U.S. patent number RE30,101 [Application Number 05/589,360] was granted by the patent office on 1979-09-25 for impedance plethysmograph.
This patent grant is currently assigned to Regents of the University of Minnesota. Invention is credited to Edwin Kinnen, William G. Kubicek, Robert P. Patterson, David A. Witsoe.
United States Patent |
RE30,101 |
Kubicek , et al. |
September 25, 1979 |
Impedance plethysmograph
Abstract
.Iadd.Cardiac output is measured by connecting excitation
electrodes at the upper and lower ends of the thorax of a mammalian
subject, connecting measuring electrodes to the thorax between the
excitation electrodes, applying a constant fluctuating excitation
current to the excitation electrodes, measuring, with appropriate
circuitry, changes in impedance within the thorax, and
simultaneously measuring the beginning and end of systole, and
determining cardiac output by measuring the maximum decreasing
impedance slope during systole..Iaddend.
Inventors: |
Kubicek; William G. (Rosemount,
MN), Kinnen; Edwin (Pittsford, NY), Patterson; Robert
P. (Minneapolis, MN), Witsoe; David A. (Rochester,
NY) |
Assignee: |
Regents of the University of
Minnesota (Minneapolis, MN)
|
Family
ID: |
27013190 |
Appl.
No.: |
05/589,360 |
Filed: |
June 23, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
390555 |
Aug 19, 1964 |
03340867 |
Sep 12, 1967 |
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Current U.S.
Class: |
600/526;
600/547 |
Current CPC
Class: |
A61B
5/026 (20130101); A61B 5/0295 (20130101); A61B
5/0535 (20130101) |
Current International
Class: |
A61B
5/026 (20060101); A61B 5/053 (20060101); A61B
005/02 () |
Field of
Search: |
;128/2.5F,2.5V,2.5R,2.1Z |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Zajic; F., et al., Physiologia Bohemoslovenica, vol. III, 1954, pp.
355-361. .
Nyboer; J., Electrical Impedance Plethysmography, 1954, Charles C.
Thomas, Spfld. Ill., Publisher, pp. 50-57. .
Bishop; S. et al., Harper Hospital Bulletin, Jul.-Aug. 1962, pp.
142-143..
|
Primary Examiner: Howell; Kyle L.
Attorney, Agent or Firm: Merchant, Gould, Smith, Edell,
Welter & Schmidt
Claims
We claim:
1. A plethysmograph for measuring cardiac output comprising in
combination excitation electrode means adapted to be connected to a
mammalian subject at the superior and inferior ends of the thorax,
a current generator means comprising an electronic oscillator
conductively connected to the electrode means for supplying a
fluctuating excitation current thereto, sensor means adapted to be
conductively connected to said thorax for carrying a sensed
electrical signal which varies as the impedance changes in the
thorax between said electrode means, a control means conductively
connected to the oscillator for balancing the current from the
oscillator with the sensed signal, a first amplifier conductively
connected to the control means, a second amplifier connected to
said sensor means whereby signals of approximately equal strength
are fed to said first and second amplifiers from said control and
said sensor means respectively and a voltage subtracting means
conductively connected to each of the amplifiers for comparing the
output of each said amplifier.
2. The apparatus of claim 1 wherein a first rectifying means is
conductively connected between the sensor means and the subtracting
means and a second rectifying means is connected between the
oscillator and the subtracting means.
3. A plethysmograph for measuring cardiac output comprising in
combination a first elongated band excitation electrode adapted to
be positioned to at least partially encircle the neck of a subject,
a second elongated band excitation electrode adapted to be
positioned to at least partially encircle the thorax of said
subject at the approximate position of the xiphisternal joint, a
source of a fluctuating current conductively connected across the
electrodes for supplying an excitation current thereto, sensor
means adapted to be conductively connected to said thorax for
receiving a sensed signal, a voltage balancing means conductively
connected to the oscillator, a first output lead conductively
connected to the balancing means, a second output lead connected to
said sensor means, and a voltage subtracting means conductively
connected to each of the output leads for comparing the voltage,
whereby the adjustment of the balancing means is adapted to feed
signals of approximately equal strength to the subtracting
means.
