b) less than 50 kHz; and, c)
less than 10 kHz.
32) A method according to claim 31, wherein the method includes, in
the processing system, using the at least one impedance measurement
as an estimate of a resistance of the subject at a zero measurement
frequency.
33) A method according to claim 29, wherein the method includes
measuring at least one second impedance value, the at least one
second impedance value being measured at a measurement frequency of
at least one of: a) greater than 200 kHz; b) greater than 500 kHz;
and, c) greater than 1000 kHz.
34) A method according to claim 33, wherein the method includes, in
the processing system, using the at least one second impedance
measurement as an estimate of a resistance of the subject at an
infinite measurement frequency.
35) A method according to claim 29, wherein the method includes, in
the processing system: a) determining at least two impedance
values; and, b) determining at least one impedance parameter value
from the at least two impedance values.
36) A method according to claim 35, wherein the method includes, in
the processing system: a) at each of three frequencies, determining
first and second parameter values for first and second impedance
parameters relating to the impedance of at least one body segment
of the subject; b) solving simultaneous equations representing a
circle defined with respect to the first and second impedance
parameters to thereby determine circle parameter values, the
equations being solved using the first and second parameter values
at each of the three frequencies; c) using the circle parameter
values to determine a third impedance parameter value at a
respective frequency; and, d) using the third impedance parameter
value to determine an indicator indicative of relative fluid levels
within the body segment of the subject.
37) A method according to claim 36, wherein the first and second
parameter values are resistance and reactance values.
38) A method according to claim 35, wherein the impedance parameter
values include at least one of: R.sub.0 which is the resistance at
zero frequency; R.sub..infin. which is the resistance at infinite
frequency; and, Z.sub.c which is the resistance at a characteristic
frequency.
39) A method according to claim 38, wherein the method includes, in
the processing system: a) determining values for impedance
parameters R.sub.0 and R.sub..infin. from the measured impedance
values; and, b) calculating the index (I) using the equation: I = R
.infin. R 0 - R .infin. ##EQU00011##
40) A method according to claim 38, wherein the method includes, in
the processing system, determining the parameter values using the
equation: Z = R .infin. + R 0 - R .infin. 1 + ( j.omega. .tau. ) (
1 - .alpha. ) ##EQU00012## where: Z is the measured impedance at
angular frequency .omega., .tau. is a time constant, and .alpha.
has a value between 0 and 1.
41) Apparatus for use in analysing impedance measurements performed
on a subject, the apparatus including a processing system for: a)
determining at least one impedance value indicative of the
impedance of at least one body segment of the subject; b)
determining an indicator using at least one impedance value, the
indicator being indicative of extracellular fluid levels in the at
least one body segment; c) determining an index using at least one
impedance value, the index being indicative of a ratio of
extracellular to intracellular fluid levels in the at least one
body segment; d) comparing the indicator to a first reference; e)
comparing the index to a second reference; and, f) providing an
indication of the results of the comparisons.
42) Apparatus according to claim 41, wherein the processing system
is for: a) causing one or more electrical signals to be applied to
the subject using a first set of electrodes; b) measuring
electrical signals across a second set of electrodes applied to the
subject in response to the applied one or more signals; and, c)
determining from the applied signals and the measured signals at
least one measured impedance value.
43) Apparatus according to claim 41, wherein the apparatus
includes: a) a signal generator for generating electrical signals;
and, b) a sensor for sensing electrical signals.
44) A method for use in distinguishing the presence of oedema and
lymphoedema in a subject, the method including, in a processing
system: a) determining at least one impedance value indicative of
the impedance of at least one body segment of the subject; b)
determining an indicator using at least one impedance value, the
indicator being indicative of extracellular fluid levels in the at
least one body segment; c) determining an index using at least one
impedance value, the index being indicative of a ratio of
extracellular to intracellular fluid levels in the at least one
body segment; d) comparing the indicator to a first reference; e)
comparing the index to a second reference; and, f) providing an
indication of the results of the comparisons, the results being
used to distinguish oedema and lymphoedema.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
use in analysing impedance measurements, and in particular, to a
method and apparatus for determining an indicator indicative of
extracellular fluid levels using impedance measurements, the
indicator being usable in identifying venous insufficiency,
lymphoedema and/or oedema.
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] Venous insufficiency is a condition characterized by an
inability for veins to adequately return blood to the heart.
Normally, when a subject is in a standing position, the blood in
the subject's leg veins is urged back towards the heart against
gravity by a combination of mechanisms, such as muscular squeezing
of the leg veins, and through the action of one-way valves in the
veins. However, conditions can arise such as increased pressure
within the veins, deep vein thrombosis (DVT), phlebitis, or the
like, which lead to blood pooling in the legs.
[0004] Chronic venous disease (CVD) is common with a 3-7%
prevalence, resulting in an economic cost US $1 billion per
annum.
[0005] Typical detection methods for venous insufficiency involve
examining for physical symptoms such as swelling in the leg or
ankle, tightness in the calves, leg tiredness, pain while walking,
or the like. Venous insufficiency may also be associated with
varicose veins.
[0006] Other techniques for assessing venous insufficiency include
measuring the ambulatory venous pressure, which is achieved by
inserting a needle into the vein on the dorsum of the foot. Whilst
this is regarded as the gold standard of haemodynamic
investigation, this is invasive, and it is therefore desirable to
find alternative non-invasive techniques. Two such methods are air
plethysmography (APG) and strain gauge plethysmography (SPG).
[0007] SPG involves placing mercury strain gauges in a silastic
band around the calf muscle which are calibrated to read percentage
leg volume changes, as described for example in Nicolaides A N
(2000) "Investigation of Chronic Venous Insufficiency: A Consensus
Statement" Circulation 102:126-163. These measurements are
typically performed during exercise regimens to allow venous
refilling time and the ejection volume to be assessed. APG uses an
air bladder which surrounds the leg from the knee to the ankle. The
bladder is inflated to a known pressure, with volume changes in the
calf muscle being determined based on changes in pressure on the
bladder during a sequence of postural changes.
[0008] However, these techniques are only of limited accuracy, and
can require extensive calibration and exercise, to allow useable
measurement to be obtained.
[0009] Lymphoedema is a condition characterised by excess protein
and oedema in the tissues as a result of reduced lymphatic
transport capacity and/or reduced tissue proteolytic capacity in
the presence of a normal lymphatic load. Acquired, or secondary
lymphoedema, is caused by damaged or blocked lymphatic vessels. The
commonest inciting events are surgery and/or radiotherapy. However,
onset of lymphoedema is unpredictable and may develop within days
of its cause or at any time during a period of many years after
that cause.
[0010] One existing technique for determining biological parameters
relating to a subject, such as fluid levels, involves the use of
bioelectrical impedance. This involves measuring the electrical
impedance of a subject's body using a series of electrodes placed
on the skin surface. Changes in electrical impedance at the body's
surface are used to determine parameters, such as changes in fluid
levels, associated with the cardiac cycle or oedema.
[0011] US2006/0111652 describes methods for enhancing blood and
lymph flow in the extremities of a human. As part of this method,
impedance measurements are used to assess segmental blood flows
within the limbs.
[0012] US2005/0177062 describes a system for measuring the volume,
composition and the movement of electroconductive body fluids,
based on the electrical impedance of the body or a body segment.
This is used primarily for electromechanocardiography (ELMEC) or
impedance cardiography (IKG) measurements for determining
hemodynamic parameters.
