U.S. patent number 3,608,543 [Application Number 04/764,748] was granted by the patent office on 1971-09-28 for physiological impedance-measuring apparatus.
This patent grant is currently assigned to Carnegie-Mellon University. Invention is credited to Wils L. Cooley, Richard L. Longini.
United States Patent |
3,608,543 |
Longini , et al. |
September 28, 1971 |
PHYSIOLOGICAL IMPEDANCE-MEASURING APPARATUS
Abstract
In a physiological impedance-measuring apparatus, the
combination comprising a pair of electrodes for spaced application
to predetermined locations of the anatomy, an additional electrode
spacedly juxtaposed to one of said first-mentioned electrodes,
circuit means for applying a measuring voltage across said
first-mentioned electrodes and a portion of said anatomy
therebetween and for applying a potential to said additional
electrode similar to that on the associated one of said
first-mentioned electrodes, and an impedance-measuring circuit
coupled to said electrodes for measuring anatomical impedance only
between a predetermined pair of said electrodes. In certain
applications wherein said circuit means include a unity-gain
amplifier for applying a substantially identical potential at lower
impedance to said additional electrode for driving the same.
Inventors: |
Longini; Richard L.
(Pittsburgh, PA), Cooley; Wils L. (Pittsburgh, PA) |
Assignee: |
Carnegie-Mellon University
(Pittsburgh, PA)
|
Family
ID: |
25071657 |
Appl.
No.: |
04/764,748 |
Filed: |
October 3, 1968 |
Current U.S.
Class: |
600/536 |
Current CPC
Class: |
A61B
5/0809 (20130101) |
Current International
Class: |
A61B
5/08 (20060101); A61b 005/05 () |
Field of
Search: |
;128/2.1,2.06,DIG.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IE.E.E. Transactions on Bio-Medical Engineering, July/October,
1965, Vol. BME-12 Nos. 3 and 4, pp. 197-198..
|
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Howell; Kyle L.
Claims
We claim:
1. In a physiological measuring apparatus, the combination
comprising a pair of electrodes adapted for spaced application to
predetermined locations of the anatomy, an additional electrode
spacedly juxtaposed to one of said first-mentioned electrodes,
circuit means for applying an impedance-measuring voltage only
across said first-mentioned electrodes and a portion of said
anatomy when juxtaposed therebetween and for applying a potential
to said additional electrode similar to that on the associated one
of said first-mentioned electrodes, and an impedance-meauring
circuit coupled to said electrodes and to said circuit means for
measuring anatomical impedance only between a predetermined pair of
said electrodes, said circuit comprising a substantially unity gain
amplifier for applying a substantially identical potential at lower
impedance to said additional electrode for driving the same.
2. The combination according to claim 1 wherein said additional
electrode is a guard ring spacedly surrounding said one
electrode.
3. The combination according to claim 1 wherein said additional
electrode includes a plurality of electrically interconnected
electrode segments.
4. The combination according to claim 1 wherein said
impedance-measuring circuit includes a phase-discriminating circuit
sensitive only to a predetermined phase of impedance changes
measured between said electrodes.
5. The combination according to claim 1 wherein said amplifier is
coupled to said additional electrode and to said one electrode.
6. The combination according to claim 1 wherein said
impedance-measuring circuit includes a phase-sensitive detector
circuit including circuit means for relating the maximum
sensitivity of said detector circuit to the phase of a
predetermined component of impedance changes between said
first-mentioned electrodes.
7. The combination according to claim 6 wherein said
impedance-measuring circuit includes an impedance bridge having a
fixed resistive and capacitive impedance and a variable
voltage-sensitive impedance, said last-mentioned impedance being
coupled in feedback relation to the output of said detector
circuit.
8. The combination according to claim 7 wherein said detector
circuit includes an amplifying and matching circuit for relating
said feedback to said impedance change component to maintain said
bridge in dynamic balance.
9. The combination according to claim 1 wherein dual
impedance-measuring paths are coupled to said circuit means and to
said impedance-measuring circuit, known impedance means are in one
of said paths, said anatomical impedance is in the other of said
paths, and means form part of said impedance-measuring circuit for
alternately connecting said paths to provide relative anatomical
impedance measurements and an absolute impedance value for
comparison purposes.