4. The apparatus according to claim 3 wherein said sensor means
comprises a pair of measuring electrodes adapted to be positioned
upon said thorax.
5. The apparatus according to claim 3 wherein said balancing means
comprises a potentiometer wired to the output of said source of
fluctuating current.
6. A plethysmograph for measuring cardiac output comprising in
combination a first elongated band excitation electrode adapted to
be positioned around the neck of said subject, a second elongated
band excitation electrode adapted to be positioned to encircle the
thorax of said subject at the approximate location of the
xiphisternal joint, an electronic oscillator means conductively
connected across the excitation electrodes for supplying an
excitation current thereto, sensor electrodes adapted to be
conductively connected to said thorax for carrying a sensed
current, a balancing means conductively connected to the
oscillator, a first amplifier conductively connected to the
balancing means, a second amplifier connected to one of said sensor
electrodes, a voltage subtracting means conductively connected to
each of the amplifiers for comparing the output thereof and a means
for rectifying the current received by the subtracting means from
each said amplifier means, whereby the adjustment of the balancing
means is adapted to feed signals of approximately equal strength to
said subtracting means.
7. A plethysmographic method for measuring cardiac output which
comprises connecting excitation electrode means at the upper and
lower ends of the thorax of a mammalian subject above the upper
border of the lungs and below the lower border of the heart and
lungs, respectively; connecting measuring electrode means to the
thorax of the subject between said excitation electrode means;
applying a constant fluctuating excitation current to said
excitation electrode means and through a balancing resistance;
amplifying, detecting and measuring the voltage across the
measuring electrode means and across the balancing resistance and
adjusting the balancing resistance to equalize the voltages;
measuring the changes in impedance within the thorax as sensed by
said measuring electrode means; and simultaneously measuring the
beginning and end of systole of the subject and determining cardiac
output therefrom.
8. A method according to claim 7 further characterized in that said
measured change in impedance and simultaneously measured beginning
and end of systole are recorded to produce a single composite
graphic image, a straight line is constructed tangential to the
portion of the graphic image at the maximum decreasing impedance
slope during systole, said line extending to intersect parallel
lines denoting beginning and end of systole, measuring the height
of the line thus constructed and determining cardiac output
therefrom.
9. The method according to claim 7 wherein said excitation
electrode means comprises a pair of encircling electrodes, one
adapted to at least partially encircle the neck and the other
adapted to at least partially encircle the thorax of the subject at
the xiphisternal joint and wherein said measuring electrode means
comprises a pair of electrodes adapted to be positioned between and
a short distance inwardy from each of said pair of excitation
electrodes.
10. A plethysmograph comprising in combination: excitation
electrode means adapted to be connected to a mammalian subject at
the superior and inferior ends of the thorax; electric generator
means for supplying a fluctuating excitation current, said
generator means being conductively connected to said electrode
means whereby said excitation current is applied to said electrode
means; measuring means; conductor means adapted to be conductively
connected to the thorax and to said measuring means for carrying an
electrical signal from the portion of the thorax between said
electrode means to said measuring means; balancing means connected
to said generator means for providing a second electrical signal
approximately equal in magnitude to said first mentioned signal;
means for rectifying each of said signals; and means for comparing
the signals after rectification.
11. A plethysmographic method for measuring cardiac output which
comprises the steps of: applying a substantially constant current,
fluctuating voltage, excitation signal between the upper and lower
ends of the thorax of a mammalian subject from above the upper
border of the lung to below the lower border of the heart and lung;
measuring impedance changes within the portion of the thorax
carrying the signal while simultaneously measuring the beginning
and the end of systole of the subject; and recording the
simultaneous measurements for determining cardiac output
therefrom.