[0013] WO00/79255 describes a method of detection of oedema by
measuring bioelectrical impedance at two different anatomical
regions in the same subject at a single low frequency alternating
current. The two measurements are analysed to obtain an indication
of the presence of tissue oedema by comparing with data obtained
from a normal population.
SUMMARY OF THE PRESENT INVENTION
[0014] The present invention seeks to ameliorate one or more
problems of the problems associated with the prior art.
[0015] In a first broad form the present invention seeks to provide
a method for use in analysing impedance measurements performed on a
subject, the method including, in a processing system: [0016] a)
determining at least one impedance value indicative of the
impedance of at least one leg segment of the subject; [0017] b)
determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least leg segment and being used in the assessment of venous
insufficiency.
[0018] Typically the method includes, in a processing system:
[0019] a) comparing the indicator to a reference; and, [0020] b)
providing an indication of the results of the comparison to allow
determination of a presence, absence or degree of venous
insufficiency.
[0021] Typically the method includes, in a processing system:
[0022] a) determining a first indicator value with the subject in a
first orientation; [0023] b) determining a second indicator value
with the subject in a second orientation; and, [0024] c)
determining an indicator change based on a difference between the
first and second fluid indicator values, the indicator change being
used in the assessment of venous insufficiency.
[0025] Typically the method includes, in the processing system:
[0026] a) comparing the indicator change to a reference; and,
[0027] b) providing an indication of the results of the comparison
to allow determination of a presence, absence or degree of venous
insufficiency.
[0028] Typically the method includes, in a processing system,
determining an index using the at least one impedance value, the
index being indicative of a ratio of extracellular to to
intracellular fluid levels in the at least leg segment, the index
being used in the assessment of venous insufficiency.
[0029] Typically the method includes, in the processing system:
[0030] a) comparing the index to a reference; and, [0031] b)
providing an indication of the results of the comparison to allow
determination of a presence, absence or degree of venous
insufficiency.
[0032] Typically the method includes, diagnosing the presence of
venous insufficiency if: [0033] a) an indicator is less than first
reference; and, [0034] b) an index is greater than a second
reference.
[0035] Typically the method includes, in a processing system:
[0036] a) determining a first index value with the subject in a
first orientation; [0037] b) determining a second index value with
the subject in a second orientation; and, [0038] c) determining an
index change based on a difference between the first and second
fluid index values, the index change being used in the assessment
of venous insufficiency.
[0039] Typically the method includes, in the processing system:
[0040] a) determining a first indicator value with the subject in a
first orientation; [0041] b) after positioning the subject in a
second orientation for a predetermined time period, determining a
second indicator value with the subject in the first orientation;
and, [0042] c) determining a difference between the first and
second fluid indicator values, the difference being used in the
assessment of venous insufficiency.
[0043] Typically the method includes, in the processing system:
[0044] a) monitoring the difference; [0045] b) determining the time
taken for the difference fall below a reference; and, [0046] c)
providing an indication of the time taken to allow determination of
a presence, absence or degree of venous insufficiency.
[0047] Typically the method includes, in a processing system:
[0048] a) determining a pre-treatment indicator value prior to
treatment of the subject; [0049] b) determining a post-treatment
indicator value following treatment of the subject for venous
insufficiency; and, [0050] c) determining an indicator change based
on a difference between the pre-treatment and post-treatment
indicator values, the indicator change being used in the assessment
of venous insufficiency.
[0051] Typically the method includes, diagnosing the presence of
venous insufficiency if the indicator change is greater than a
reference.
[0052] Typically the reference includes using a reference that is
at least one of: [0053] a) an indicator or index determined for
another limb of the subject; [0054] b) a reference determined from
a sample population; and, [0055] c) a previous indicator or index
determined for the subject.
[0056] Typically the method includes, in the processing system,
displaying at least one of: [0057] a) an indicator; [0058] b) an
index ratio; [0059] c) an index; [0060] d) an indicator change;
[0061] e) an index change; [0062] f) one or more impedance
parameter values; and, [0063] g) results of a comparison.
[0064] Typically the first impedance is measured at a measurement
frequency of at least one of: [0065] a) less than 100 kHz; [0066]
b) less than 50 kHz; and, [0067] c) less than 10 kHz.
[0068] Typically the method includes, in the processing system,
using the at least one impedance measurement as an estimate of a
resistance of the subject at a zero measurement frequency.
[0069] Typically the method includes measuring at least one second
impedance value, the at least one second impedance value being
measured at a measurement frequency of at least one of: [0070] a)
greater than 200 kHz; [0071] b) greater than 500 kHz; and, [0072]
c) greater than 1000 kHz.
[0073] Typically the method includes, in the processing system,
using the at least one second impedance measurement as an estimate
of a resistance of the subject at an infinite measurement
frequency.
[0074] Typically the method includes, in the processing system:
[0075] a) determining a plurality of impedance values; and, [0076]
b) determining at least one impedance parameter value from the
plurality of impedance values.
[0077] Typically the impedance parameter values include at least
one of [0078] R.sub.0 which is the resistance at zero frequency;
[0079] R.sub..infin. which is the resistance at infinite frequency;
and, [0080] Z.sub.c which is the resistance at a characteristic
frequency.
[0081] Typically the method includes, in the processing system:
[0082] a) determining values for impedance parameters R.sub.0 and
R.sub..infin. from the measured impedance values; and, [0083] b)
calculating the index (I) using the equation:
[0083] I = R .infin. R 0 - R .infin. ##EQU00001##
[0084] Typically the method includes, in the processing system,
determining the parameter values using the equation:
Z = R .infin. + R 0 - R .infin. 1 + ( j.omega. .tau. ) ( 1 -
.alpha. ) ##EQU00002## [0085] where: [0086] Z is the measured
impedance at angular frequency .omega., [0087] .tau. is a time
constant, and [0088] .alpha. has a value between 0 and 1.
[0089] Typically the method includes, in the computer system,
causing the impedance measurements to be performed.
[0090] Typically the method includes, in the computer system:
[0091] a) causing one or more electrical signals to be applied to
the subject using a first set of electrodes; [0092] b) measuring
electrical signals across a second set of electrodes applied to the
subject in response to the applied one or more signals; and, [0093]
c) determining from the applied signals and the measured signals at
least one measured impedance value.
[0094] In a second broad form the present invention seeks to
provide apparatus for use in analysing impedance measurements
performed on a subject, the apparatus including a processing system
for: [0095] a) determining at least one impedance value, indicative
of the impedance of at least leg segment of the subject; [0096] b)
determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least leg segment and being used in the assessment of venous
insufficiency.
[0097] Typically the apparatus includes a processing system for:
[0098] a) causing one or more electrical signals to be applied to
the subject using a first set of electrodes; [0099] b) measuring
electrical signals across a second set of electrodes applied to the
subject in response to the applied one or more signals; and, [0100]
c) determining from the applied signals and the measured signals at
least one measured impedance value.
[0101] Typically the apparatus includes: [0102] a) a signal
generator for generating electrical signals; and, [0103] b) a
sensor for sensing electrical signals.
[0104] In a third broad form the present invention seeks to provide
a method for use in assessing the presence, absence or degree of
venous insufficiency, the method including, in a processing system:
[0105] a) determining at least one impedance value, indicative of
the impedance of at least leg segment of the subject; and, [0106]
b) determining an indicator using the at least one impedance value,
the indicator being indicative of extracellular fluid levels in the
at least leg segment and being used in the assessment of venous
insufficiency.