10. The combination according to claim 9 wherein said paths include
known impedances in series and parallel respectively with said
anatomical impedance, and said connecting means include shorting
circuit means around each of said impedances for selectively
coupling and discoupling said impedances so that series and
parallel value bases can be alternatively and selectively indicated
at any point along an output curve of said impedance-measuring
circuit.
11. The combination according to claim 10 wherein said series
impedances are relatively small and said parallel impedance is
relatively large so that absolute value bases of impedance and
conductances can be indicated.
12. The combination according to claim 1 wherein said amplifier is
provided with a power supply which is independent of said
impedance-measuring voltage.
13. The combination according to claim 1 wherein said additional
electrode is a guard ring spacedly surrounding said one
electrode.
14. In a physiological measuring apparatus, the combination
comprising a pair of electrodes adapted for spaced application to
predetermined locations of the anatomy, an additional electrode
spacedly juxtaposed to one of said first-mentioned electrodes,
circuit means for applying an impedance-measuring voltage only
across said first-mentioned electrodes and a portion of said
anatomy when juxtaposed therebetween and for applying a potential
to said additional electrode similar to that on the associated one
of said first-mentioned electrodes, and an impedance-measuring
circuit coupled to said electrodes and to said circuit means for
measuring anatomical impedance only between a predetermined pair of
said electrodes, said impedance-measuring circuit including an
impedance bridge having fixed and variable impedances coupled to
said first-mentioned electrodes, said variable impedance being
coupled in feedback relation to an output circuit of said bridge
for dynamically balancing said bridge.
15. The combination according to claim 14 wherein said additional
electrode is a guard ring spacedly surrounding said one electrode
and having a width at least equal to about the thickness of the
thoracic wall structure being measured.
Description
The present invention relates to apparatus for making physiological
measurements and more particularly to apparatus of the character
described for measuring electrical impedances of the thoracic
regions.
Although our invention is adaptable for measuring electrical
impedances of various parts of the anatomy, it will be described
primarily in connection with the thoracic regions of the body, as
the measurement of respiration is desirable in a large number of
diagnostic situations. Useful measurements extend from simple
detection of apnea to accurate, volumetric determinations for
physiological studies of respiration. A good indication of cardiac
output can also be obtained.
A reliable impedance pneumograph for measuring the electrical
impedance of the human lung has eluded researchers for many years.
It is well known that lung impedance changes by about 50 percent
during inspiration and expiration. As this impedance is a strong
function of the physiological condition of a subject, a reliable
measurement of lung impedance, therefore, would be extremely useful
in diagnostic and physiological studies. Previous impedance
pneumographs have been unreliable both as diagnostic instruments or
long term monitors owing to the presence of other significant
nonpulmonary, transthoracic impedances. Previously proposed
pneumographs have been unable to discriminate accurately among the
several transthoracic impedances for the purpose of yielding an
accurate measurement of lung impedance.
Certain prior pneumographs use small bilateral electrodes and
detect impedance changes of only about 2 to 10 percent of the
resting impedance for a maximum breath. During normal respiration,
the breathing signal is very small and is easily contaminated by
spurious signals resulting from other transthoracic impedance
variations.
Owing to these difficulties, known impedance pneumographs are not
amenable to situations requiring accurate pneumographic
measurements. Such measurements have been made on the other hand by
accepted volumetric or flow techniques. Because of the discomfort
and inconvenience to the patient, the number of routine
measurements which can be made by such means is very limited.
We overcome these difficulties of the prior art with the provision
of physiological electrical impedance-measuring apparatus capable
of distinguishing among several bodily impedances. Specifically,
our measuring apparatus is capable of measuring changes in the
impedance of lung tissue, which fills most of the thoracic cavity,
against a background of much lower and hitherto interfering
impedances of the thoracic wall structures. When used as an
impedance pneumograph our measuring apparatus, therefore, is
capable of sensitive and accurate measurements of lung tissue
ventilation with very little discomfort and inconvenience. Most
importantly, interference or disturbance of rhythmic breathing
patterns is considerably reduced.
We accomplish these desirable results by providing in a
physiological electrical impedance-measuring apparatus, the
combination comprising a pair of electrodes for spaced application
to predetermined locations of the anatomy, an additional electrode
spacedly juxtaposed to one of said first-mentioned electrodes,
circuit means for applying a measuring voltage across said
first-mentioned electrodes and a portion of said anatomy
therebetween, an active means for applying a potential to said
additional electrode similar to that on the associated one of said
first-mentioned electrodes, and an impedance-measuring circuit
coupled to said electrodes for measuring anatomical impedance only
between a predetermined pair of said electrodes.