12. A plethysmographic method for measuring cardiac output which
comprises the steps of: applying a substantially constant current,
fluctuating voltage, excitation signal between the upper and lower
ends of the thorax of a mammalian subject from above the upper
border of the lung to below the lower border of the heart and lung;
and measuring impedance changes within the portion of the thorax
carrying the signal relative to a period from beginning to end of
systole of the subject for determining cardiac output
therefrom.
13. A plethysmograph comprising: electrical means adapted to induce
a fluctuating excitation current between the superior and inferior
ends of the thorax of a mammalian subject; and means for measuring
impedance changes in said thorax, in the presence of said
fluctuating excitation current, and for measuring, simultaneously,
the beginning and the end of systole of the subject. .Iadd.
14. A plethysmograph for measuring cardiac output comprising in
combination:
(a) excitation electrode means adapted to be connected to a
mammalian subject at the superior and inferior ends of the
thorax,
(b) a current generator means comprising an electronic oscillator
conductively connected to the electrode means for supplying a
fluctuating excitation current thereto,
(c) sensor means adapted to be conductively connected to said
thorax for carrying a sensed electrical signal which varies as the
impedance changes in the thorax between said electrode means,
(d) a control means conductively connected to the oscillator for
balancing the current from the oscillator with the sensed
signal,
(e) a first amplifier conductively connected to the control
means,
(f) a second amplifier connected to said sensor means whereby
signals of approximately equal strength are fed to said first and
second amplifiers from said control and sensor means
respectively,
(g) a voltage subtracting means conductively connected to each of
the amplifiers for comparing the output of each said amplifier,
(h) and means connected to said subtracting means for receiving an
output therefrom whereby the maximum decreasing slope of said
impedance changes in the thorax may be determined. .Iaddend.
.Iadd.
15. a plethysmograph for measuring cardiac output comprising in
combination:
(a) excitation electrode means adapted to be connected to a
mammalian subject at the superior and inferior ends of the
thorax,
(b) a current generator means comprising an electronic oscillator
conductively connected to the electrode means for supplying a
fluctuating excitation current thereto,
(c) sensor means adapted to be conductively connected to said
thorax for carrying a sensed electrical signal which varies as the
impedance changes in the thorax between said electrode means,
(d) a control means conductively connected to the oscillator for
balancing the current from the oscillator with the sensed
signal,
(e) a first amplifier conductively connected to the control
means,
(f) a second amplifier connected to said sensor means whereby
signals of approximately equal strength are fed to said first and
second amplifiers from said control and sensor means
respectively,
(g) a voltage subtracting means conductively connected to each of
the amplifiers for comparing the output of each said amplifier,
(h) and means connected to said subtracting means for receiving an
output therefrom whereby the maximum decreasing slope of said
impedance changes in the thorax and .DELTA.Z may be determined.
.Iaddend. .Iadd.
16. A plethysmographic method for measuring cardiac output which
comprises:
(a) connecting excitation electrode means at the upper and lower
ends of the thorax of a mammalian subject above the upper border of
the lungs and below the lower border of the heart and lungs,
respectively;
(b) connecting measuring electrode means to the thorax of the
subject between said excitation electrode means;
(c) applying a constant fluctuating excitation current to said
excitation electrode means and through a balancing resistance;
(d) amplifying, detecting, and measuring the voltage across the
measuring electrode means and across the balancing resistance and
adjusting the balancing resistance to equalize the voltages;
(e) measuring the changes in impedance within the thorax as sensed
by said measuring electrode means;
(f) and simultaneously measuring the beginning and end of systole
of the subject, determining the maximum decreasing impedance slope
during systole, and determining cardiac output therefrom. .Iaddend.
.Iadd.