[0107] In a fourth broad form the present invention seeks to
provide a method for use in analysing impedance measurements
performed on a subject, the method including, in a processing
system: [0108] a) determining at least one impedance value
indicative of the impedance of at least one body segment of the
subject; [0109] b) determining an indicator using at least one
impedance value, the indicator being indicative of extracellular
fluid levels in the at least one body segment; [0110] c)
determining an index using at least one impedance value, the index
being indicative of a ratio of extracellular to intracellular fluid
levels in the at least one body segment; [0111] d) comparing the
indicator to a first reference; [0112] e) comparing the index to a
second reference; and, [0113] f) providing an indication of the
results of the comparisons.
[0114] Typically the method includes, in the processing system,
determining the indicator using an impedance measurement performed
at a single low frequency.
[0115] Typically the at least one impedance measurement is measured
at a measurement frequency of at least one of: [0116] a) less than
100 kHz; [0117] b) less than 50 kHz; and, [0118] c) less than 10
kHz.
[0119] Typically the method includes, in the processing system,
using the at least one impedance measurement as an estimate of a
resistance of the subject at a zero measurement frequency.
[0120] Typically the method includes measuring at least one second
impedance value, the at least one second impedance value being
measured at a measurement frequency of at least one of: [0121] a)
greater than 200 kHz; [0122] b) greater than 500 kHz; and, [0123]
c) greater than 1000 kHz.
[0124] Typically the method includes, in the processing system,
using the at least one second impedance measurement as an estimate
of a resistance of the subject at an infinite measurement
frequency.
[0125] Typically the method includes, in the processing system:
[0126] a) determining a plurality of impedance values; and, [0127]
b) determining at least one impedance parameter value from the
plurality of impedance values.
[0128] Typically the method includes, in the processing system:
[0129] a) at each of three frequencies, determining first and
second parameter values for first and second impedance parameters
relating to the impedance of at least one body segment of the
subject; [0130] b) solving simultaneous equations representing a
circle defined with respect to the first and second impedance
parameters to thereby determine circle parameter values, the
equations being solved using the first and second parameter values
at each of the three frequencies; [0131] c) using the circle
parameter values to determine a third impedance parameter value at
a respective frequency; and, [0132] d) using the third impedance
parameter value to determine an indicator indicative of relative
fluid levels within the body segment of the subject.
[0133] Typically the first and second parameter values are
resistance and reactance values.
[0134] Typically the impedance parameter values include at least
one of: [0135] (1) R.sub.0 which is the resistance at zero
frequency; [0136] (2) R.sub..infin. which is the resistance at
infinite frequency; and, [0137] (3) Z.sub.c which is the resistance
at a characteristic frequency.
[0138] Typically the method includes, in the processing system:
[0139] a) determining values for impedance parameters R.sub.0 and
R.sub..infin. from the measured impedance values; and, [0140] b)
calculating the index (I) using the equation:
[0140] I = R .infin. R 0 - R .infin. ##EQU00003##
[0141] Typically the method includes, in the processing system,
determining the parameter values using the equation:
Z = R .infin. + R 0 - R .infin. 1 + ( j.omega..tau. ) ( 1 - .alpha.
) ( 1 ) ##EQU00004## [0142] (a) where: [0143] 1. Z is the measured
impedance at angular frequency .omega., [0144] 2. .tau. is a time
constant, and [0145] 3. .alpha. has a value between 0 and 1.
[0146] In a fifth broad form the present invention seeks to provide
apparatus for use in analysing impedance measurements performed on
a subject, the apparatus including a processing system for: [0147]
a) determining at least one impedance value indicative of the
impedance of at least one body segment of the subject; [0148] b)
determining an indicator using at least one impedance value, the
indicator being indicative of extracellular fluid levels in the at
least one body segment; [0149] c) determining an index using at
least one impedance value, the index being indicative of a ratio of
extracellular to intracellular fluid levels in the at least one
body segment; [0150] d) comparing the indicator to a first
reference; [0151] e) comparing the index to a second reference;
and, [0152] f) providing an indication of the results of the
comparisons.
[0153] Typically the processing system is for: [0154] a) causing
one or more electrical signals to be applied to the subject using a
first set of electrodes; [0155] b) measuring electrical signals
across a second set of electrodes applied to the subject in
response to the applied one or more signals; and, [0156] is c)
determining from the applied signals and the measured signals at
least one measured impedance value.
[0157] Typically the apparatus includes: [0158] a) a signal
generator for generating electrical signals; and, [0159] b) a
sensor for sensing electrical signals.
[0160] In a sixth broad form the present invention seeks to provide
a method for use in distinguishing the presence of oedema and
lymphoedema in a subject, the method including, in a processing
system: [0161] a) determining at least one impedance value
indicative of the impedance of at least one body segment of the
subject; [0162] b) determining an indicator using at least one
impedance value, the indicator being indicative of extracellular
fluid levels in the at least one body segment; [0163] c)
determining an index using at least one impedance value, the index
being indicative of a ratio of extracellular to intracellular fluid
levels in the at least one body segment; [0164] d) comparing the
indicator to a first reference; [0165] e) comparing the index to a
second reference; and, [0166] f) providing an indication of the
results of the comparisons, the results being used to distinguish
oedema and lymphoedema.
[0167] It will be appreciated that the broad forms of the invention
may be used individually or in combination, and may be used for
assessing venous insufficiency as well as diagnosing the presence,
absence or degree of a range of conditions in addition to and
including oedema, lymphodema, body composition, or the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0168] An example of the present invention will now be described
with reference to the accompanying drawings, in which:
[0169] FIG. 1 is a schematic diagram of a first example of
impedance measuring apparatus;
[0170] FIG. 2 is a flowchart of an example of a process for use in
analysing impedance measurements;
[0171] FIG. 3 is a schematic diagram of a second example of
impedance measuring apparatus;
[0172] FIG. 4 is a schematic diagram of an example of a computer
system;
[0173] FIG. 5 is a flowchart of an example of a process for
performing impedance measurements;
[0174] FIG. 6A is a schematic of an example of a theoretical
equivalent circuit for biological tissue;
[0175] FIG. 6B is an example of a locus of impedance known as a
Wessel plot;
[0176] FIG. 7 is a flowchart of a first specific example of a
process for analysing impedance measurements to allow assessment of
venous insufficiency;
[0177] FIG. 8 is a flowchart of a second specific example of a
process for analysing impedance measurements to allow assessment of
venous insufficiency;
[0178] FIG. 9 is a flowchart of a third specific example of a
process for analysing impedance measurements to allow assessment of
venous insufficiency;
[0179] FIG. 10 is a flowchart of a fourth specific example of a
process for analysing impedance measurements to allow assessment of
venous insufficiency; and,
[0180] FIG. 11 is a flowchart of a fifth specific example of a
process for analysing impedance measurements to allow assessment of
venous insufficiency.
DETAILED DESCRIPTION OF THE DRAWINGS
[0181] An example of apparatus suitable for performing an analysis
of a subject's bioelectric impedance will now be described with
reference to FIG. 1.
[0182] 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.
[0183] 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 therefore act as sense electrodes, to
allow signals induced across the subject S to be sensed.
[0184] 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 therefore be
integrated into the measuring device 100.
[0185] 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. By performing
this, the length of any connections between the signal generators
117A, 117B and the sensors 118A, 118B, and the corresponding
electrodes 113A, 113B, 115A, 115B can be reduced. This minimises
any parasitic capacitances between the connections, the connections
and the subject, and the connections and any surrounding articles,
such as a bed on which the subject is provided, thereby reducing
measurement errors.
[0186] The above described system can be described as a two channel
device, 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, and any number of channels may be provided, as
required.
[0187] 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.