We also provide similar measuring apparatus wherein said circuit
means include a driving circuit for said additional electrode for
driving said additional electrode at a potential substantially
identical to that on said one electrode.
We also provide similar measuring apparatus wherein said circuit
means include a substantial unity gain amplifier for applying a
substantially identical potential at lower impedance to said
additional electrode for driving the same.
We also provide similar measuring apparatus wherein said
impedance-measuring circuit includes a phase-discriminating circuit
sensitive only to a predetermined phase of impedance changes
between said electrodes.
FIG. 1 is a schematic view of current flow in one quadrant of a
simplified model of a human thorax;
FIG. 2 is a structural and schematic diagram of electrode and
circuit components of our measuring apparatus;
FIG. 3 is a phase diagram illustrating the monitoring of changes in
the resistive component of transthoracic impedance;
FIG. 4 is a logarithmic chart of test results with corresponding
electrode structures;
FIGS. 4A and 4B and 4C are plan views of electrode structures
forming part of our measuring apparatus;
FIG. 5 is a graphical representation of pneumographic impedance
changes for differing conditions;
FIG. 6 is a schematic circuit diagram of one form of guard ring
amplifier suitable for use in the circuit of FIG. 2;
FIG. 7 is a schematic circuit diagram of one form of differential
amplifier suitable for use in the circuit of FIG. 2; and
FIG. 8 is a circuit schematic of one form of phase sensitive
detector suitable for use in the circuit of FIG. 2.
Referring now to FIG. 1 of the drawings, the problem and a partial
solution involved in measuring thoracic impedances are illustrated.
A simplified model of the thoracic region, used for computational
purposes, is denoted by reference character 10. The model
illustrates a thoracic peripheral structure 12, lung tissue 14, and
heart structure 16. For diagnostic and other physiological
measurements, it is desirable to measure exclusively, if possible,
the thoracic core impedance Z.sub.1 in the region denoted by the
reference character 18. Unless some means are provided for
eliminating substantial inclusion of the much lower thoracic wall
impedance Z.sub.R spurious signals or measurements are obtained.
Known pneumographic-measuring apparatus applies a pair of spaced
electrodes to the thoracic region with the result that the output
signal reflects primarily the much lower impedance combination,
Z.sub.R Z.sub.1 (Z.sub.R +Z.sub.1).sup..sup.-1, the combined effect
of Z.sub.R and Z.sub.1. As the undesired peripheral impedance
Z.sub.R predominates, respirational changes in Z.sub.1 do not
effect a significant change in output in conventional measurements.
See FIG. 4, arrow 86 and subsequent description.
We obviate this condition by providing a third electrode 20
spacedly juxtaposed to and desirably surrounding one of primary
electrodes 22, 24 with the result that the measuring signal passes
centrally through the thoracic region between the primary
electrodes 22, 24 when applied thereto (FIGS. 1 and 2). The guard
ring electrode 20 in this example surrounds the associated primary
electrode 22 (FIGS. 4A, 4B, 4C) and is maintained at a potential
similar to that of the associated primary electrode 22. Measuring
currents between the primary electrodes 22, 24 thus are restricted
so that they cannot bypass the lung tissue by flowing
circumferentially through the peripheral thoracic structure.
Accordingly, primarily the thoracic core impedance Z.sub.1 is
measured rather than a composite impedance reflecting primarily the
much lower impedance Z.sub.R of the thoracic periphery. In making
such measurements it is necessary to obviate, as explained below,
the effect of the skin and electrode paste impedance Z.sub.M across
space 25 between the primary electrode 22 and its guard electrode
20.
An exemplary bridge circuit for performing this measurement is
illustrated in FIG. 2 of the drawings. The bridge and detecting
circuit 26 includes three known impedances 28, 30 and 32 in three
branches of the bridge network 34, with the thoracic core impedance
Z.sub.1 in the fourth branch when the electrodes 20-24 are applied
to the thoracic region. Analogous impedances can be similarly
measured in other areas of the anatomy. Desirably, high frequency
potential is applied to the impedance bridge 34 on conductors 36,
38 from a conventional oscillator 40 capable of supplying a
relatively high frequency alternating voltage. Suitable voltage
sources (not shown) are provided for the oscillator 40 and other
circuit components. In this arrangement the frequency of the source
40 is in the neighborhood of 100 kHz., which is sufficiently high
to avoid stimulation of the body tissues, electrode polarization,
and excessive skin impedance. The power level is well below that
which would cause any perceptible heating of the tissues.