[0188] 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.
[0189] 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 indicators of the presence, absence or degree of
venous insufficiency, other conditions, or the like.
[0190] 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.
[0191] 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, assessment of venous insufficiency, or the
like.
[0192] Once the electrodes are positioned, one or more alternating
signals are applied to the subject S, via 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.
[0193] For example, the system can use Bioimpedance Analysis (BIA)
in which a single low frequency signal is injected into the subject
S, with the measured impedance being used directly in the
determination of biological parameters, such as extracellular fluid
levels, which can be indicative of oedema, and hence of venous
insufficiency.
[0194] In one example, the applied signal has a relatively low
frequency, such as below 100 kHz, more typically below 50 kHz and
more preferably below 10 kHz. In this instance, such low frequency
signals can be used as an estimate of the impedance at zero applied
frequency, commonly referred to as the impedance parameter value
R.sub.0, which is in turn indicative of extracellular fluid
levels.
[0195] Alternatively, the applied signal can have a relatively high
frequency, such as above 200 kHz, and more typically above 500 kHz,
or 1000 kHz. In this instance, such high frequency signals can be
used as an estimate of the impedance at infinite applied frequency,
commonly referred to as the impedance parameter value R.sub..infin.
which is in turn indicative of a combination of the extracellular
and intracellular fluid levels, as will be described in more detail
below.
[0196] In contrast Bioimpedance Spectroscopy (BIS) devices perform
impedance measurements at multiple frequencies over a selected
frequency range. Whilst any range of frequencies may be used,
typically frequencies range from very low frequencies (4 kHz) to
higher frequencies (15000 kHz). Similarly, whilst any number of
measurements may be made, in one example the system can use 256 or
more different frequencies within this range, to allow multiple
impedance measurements to be made within this range.
[0197] When impedance measurements are made at multiple
frequencies, these can be used to derive one or more impedance
parameter values, such as values of R.sub.0, Z.sub.c,
R.sub..infin., which correspond to the impedance at zero,
characteristic and infinite frequencies. These can in turn be used
to determine information regarding both intracellular and
extracellular fluid levels, as will be described in more detail
below.
[0198] A further alternative is for the system to use Multiple
Frequency Bioimpedance Analysis (MFBIA) in which multiple signals,
each having a respective frequency are injected into the subject S,
with the measured impedances being used in the assessment of fluid
levels. In one example, four frequencies can be used, with the
resulting impedance measurements at each frequency being used to
derive impedance parameter values, for example by fitting the
measured impedance values to a Cole model, as will be described in
more detail below. Alternatively, the impedance measurements at
each frequency may be used individually or in combination.
[0199] 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.
[0200] 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.
[0201] In one example, the voltage source is typically
symmetrically and/or differentially arranged, with each of the
signal generators 117A, 117B being independently controllable, to
allow the potential across the subject to be varied. This can be
performed to reduce the effects of any imbalance, which occurs when
the voltages sensed at the electrodes are unsymmetrical (a
situation referred to as an "imbalance"). In this instance, any
difference in the magnitude of signals within the leads can lead to
differing effects due to noise and interference.
[0202] Whilst applying the voltage symmetrically, can reduce the
effect, this is not always effective if the electrode impedances
for the two drive electrodes 113A, 113B are unmatched, which is
typical in a practical environment. However, by adjusting the
differential drive voltages applied to each of the drive electrodes
113A, 113B, this compensates for the different electrode
impedances, and restores the desired symmetry of the voltage at the
sense electrodes 115A, 115B. This can be achieved by measuring the
voltages at the sense electrodes, and then adjusting the magnitude
and/or phase of the applied signal to thereby balance the magnitude
of the sensed voltages. This process is referred to herein as
balancing and in one example is performed by minimizing the
magnitude of any common mode signal.
[0203] A potential 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 potential at each second electrode 115A, 115B
and therefore need only measure half of the potential as compared
to a single ended system.
[0204] The acquired signal and the measured signal will be a
superposition of potentials generated by the human body, such as
the ECG (electrocardiogram), potentials 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.
[0205] 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.
[0206] 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 correlating the signal.
This can be achieved by 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.
[0207] Other suitable digital and analogue demodulation techniques
will be known to persons skilled in the field.
[0208] In the case of BIS, impedance or admittance measurements can
be determined from the signals at each frequency using the recorded
voltage across and current flow through the subject. The
demodulation algorithm can then produce an amplitude and phase
signal at each frequency. This can then be used to derive one or
more impedance parameter values, if required.
[0209] As part of the above described process, the position of the
second electrodes may be measured and recorded. Similarly, other
parameters relating to the subject (subject parameters) 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 venous insufficiency and/or oedema, to assess body
composition, or the like.
[0210] An example of the process of analysing impedance
measurements operation of the apparatus of FIG. 1 to perform this
will now be described with reference to FIG. 2.
[0211] At step 200, at least one first impedance value indicative
of the impedance of at least one segment of the subject's leg is
determined. This may be achieved by having the signal generators
117A, 117B, apply at least one first signal to the subject S, via
the first electrodes 113A, 113B, with voltage signals being
measured across the subject S by the sensors 118A, 118B, via the
second electrodes 115A, 115B. An indication of the current flow
through and voltage across the subject is provided to the
processing system 102, allowing the impedance, or an impedance
parameter value to be determined.
[0212] At step 210, an indicator is determined using the first
impedance. The indicator is typically indicative of the
extracellular fluid levels within the subject. Accordingly, in one
example, the impedance measurement is performed at a single low
frequency, such as below 100 kHz, and in one example, at 5 kHz,
allowing the indicator to be based on the measured value directly.
Alternatively, multiple measurements may be performed at multiple
frequencies, with the indicator being based on an appropriate
impedance parameter value derived therefrom, such as the impedance
at zero applied frequency R.sub.0, as will be described in more
detail below.
[0213] Optionally, at step 220, the indicator can be used in the
assessment of venous insufficiency, or other conditions, such as
oedema or lymphoedema. In this regard, high extracellular fluid
levels in the leg segment are indicative of oedema in the leg,
which is in turn an indicator that venous insufficiency may be
present. In one example, the indicator can be compared to a
reference, such as an oedema reference, to allow the presence,
absence or degree of oedema to be determined, as will be described
in more detail below.
[0214] As the determination of the presence of oedema alone may not
be conclusive as to the presence, absence or degree of venous
insufficiency, or lymphoedema, additional steps may optionally be
performed.
[0215] In one example, at step 230, the subject can be treated for
venous insufficiency. This can be performed in any one of a number
of manners, such as by performing ablation, or the like. Following
this, at step 240, the impedance measurement described above is
repeated to allow a post-treatment indicator to be determined. Any
difference between the pre-treatment indicator determined prior to
treatment, and the post-treatment indicator, can be used to
determine if there has been a reduction in the extra-cellular fluid
levels. Any reduction in fluid levels indicates that the treatment
has been at least partially successful, thereby allowing the
presence of venous insufficiency to be confirmed at step 250.
[0216] In this example, both measurements are typically made with
the subject in a standing or equivalent position, such as leaning
or sitting with their leg hanging in a substantially vertical
position, to thereby enhance the impact of blood pooling caused by
venous insufficiency on extracellular fluid levels. For the purpose
of the remaining description, the term standing will be understood
to encompass any position that maximises or enhances pooling of
blood in the subject's leg.
[0217] In another example, at step 260, the subject is reorientated
from an orientation used to determine the first indicator, allowing
a change in indicator between different orientations to be
determined at step 270.