The bridge output is applied along conductors 42, 44 to
differential amplifying circuit 45. The amplifying circuit 45 (FIG.
7) includes a differential input circuit 46, a differential,
high-gain amplifier 48, and a gain control circuit denoted
generally by reference character 50. A suitable integrated circuit
differential amplifier 48 is available from Radio Corporation of
America, catalog number CA3030. Variable inductor 51 is adjusted
for maximum gain at the frequency of the bridge input voltage on
conductors 36, 38. A clipper circuit 53 limits the peak-to-peak
output signal (by clipping both positive and negative peaks)
preparatory to shaping the signal to a square wave as described
below relative to FIG. 8.
The output from the differential amplifying circuit 45 is supplied
on conductor 52 to a phase-sensitive detector 47 (FIG. 8). The
detector circuit 47 compares the bridge unbalance signal supplied
to it on conductor 52 with a reference input signal on conductor
96, supplied through a phase shifting network 97 of conventional
construction from supply oscillator 40, to produce an output on
conductor 54. The detector output is fed back on conductor 56 to
the voltage-controlled impedance 32 (FIG. 2) to maintain the bridge
circuit 34 in dynamic balance. Because changes occur in both
amplitude and phase of the transthoracic impedance with breathing,
a complete dynamic rebalance of a bridge ordinarily requires
adjustment of both the resistive and reactive components of the
balancing impedance. The principles on which balancing of the
bridge 34 relative to a single preselected phase (in this case that
of the real or resistive component R.sub.1 of Z.sub.1) are
elaborated upon below in connection with FIGS. 3 and 8 of the
drawings.
In order to exclude the thoracic peripheral impedance Z.sub.R and
the skin (epidermic) impedance Z.sub.M across the electrode gap 25
and thereby to limit measured impedance to the thoracic core
impedance Z.sub.1, the potential of electrode 22 is sensed and
coupled to the guard electrode 20 through guard ring amplifier 57.
Desirably the guard electrode is driven in a manner that maintains
its equipotential status (to the center electrode 22) although the
guard electrode is in a considerably lower impedance path. A
suitable guard ring amplifying or driving circuit is shown in FIG.
6 and is provided with input and output conductors 58, 60 connected
to the electrode 22 (conductors 44, 58) and to the guard ring
electrode 20 (conductor 60) respectively.
The guard ring amplifying circuit 57 includes a DC level adjustment
62, a conventional, high-gain differential amplifier 64 which can
be similar to the amplifier 48 of F6 (RCA-CA3030) amplification and
an output stage 66. Resistances 65 are for current-limiting
purposes. The amplifier output on conductor 60 is fed back on
conductor 61 to one of the differential input terminals 63 of the
amplifier 64. Because the gain of the amplifier is very high and
because the output stage 66 is in both the output and feedback
circuits, the net gain of the amplifying circuit 57 is very nearly
unity. The circuit 57, then, becomes an impedance transformer to
drive the guard electrode 20 (which is in the considerably lower
impedance circuit including the peripheral thorax structure) with a
voltage which is maintained approximately identical with that on
the associated primary electrode 22. Electrode 22 potential thus
applied to the guard ring 20 ensures that the comparatively low
impedance Z.sub.R representing the peripheral thoracic impedance is
confined between the guard ring 20 and the opposite primary
electrode 24 (FIG. 2). On the other hand, the comparatively high
impedance Z.sub.1 representing the impedance of the thoracic core
only, is confined to the primary electrode 22, 24 for sensing by
the bridge 34 and related circuitry.
The unity-gain amplifying or driving circuit 57 thus applies a
potential to the guard ring 20 which is substantially identical to
that of the associated primary electrode 22. In this example, the
voltage on the guard electrode 20 is made as identical as possible
with the voltage on electrode 22, as aforesaid. The driving circuit
is essentially an impedance transformer which ensures substantial
bypassing current through the peripheral thoracic structure. In
consequence, the thoracic core and peripheral impedances (Z.sub.1,
Z.sub.R) are isolated between electrodes 22, 24 and 20, 24
respectively.
As shown in FIG. 3, vector 70 representing the reactive changes in
core impedance is 90.degree. out of phase with vector 68
representing the resistive changes in thoracic core impedance. This
phase relationship is exact only at the condition of perfect bridge
balance, i.e., of amplitude null. When the phase-sensitive
detecting circuit 47 (FIG. 8) is properly aligned or tuned, it is
completely insensitive to the orthogonal component represented by
vector 70 (FIG. 3), so that 2V/2x.sub.1 =0, where V denotes
detector output signal voltage. Thus, the detector 47 is completely
insensitive to the change in the reactive or imaginary component
X.sub.1 of the core impedance Z.sub.1. On the other hand, the
detector 47 has maximum sensitivity to the change in thoracic
resistive component R.sub.1 represented by the vector 68, i.e.
2V/2R.sub.1 is a maximum. Vector 72 denotes the input voltage phase
while angle 74 relates the phase lag of the reference input 96
through conventional phase shifting network 97 of the detector 47
to the signal voltage on conductor 52.
The bridge circuit 34 is maintained in dynamic balance, as
aforesaid, by varying the impedance 32 (FIG. 2). The variable
impedance 32 in this example is a field-effect transistor 76, the
control electrode of which is coupled to feedback conductor 56 of
the detecting circuit 47. Thus, the effective impedance of the
variable impedance 32 is varied automatically in response to the
bridge output. An operator of our measuring apparatus, therefore,
is not required to add compensating capacitors across the bridge 34
to balance the capacitance of individual patients. Our measuring
apparatus eliminates the need for a highly skilled operator and it
is much less subject to operational error.
The phase-related detector 47 includes an input clipper stage 98
which cooperates with the output clipper stage 53 of the
differential amplifying circuit 45 to shape the differential output
signal into a substantially square waveform on conductor 100. At
the same time, the reference input voltage on conductor 96 is
similarly shaped by a Schmidt trigger circuit 102 to provide
likewise a square waveform on conductor 104. The symmetries of the
square waveforms on conductors 100, 104 are controlled by
potentiometers 103, 105 respectively.
The reference square wave is applied to the base electrode of
transistor 106, the emitter and collector circuits of which are
supplied respectively with positive and negative supply voltages
through equal load resistances 108, 110. As a result, a synchronous
square wave is developed on conductor 112 connected to the
collector circuit of the transistor 106 while a square waveform
which is similar but 180.degree. out of phase is developed on
conductor 114 connected to the emitter circuit of the transistor
106.
These waveforms (conductors 112, 114) are applied respectively to
the gate electrodes of a pair of field-effect transistors 116, 118.
The transistors 116, 118 form a series-shunt chopper, with the
transistors 116, 118 acting as electronic switches which open and
close in alternation. As noted previously, the reference input
voltage on conductor 96 is 90.degree. out of phase (or in some
predetermined phase relationship) with the input signal on
conductor 52 by operation of conventional phase-shifting network
97.
The alternate switching action of the series-shunt chopper
alternately charges capacitance 120 through transistor 116 and
discharges capacitance 120 through transistor 118 to ground. In
this process the average DC voltage across the capacitance 120
becomes an indication of the phase relationship between the
voltages on conductors 52, 96.
The switching of transistor 118 is controlled by a fixed voltage
clamp consisting of diode 124. On the other hand transistor 116 is
controlled by a variable voltage clamp composed of diode 126 and
transistor 128. A variable voltage clamp is necessary owing to the
presence of the varying output signal through transistor 116 and
across capacitance 120 which is directly connected thereto.
The average DC voltage across capacitance 120 is applied to an
amplifying and level-shifting circuit 127 to supply a usable output
signal on conductor 54. In addition, any tendency of the
capacitance voltage to deviate as a result of a similar deviation
in phase of the input signal on conductor 52 is translated into
changes in feedback voltage on conductor 56 which oppose such phase
deviations and maintain the bridge 34 in dynamic balance. The
feedback voltage is connected to the control electrode (FIG. 2) of
the transistor 76 which constitutes the impedance 32 of the bridge
34. The transistor 76, therefore, acts as a variable impedance to
maintain the bridge 34 in balance.