[0218] Thus, the first indicator can be determined with the subject
provided in a first orientation, to allow a baseline reading to be
established. In one example, this is performed with the subject in
an orientation designed to reduce or minimise blood pooling, such
as with the subject in a supine position, and optionally with their
leg elevated to a height of up to 20 cm above the level of their
heart. For the purpose of the remaining description, the term
supine will be understood to encompass any position that minimises
pooling of blood in the subject's leg.
[0219] The subject then stands, leans or sits with their leg
hanging in a substantially vertical position, allowing a second
indicator to be determined, with a change in indicator values being
indicative of the change of extracellular fluid levels within the
leg, which in turn can be used in venous insufficiency
assessment.
[0220] Alternatively, the subject can be allowed to stand to allow
pooling of blood. Following this, the subject is returned to the
supine position allowing the indicator value to be monitored. The
time taken for this return to the baseline, or to within a range of
the threshold, can then be used in venous insufficiency
assessment.
[0221] In another example, at step 280, second impedance
measurements are performed to allow an index to be determined at
step 290. The index is typically indicative of a ratio between
intracellular and extracellular fluid levels in the leg, or a leg
segment, which can in turn be used to assess the presence of
lymphoedema. Being able to distinguish between oedema and
lymphoedema can assist in assessing venous insufficiency.
[0222] In the above described examples, the measurements are
performed on the subject's leg as this maximises the effect of any
blood pooling, thereby maximising the effectiveness of the
measurement procedure to determine indicators that can be used in
identifying venous insufficiency. However, if the technique is
being used for other identifying conditions such as lymphoedema,
then the process can be applied to other body segments, such as
arms, as will be described in more detail below.
[0223] A specific example of the apparatus will now be described in
more detail with respect to FIG. 3.
[0224] 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.
[0225] 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.
[0226] 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.
[0227] A controller, 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 instructions executed by the
processing system 330.
[0228] 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.
[0229] 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 310 can be
used to allow at least part of the analysis of the impedance
measurements to be performed.
[0230] 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.
[0231] 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.
[0232] 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.
[0233] 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.
[0234] 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.
[0235] Additionally, the computer system 310 can operate to analyse
impedance determined by the processing system 330, to allow
biological parameters to be determined.
[0236] 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.
[0237] Firstly, the use of the processing system 330 more easily
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.
[0238] Secondly, the use of a custom configured processing system
330 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
generating a "Wessel" plot, using the impedance values to determine
parameters relating to cardiac function, as well as determining the
presence or absence of lymphoedema.
[0239] 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.
[0240] In use, the processing system 330 generates digital control
signals, which are converted to analogue voltage drive signals
V.sub.D by the DACs 329, and transferred to the signal generators
117. Analogue signals representing the current of the drive signal
I.sub.D applied to the subject and the subject voltage V.sub.S
measured at the second electrodes 115A, 115B are received from the
signal generators 117 and the sensors 118 and are digitised by the
ADCs 327, 328. The digital signals can then be returned to the
processing system 330 for preliminary analysis.
[0241] 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 respectively. This therefore represents a two
channel device, each channel being designated by the reference
numerals A, B.
[0242] 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 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.
[0243] An example of the process for performing impedance
measurements will now be described with reference to FIG. 5.
[0244] At step 500, the electrodes are positioned on the subject as
required. The general arrangement to allow impedance of a leg to be
determined is to provide drive electrodes 113A, 113B on the hand at
the base of the knuckles and on the feet at the base of the toes,
on the side of the body being measured. Sense electrode 115A are
also positioned at the front of the ankle on the leg being
measured, with the sense electrode 115B being positioned anywhere
on the contra-lateral leg.
[0245] It will be appreciated that this configuration uses the
theory of equal potentials, allowing the electrode positions to
provide reproducible results for impedance measurements. This is
advantageous as it greatly reduces the variations in measurements
caused by poor placement of the electrodes by the operator.
[0246] Alternatively however other arrangements can be used. Thus
for example, the sense electrodes can be provided anywhere on the
leg of interest, allowing the impedance measurements to be made
along the entire leg, or for a part of the leg (generally referred
to as a leg segment), such as a calf segment, or the like.
[0247] At step 510, an impedance measurement type is selected using
the computer system 310, allowing the processing system to
determine an impedance measurement protocol, and configure the
processing system 330 accordingly. This is typically achieved by
configuring firmware or software instructions within the processing
system 330, as described above.
[0248] At step 520, the processing system 300 selects a next
measurement frequency f.sub.i, and causes the signal generators
117A, 117B to apply a first signal to the subject at the selected
frequency at step 530. At step 540, the signal generators 117A,
117B and sensors 118A, 118B provide an indication of the current
through and the voltage across the leg segment to the processing
system 330.
[0249] At step 550, the processing system 330 determines if all
frequencies are complete, and if not returns to step 520 to select
the next measurement frequency. At step 560, one or more measured
impedance values are determined, by the computer system 310, the
processing system 330, or a combination thereof, using the
techniques described above. One or more impedance parameter values
may optionally be derived at step 570.
[0250] In this regard, FIG. 6A is an example of an equivalent
circuit that effectively models the electrical behaviour of
biological tissue. The equivalent circuit has two branches that
represent current flow through extracellular fluid and
intracellular fluid, respectively. The extracellular fluid
component of biological impedance is represented by an
extracellular resistance R.sub.e, whilst the intracellular fluid
component is represented by an intracellular resistance R.sub.i and
a capacitance C representative of the cell membranes.
[0251] The relative magnitudes of the extracellular and
intracellular components of impedance of an alternating current
(AC) are frequency dependent. At zero frequency the capacitor acts
as a perfect insulator and all current flows through the
extracellular fluid, hence the resistance at zero frequency,
R.sub.0, equals the extracellular resistance R.sub.e. At infinite
frequency the capacitor acts as a perfect conductor and the current
passes through the parallel resistive combination. The resistance
at infinite frequency R.sub..infin. is given by:
R .infin. = R e R i R e + R i ( 1 ) ##EQU00005##
[0252] Accordingly, the impedance of the equivalent circuit of FIG.
6A at an angular frequency .omega., where .omega.=2.pi.*frequency,
is given by:
Z = R .infin. + R 0 - R .infin. 1 + ( j.omega. .tau. ) ( 2 )
##EQU00006##
where: [0253] R.sub..infin.=impedance at infinite applied frequency
[0254] R.sub.0=impedance at zero applied frequency=R.sub.e and,
[0255] .tau. is the time constant of the capacitive circuit.
[0256] However, the above represents an idealised situation which
does not take into account the fact that the cell membrane is an
imperfect capacitor. Taking this into account leads to a modified
model in which:
Z = R .infin. + R 0 - R .infin. 1 + ( j.omega. .tau. ) ( 1 -
.alpha. ) ( 3 ) ##EQU00007##
where: [0257] .alpha. has a value between 0 and 1 and can be
thought of as an indicator of the deviation of a real system from
the ideal model.
[0258] The values of impedance parameters R.sub.0, R.sub..infin. or
Z.sub.c may be determined in any one of a number of manners such as
by: [0259] estimating values based on impedance measurements
performed at selected respective frequencies; [0260] solving
simultaneous equations based on the impedance values determined at
different frequencies; [0261] using iterative mathematical
techniques; [0262] extrapolation from a "Wessel plot" similar to
that shown in FIG. 6B; [0263] performing a function fitting
technique, such as the use of a polynomial function.