Thus, the phase relationship between the voltages on conductors 52
and 96 is substantially maintained so that the small phase
deviations and related deviations of capacitance 120 voltage are a
measure of the changes in impedance Z.sub.1. By maintaining the
phase relationship between the voltage on conductors 52 and 96
substantially at 90.degree., the sensitivity of the detecting
circuit 47 is maximized at a phase relationship corresponding, for
example to the phase of the real R.sub.1 component of thoracic core
impedance Z.sub.1 and insensitive to the component of impedance
Z.sub.1 at right angles to it, the reactive component X.sub.1 in
this case, as explained above in the description of FIG. 3.
A pair of standard resistances 130, 132 are connected in the bridge
34 (FIG. 2) momentarily by closing switch 134 and opening switch
136 respectively. Resistance 132 is very low compared to impedance
130. Resistances 130, 132 can be individually switched to parallel
and series respectively with the subject. As the measured impedance
is relative only changes therein are normally indicated. Momentary
interjection of the known resistances 130, 132 into the
impedance-measuring circuit therefore yields absolute bases of
comparing subject impedance and conductivity respectively at any
given point along the output curve (such as the curves 88, 92 of
FIG. 5) of the impedance-measuring circuit. Alternatively, the
operation of the switches 134, 136 can be reversed with the
resistances 130, 132 being then parallel in the circuit, so that
reference values can be established by momentarily shorting
resistances out of the measuring circuit. Thus, the resistances
130, 132 relate lung conductivity, in the specific application of
our apparatus described herein, to lung resistance for the subject
being studied. Once the feedback from the amplifying and matching
circuit 126 is thus adjusted, the bridge 34 is subsequently
maintained in dynamic balance by the phase-related detector 47 and
the variable bridge impedance 32.
The improvements in pneumographic measurements made possible by our
apparatus are illustrated in FIG. 4. Arrow 78 represents a 27
percent change in a measured thoracic impedance level of about
1,000 ohms utilizing the electrode assembly shown in FIG. 4B. In
the latter arrangement, the guard ring electrode 20' has a width
(dimensional arrow 80) equal to approximately twice the the
peripheral wall thickness of the thoracic region. A similar
percentage change (arrow 82 FIG. 4) at a lower measured impedance
level of about 600 ohms was obtained with the electrode structure
of FIG. 4A. In the latter structure the width 84 of the guard ring
20 is about equal to the thickness of the thoracic wall 12 (FIG.
1). In contrast, arrow 86 (FIG. 4) denotes the usual and unreliable
change of about 4 percent measured by conventional, two-electrode
impedance pneumographs during the respirational cycle, where no
guard ring is used.
The electrodes 20 or 20' and 22 of FIGS. 4A and 4B are made from
flexible metallic foil or sheet. Tin is suitable for this purpose
and the electrodes are usually applied with an interposed layer of
conductive grease. The guard electrode 20 need not be circular or
even continuous. As can be seen in FIG. 4C, the guard electrode can
be segmented or provided as discrete but electrically
interconnected electrodes 21. Although four such electrodes are
shown in equally spaced array, obviously a different number or
arrangement can be used, depending on the application of the
invention.
In FIG. 5 curve 88 is indicative of variations in phase detector
output on conductor 90 (FIGS. 2, 8), which in turn indicates
respiratory thoracic impedance changes of a walking subject. Curve
92 of FIG. 5 similarly represents thoracic impedance changes in a
subject who is nearly asleep. The sensitivity of an output signal
recorder 98 can be adjusted by trimmer-resistance 94 (FIG. 8).
It will be understood that the bridge and detecting circuit 26 can
be connected to indicate impedance between another pair of the
electrodes 20-24. Thus, the impedance Z.sub.R between the guard
ring and the primary electrode 24 or 22, respectively can be
similarly and exclusively measured or monitored.
From the foregoing it will be apparent that novel and efficient
forms of physiological impedance-measuring apparatus have been
disclosed herein. Although our invention is adaptable for measuring
various parts of the anatomy, it will be described primarily in
connection with the thoracic regions of the body, as the lungs are
most sensitive in their electrical conduction processes and are
most symptomatic of body conditions. Thus, such measurements will
yield information which is highly desirable in a large number of
diagnostic situations. While we have shown and described certain
presently preferred embodiments of the invention and have
illustrated certain presently preferred methods of practicing the
same, it is to be distinctly understood that the invention is not
limited thereto but may be otherwise variously embodied and
practiced within the scope of the following claims.
* * * * *