[0264] For example, the Wessel plot is often used in BIS
(Bioimpedance Spectroscopy) Bioimpedance Spectroscopy (BIS)
devices, which perform multiple measurements over a range of
frequencies, such as from 4 kHz to 1000 kHz, using 256 or more
different frequencies within this range. A regression procedure is
then used to fit the measured data to the theoretical semi-circular
locus, allowing values for R.sub..infin. and R.sub.0 to be
calculated.
[0265] The regression analysis is computationally expensive,
requiring a device with significant processing power to perform the
calculations, which in turn results in relatively high power usage
by the apparatus, requiring a larger battery, and adding to the
weight and size of the apparatus.
[0266] A further issue is that a large number of data points are
required to perform the regression analysis, and as measurements
are typically performed at each frequency sequentially, the
measurement process takes a significant amount of time, such as
several seconds. This is undesirable as remaining still for long
periods of time can cause discomfort for the subject. Additionally,
the subject may move during the measurement procedure, which can
affect the measured impedance values, for example due to changes in
capacitive and/or inductive coupling between the subject and
environment, leads and electrodes. This can lead to inaccuracies in
the measured values.
[0267] A circle may be described by the equation:
(x-i).sup.2+(y-j).sup.2=r.sup.2 (3) [0268] where: i and j are the
centre of the circle and r is the radius.
[0269] Additionally, a circle may be uniquely defined by the
co-ordinates of three points (x.sub.1-3, y.sub.1-3) located on the
locus, as shown in FIG. 4. Accordingly, three simultaneous
equations can be defined, one for each of three loci that describe
the circle that fits these points, as shown by equations (4)
below.
(x.sub.1-i).sup.2+(y.sub.1-j).sup.2=r.sup.2
(x.sub.2-i).sup.2+(y.sub.2-j).sup.2=r.sup.2
(x.sub.3-i).sup.2+(y.sub.3-j).sup.2=r.sup.2 (4)
[0270] Solving these three simultaneous equations allows
calculation of the radius (r) and the co-ordinates of the centre of
the circle (i, j). From these data, R.sub.0 and R.sub..infin. are
readily computed from geometric first principles.
[0271] Accordingly, this technique allows a value for R.sub.0 and
optionally R.sub..infin. to be derived in a computationally less
expensive manner than if a regression analysis is performed.
Additionally, this also requires a reduced number of data points.
This allows a value of R.sub.0 to be determined more rapidly, and
with a more basic processor than can be achieved using BIS and
regression analysis, which in turn renders the device required to
determine a value of R.sub.0 less expensive to manufacture.
[0272] In particular, this is achieved by performing impedance
measurements at least three frequencies. Indications of the signals
are used to determine first and second impedance parameter values
at each of the frequencies. The nature of the impedance parameter
values will vary depending on the preferred implementation. Thus,
for example the impedance parameter values could include magnitude
and phase information relating to the measured signals. However, in
one example the impedance parameter values are indicative of the
resistance and reactance, as derived from the magnitude and phase
signals.
[0273] Once this is completed, simultaneous equations are solved
using the first and second impedance parameter values determined at
each of the three frequencies, thereby allowing circle parameters
to be determined. The circle parameters are used to define a locus
corresponding to at least part of an arc of a circle in a space
defined by the parameter values. Thus, in one example, the
simultaneous equations represent a circular locus provided in a
reactance/resistance space, similar to the Wessel plot described
above.
[0274] Theoretical impedance parameter values, such as R.sub.0 and
R.sub..infin. can then be determined from the circle
parameters.
[0275] One potential disadvantage of the use of simultaneous
equations is that if one of the impedance measurements is
inaccurate for any reason, this can lead to a large deviation in
the calculated value of R.sub.0. Accordingly, in one example,
impedance measurements are performed at more than three
frequencies, with circle parameters for all possible combinations
of impedance measurements at three frequencies being calculated.
The average can be provided along with the standard deviation as a
measure of the goodness of fit of the data to the Cole model. In
the event that one of the measurements is inaccurate, this can be
accounted for by excluding one or more outlier measurements, such
as measurements that deviates the greatest amount from the mean, or
measurements differing by more than a set number of standard
deviations from the mean, allowing the mean to be recalculated,
thereby providing more accurate values.
[0276] Whilst this process uses additional measurements, such as
four or five measurements, this is still significantly less than
the 256 or more frequencies typically performed using a BIS
measurement protocol, allowing the measurement process to be
performed more quickly.
[0277] In one example, the frequencies used are in the range 0 kHz
to 1000 kHz, and in one specific example, four measurements are
recorded at frequencies of 25 kHz, 50 kHz, 100 kHz, and 200 kHz,
although any suitable measurement frequencies can be used.
[0278] A further alternative for determining impedance parameter
values such as R.sub.0 and R.sub..infin. is it perform impedance
measurements at a single frequency, and use these as an estimate of
the parameter values. In this instance, measurements performed at a
single low frequency can be used to estimate R.sub.0, whilst
measurements at a single high frequency can be used to estimate
R.sub..infin..
[0279] The above described equivalent circuit models the
resistivity as a constant value and does not therefore accurately
reflect the impedance response of a subject, and in particular does
not accurately model the change in orientation of the erythrocytes
in the subject's blood stream, or other relaxation effects. To more
successfully model the electrical conductivity of the human body,
an improved CPE based model may alternatively be used.
[0280] In any event, it will be appreciated that any suitable
technique for determination of the parameter values such as
R.sub.0, Z.sub.c and R.sub..infin. may be used.
[0281] A first specific example of a process for analysing
impedance measurements to allow assessment of venous insufficiency
will now be described with reference to FIG. 7.
[0282] In this example, at step 700, at least one first impedance
value is determined using the method described above. The
measurement is typically performed with the subject in a specific
orientation, such as in a supine or standing position. This is
performed to either maximise or minimise the effect of blood
pooling, and this will depend on the analysis performed.
[0283] In this first specific example, the subject is made to stand
for a set time period such as between five and fifteen minutes to
maximize the effect of any blood pooling. In general, a marked
increase in blood pooling is achieved after five minutes, with the
blood levels reaching a relatively static maximum after
approximately fifteen minutes. Accordingly, whilst it is preferable
for the subject to stand for fifteen minutes to thereby maximise
blood pooling, even after five minutes sufficient pooling occurs to
allow measurements to be performed. It will be appreciated from
this that the length of time selected may depend on factors such as
the amount of time available for the measurement process and the
ability of the subject to remain in standing position.
[0284] Furthermore, the subject may be required to lay in a supine
position for a set time period, such as five to fifteen minutes
prior to standing. This can be performed to minimise any blood
pooling before standing, so as to provide a more accurate baseline
status for the subject prior to measurements being performed.
Again, a marked reduction in pooling is achieved after five
minutes, with the level of pooling typically reaching a reasonably
static minimum after approximately fifteen minutes, so the length
of time used will depend on factors such as the amount of time
available to make a measurement.
[0285] At step 710 an impedance parameter value R.sub.0 is
optionally determined. This can be performed if multiple impedance
values are determined. Otherwise, a single impedance measurement
can be made at a low frequency, such as below 10 kHz, as this
provides a reasonably close approximation of R.sub.0.
[0286] At step 720, an indicator that is indicative of the
extracellular R.sub.e fluid levels within the subject is
determined, with this being displayed to the user at step 730.
[0287] The indicator can be any form of suitable indicator such as
a numerical value based on the value of the impedance parameter
value R.sub.0. The indicator may also be scaled to provide a
numerical value that is indicative of the presence, absence or
degree of venous insufficiency or oedema. The indicator can also be
based on the results of a comparison of a numerical value to a
reference, such as an oedema reference.
[0288] The oedema reference could be any suitable form of
reference. Thus, in one example, the oedema reference can be based
on an equivalent impedance parameter value determined for a
different limb of the subject, such as an arm. This is possible,
as, for a subject not suffering from venous insufficiency, there is
a predictable relationship between the extracellular fluid levels
between different limbs. Thus, for example, if the subject is
suffering from a condition other than venous insufficiency, which
causes a general change in extracellular fluid levels, then this
should affect body segments in an assessable manner, thereby
allowing venous insufficiency to be identified.
[0289] Minor variations in tissue may occur between different body
segments in a healthy subject, and this can be accounted for by
providing a tolerance to the comparison. Thus, for example, this
could take into account naturally expected variations between
different limbs in normal healthy subjects, for example due to limb
dominance, previous analysis for the subject, or the like. The
tolerance may also depend on a number of factors, such as the
subject's age, weight, sex and height, and again a respective range
can be selected based on these factors.
[0290] Alternatively, the oedema reference can be based on a
reference derived from sample populations, or the like. The oedema
reference can be selected based on the subject parameters, so that
the value of the indicator is compared to values of the indicator
derived from a study of a sample population of other individuals
having similar subject parameters.
[0291] As a further alternative, the oedema reference can be based
on a previously measured reference for the subject, for example
determined before the subject suffered from venous insufficiency or
oedema. This allows a longitudinal analysis to be performed,
thereby allowing the onset or progression of venous insufficiency
to be assessed.
[0292] The indicator can additionally and/or alternatively be
displayed on a graphical linear or non-linear scale, with the
position of a pointer on the scale being indicative of
extracellular fluid levels and or the presence, absence or degree
of oedema or venous insufficiency. In one example, the linear scale
can include thresholds at values representing ranges indicative of
the presence or absence of oedema or venous insufficiency, as
derived from sample population data, or other references.
[0293] At step 740, the user can use the indicator to assess
whether further investigation is required. In this regard, a high
extra-cellular fluid level indicative of the presence of oedema is
a good indication that the subject has venous insufficiency, but
this may need to be confirmed with further measurements, and/or
analysis.
[0294] The above described example allows for a rapid assessment of
the presence of venous insufficiency. This can be performed using
BIA, which allows relatively simple apparatus and processing to be
used, thereby reducing the cost of equipment required to assess
venous insufficiency compared to more complex techniques. Despite
this, the process is more reliable than current non-invasive
techniques such as SPG and APG. In this regard, changes in fluid
levels can typically be detected using impedance measurements
before the fluid level changes have a noticeable impact on limb
volume, thereby making the impedance measurement process more
sensitive than other techniques such as SPG or APG.
[0295] Examples for performing further investigation will now be
described in more detail.
[0296] In the specific example of FIG. 8, at step 800 a first
indicator value is determined with the subject in a standing
position, as described with respect to FIG. 7, to maximise blood
pooling. This is performed before the subject is treated so that
the first indicator acts as a pre-treatment indicator. At step 810,
the subject is treated for venous insufficiency, by performing
ablation, or the like. Following this, at step 820, a second
indicator value is determined using a similar technique, (i.e. with
the subject in a standing position) which acts as a post-treatment
indicator.
[0297] The processing system 102 then determines any difference
between the first and second indicator values at step 830, with the
difference being compared to a reference, such as a treatment
reference at step 840, thereby allowing the relevance of any change
to be assessed. If the comparison indicates that there is a
reduction in the extra-cellular fluid levels greater than a
threshold amount, then this indicates that the treatment is
successful or has at least had an impact, thereby allowing the
presence of venous insufficiency to be confirmed at step 850. The
magnitude of any difference may also be used to determine the
degree of any venous insufficiency, and/or the effectiveness of the
treatment.
[0298] Again, the treatment reference can be derived in any one of
a number of manners. For example, treatment reference can be
obtained from data collected from a sample population of subjects,
which acts as a pool of data from which normalised expected
differences for successfully treated, untreated and/or healthy
subjects can be determined. The treatment reference is then
generated by selecting reference values that are determined to be
relevant to the test subject based on the subject parameters such
as age, sex, height, weight, race, interventions, or the like.
[0299] In this instance, given a preliminary indication that venous
insufficiency is present following the procedure of FIG. 7,
treatment of the subject is performed, with the presence of venous
insufficiency being confirmed if the treatment is successful in the
sense that it results in a reduction in extra-cellular fluid
levels. This therefore allows a more reliable assessment of venous
insufficiency, but advantageously also simultaneously treats the
venous insufficiency, ensuring that the subject is treated as
rapidly as possible, without having to await further analysis. In
the instance that the subject does not have venous insufficiency,
then there is no negative effect of performing the treatment. It
will also be appreciated that this allows the analysis to be
confirmed using the same apparatus as used to perform the initial
assessment, thereby simplifying the analysis for the relevant
health professional performing the assessment.
[0300] In the specific example of FIG. 9, at step 900 a first
indicator value is determined with the subject in a supine
position. This measurement is typically performed after the subject
has been allowed to rest for some set time, such as five to fifteen
minutes. This reduces the effect of any blood pooling, allowing a
baseline reading to be established. The subject then stands for a
predetermined time period, such as five to fifteen minutes, to
maximize blood pooling before a second indicator value is
determined using the technique described above, at step 910. Again,
the first and second indicator values are indicative of
extra-cellular fluid levels, and therefore could be based on one or
more low frequency impedance measurements or the impedance
parameter value R.sub.0, as derived from impedance measurements in
some manner.
[0301] At step 920, the processing system 102 then determines any
difference between the first and second indicator values, with the
difference being compared to a reference, such as a pooling
reference, at step 930.
[0302] Again, the pooling reference can be based on similar
indicator values derived from sample populations, or the like.
Alternatively, the pooling reference can be based on first and
second indicator values previously determined for the subject, for
example prior to the onset of venous insufficiency, allowing
longitudinal analysis to be performed.
[0303] In one example, the reference is determined as a percentage
change from the baseline reading, so that a change of less than a
predetermined amount, such as 15%, indicates that the subject does
not have venous insufficiency. For a percentage change greater than
the predetermined amount, such as 15%, this indicates that the
level of pooling is abnormal, and hence that the subject may have
venous insufficiency, with the magnitude of the percentage change
being indicative of the degree of venous insufficiency. Thus, for
example, a change of greater than 20%, would typically be
indicative of venous insufficiency.
[0304] Results of the comparison can be displayed at step 940, to
allow the relevance of any change to be assessed. In this regard,
if the comparison indicates that there is an increase in the
extra-cellular fluid levels greater than an amount determined from
the pooling reference, then this indicates that there is
significant blood pooling within the subject, which is in turn
indicative of venous insufficiency. It will be appreciated from
this, that the magnitude of the difference between the first and
second indicator values can be indicative of the degree of venous
insufficiency.
[0305] In this instance, given a preliminary indication that venous
insufficiency is present following the procedure of FIG. 7, the
subject is reorientated and the measurement re-performed. In this
regard, if the indicator derived in the process of FIG. 7 is made
in the standing position, then this can act as the second
indicator, with the subject being reorientated to allow the first
indicator to be determined. Accordingly, it will be appreciated
that the terms "first" and "second" are used to identify respective
indicators and do not necessarily imply or require an order to the
measurements.
[0306] However, in general the first indicator is measured first to
allow an accurate baseline to be established, although this is not
essential. This allows a more reliable assessment of venous
insufficiency than achievable with prior art techniques, and allows
the analysis to be confirmed using the same apparatus as used to
perform the initial assessment. This also allows assessment to be
confirmed without requiring treatment of the subject, which can
reduce the cost burden of the assessment.
[0307] In the specific example of FIG. 10, at step 1000 a first
indicator value is determined with the subject in a supine
position. As in the previous example, this is performed to allow a
baseline reading to be established. The subject then stands for a
predetermined time period, such as five to fifteen minutes, to
maximize blood pooling at step 1010, before returning to a supine
position to allow a second indicator value to be determined using
the technique described above, at step 1020.
[0308] At step 1030, the processing system 102 then determines any
difference between the first and second indicator values, before
determining if the difference is below a reference, such as a
return reference, at step 1040. If not, the process returns to step
1020, allowing a new second indicator to be determined.
[0309] The reference may be derived from sample populations,
previous measurements for the subject, or the like. Alternatively,
the reference is deemed to be a certain percentage variation from
the baseline reading. Thus, in this example, the time taken to
return to within 5-10% of the baseline reading can be
determined.
[0310] Steps 1020 to 1040 are repeated until the difference between
the first and second indicators is below the return reference. At
step 1050, the length of time taken for the difference to fall
below the pooling reference is determined, with this being
displayed to the user at step 1060.
[0311] If the time taken to return to the baseline (or the
reference), is less than a reference amount, such as fifteen
minutes, then this is indicative that the subject does not have
venous insufficiency. However, if the time taken is greater than
the reference time, then this is indicative that the subject does
have venous insufficiency.
[0312] In this regard, in a healthy subject, the amount of blood
will return to the baseline amount relatively quickly, whilst in a
subject with venous insufficiency, this can take several minutes.
Accordingly, the time taken for blood pooling in the leg to be
reduced is in turn indicative of the presence, absence or degree of
venous insufficiency. The return reference is used to take into
account natural variation and to allow the measurement to be
performed in a reasonable time period.
[0313] In this instance, by measuring the time taken for the
subject to return to a baseline reading, this provides a direct
correlation for the ability of the venous system to counteract any
blood pooling, which is directly indicative of the degree of venous
insufficiency. This therefore provides an accurate technique for
assessing the severity of venous insufficiency without requiring
treatment or complex apparatus.
[0314] An example of a process for allowing oedema and lymphoedema
to be distinguished will now be described with reference to FIG.
11. In one example, this process can be used to help assess the
presence of venous insufficiency. For example, if measurements are
performed on the subject's leg, and it is determined that the
subject has oedema but not lymphoedema, then this is indicative
that the subject has venous insufficiency. Alternatively however,
this technique can be used to distinguish between oedema and
lymphoedema in other body segments, such as arms. This can be used
in helping to identify optimum management programmes depending on
the conditions suffered by the subject.
[0315] Accordingly, for the purpose of this example, the technique
will be described as being generally applied to a body segment to
allow oedema and lymphoedema to be distinguished. It will be
appreciated that in the event that this technique is used for
identifying venous insufficiency, then the process will be
performed on the subject's leg, but that the process could also be
applied to other body segments.
[0316] In the specific example of FIG. 11, at step 1100 impedance
values are determined for the body segment at a number of different
frequencies. If this technique is being used to determine
indicators that can be used in identifying venous insufficiency,
then these measurements are typically performed with the subject in
a standing position to maximise blood pooling.
[0317] At step 1110, the processing system 102 determines values
for the impedance parameters R.sub.0, R.sub..infin. as described in
more detail above. Thus, this could be performed using regression
analysis applied to BIS measurements. Alternatively, the process
could use MFBIA, and solving of simultaneous equations to determine
the impedance parameter values. Other techniques could also be used
as appropriate in which combinations of individual measurements at
selected frequencies are used. Thus, for example, single
measurements at low and high frequencies respectively could be used
as an estimate of R.sub.0 and R.sub..infin..
[0318] At step 1120, a first indicator value is determined using
the impedance parameter value R.sub.0. It will be appreciated that
this can alternatively be derived from a single low frequency
impedance measurement described above, although in another example,
measurements at multiple frequencies can be used for subsequent
steps, in which case the need to make additional single frequency
measurements can be avoided.
[0319] At step 1130, an index is determined for the body segment
which provides an indication of the distribution between intra and
extracellular fluids. In one example, the index is indicative of
the ratio of extracellular fluid to intracellular fluid
R.sub.i/R.sub.e. In this regard, using the values for the
extracellular fluid resistance R.sub.e and intracellular fluid
resistance R.sub.i above, the index I is given by the equation:
I = R i R e = R .infin. R 0 - R .infin. ( 4 ) ##EQU00008##
[0320] At step 1140, the first indicator value is compared to a
first reference, such as the oedema reference described above, with
the results of the comparison allowing the presence of oedema to be
determined.
[0321] At step 1150, the index value is compared to a second
reference, with the results of the comparison allowing
determination of whether the subject has lymphoedema. The second
reference can be determined in any one of a number of manners.
[0322] In one example, the index is compared to a reference index
value determined for another limb. This is possible, as, for a
subject not having lymphoedema, there is generally a degree of
similarity of intra- and extra-cellular fluid levels, even between
different body segments. Typically minor variations in tissue will
occur between different body segments, for example due to inherent
differences between different types of limb, or due to limb
dominance, and this can be accounted for using appropriate
tolerances in a manner similar to that described above.
Additionally, and/or alternatively, different references can be
used, such as references derived from sample populations, previous
measurements made for the subject, or the like.
[0323] The results of the comparison can be displayed to the user
to allow assessment of whether the subject has oedema and/or
lymphoedema.
[0324] In the event that measurements are performed on the
subject's leg, whilst the subject is in a standing position, and
this indicates that the subject is likely to have oedema which is
not lymphoedema, then this is indicative that the subject has
venous insufficiency. In this instance, the magnitude of the
indicator and index values can be used to assess the degree of
venous insufficiency. Alternatively, this technique can be used on
other parts of the body to distinguish between oedema and
lymphoedema.
[0325] In a further example, the above described process can be
repeated with the subject in supine and standing positions. In this
instance, changes in the indicator and index values can further
assist in assessing the presence, absence or degree of venous
insufficiency.
[0326] In this example, whilst more complex BIS apparatus is
required, this can provide a more accurate indication of venous
insufficiency severity than can be achieved using the prior art or
other techniques. Furthermore, by using the BIS apparatus described
above, the assessment can be performed rapidly, without requiring
the subject to undergo exercise regimes, which may not be
appropriate for some subjects.
[0327] In the above described examples, the term impedance
generally refers to a measured impedance value or impedance
parameter value derived therefrom. The term resistance refers to
any measured value relating to the impedance, such as admittance of
reactance measurements.
[0328] The term processing system is intended to include any
component capable of performing processing and can include any one
or more of a processing system and a computer system.
[0329] 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.
[0330] Features from different examples above may be used
interchangeably where appropriate. Thus, for example, multiple
different indicators may be determined and compared to respective
thresholds.
[0331] Furthermore, whilst the above examples have focussed on a
subject 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 race horses, or the like.
[0332] The above described processes can be used in determining
biological indicators, which in turn can be used for diagnosing the
presence, absence or degree of a range of conditions and illnesses,
including, but not limited to oedema, lymphodema, body composition,
or the like.
[0333] Furthermore, whilst the above described examples have
focussed on the application of a voltage signal to cause a current
to flow through the subject, this is not essential and the process
can also be used when applying a current signal.
[0334] It will also be appreciated that the term impedance
measurement covers admittance and other related measurements.
* * * * *