17. A plethysmographic method for measuring cardiac output which
comprises:
(a) connecting excitation electrode means at the upper and lower
ends of the thorax of a mammalian subject above the upper border of
the lungs and below the lower border of the heart and lungs,
respectively;
(b) connecting measuring electrode means to the thorax of the
subject between said excitation electrode means;
(c) applying a constant fluctuating excitation current to said
excitation electrode means and through a balancing resistance;
(d) amplifying, detecting, and measuring the voltage across the
measuring electrode means and across the balancing resistance and
adjusting the balancing resistance to equalize the voltages;
(e) measuring the changes in impedance within the thorax as sensed
by said measuring electrode means;
(f) and simultaneously measuring the beginning and end of systole
of the subject, determining the maximum decreasing impedance slope
during systole and .DELTA.Z, and determining cardiac output
therefrom..Iaddend.
Description
The present invention relates to plethysmographs and particularly
to an impedance plethysmograph and process of using the same. The
invention is particularly useful in determining cardiac output.
In accordance with the present invention, a current flux is
distributed in the mammalian thorax by the placement of electrodes
at the neck and lower thorax. According to one form of the
invention the electrodes have the form of bands positioned to
encircle the neck and thorax. To these electrodes is applied a
fluctuating excitation current. The impedance of the thorax is then
measured with either the same electrodes or with different
electrodes to obtain information concerning cardiac activity and
particularly cardiac output.
In evaluating the invention, measurements have been made which
indicate that the major portion of excitation current flux passed
through the lung tissues, rather than through the lower resistivity
volume of the major thoracic arteries, veins and the heart.
Accordingly, it is possible to measure the blood volume changes of
the lungs and derive cardiac output from the impedance changes.
The evaluations were based upon measurements of current densities
around different regions on the circumference of the electrodes,
equipotential surfaces constructed from both surface and interior
potential measurements, externally observed directed flux impedance
waveforms and tests conducted with a model representing the thorax.
The impedance plethysmographic waveforms obtained between the
electrodes appeared to monitor the pulmonary flow as reflected by
impedance changes in the pulmonary vascular bed.
The invention is based on the measurement of electrical impedance
changes in the thorax during application of a fluctuating current
(such as a 100 kc. current having RMS value of 5.0 ma.) between
electrodes placed on the surface of the thorax. This procedure
provides the advantage of minimum subject preparation and
constraint.
A variety of electrodes have been investigated for potential use.
Results obtained with four braided band electrodes, two positioned
around the neck and two placed around the subject's midsection have
been the most acceptable. The experimentally determined values of
cardiac output obtained from these impedance measurements have been
found to be significantly correlated to simultaneously performed
studies using the Fick and dye dilution procedures as reported, for
example, in Circulatory Physiology: Cardiac Output and Its
Regulation, Arthur C. Greyton, W. B. Saunders Co., 1963, pages
21-71. The system according to the present invention has also been
shown to be relatively insensitive to body type and lung air
volume.
To verify the determination of cardiac output from impedance
measurements, an understanding of the physiological phenomena
responsible for the measured variations must be obtained. In
particular the excitation current flux paths in the thorax between
the excitation electrodes must be established.
It has been found that the only tenable theoretical flux
distribution path is that in which the majority of .[.he.].
.Iadd.the .Iaddend.current flux passes from the band electrodes
into the lung volume and tends to avoid the heart blood volume
regions. Consequently, this current flux distribution implies that
the measured impedance is an indication of the total movement of
blood in the pulmonary vascular bed.
The impedance waveforms obtained with two band electrodes, one
positioned at the neck and the other about 2 cm. below the
xiphisternal joint, appeared to reflect the pulmonary blood
pulsations in the lungs rather than the direct ventricular volume
change. The equipotential surfaces sketched from thorax potential
measurements indicated a flow of current from the blood volume
regions. The largest density of the current leaving a band
electrode positioned at the midsection of the thorax was found at
the base of the lungs on the posterior thorax.
Each of these results would indicate that in an electrode
configuration including one band electrode about the neck and
another about the thorax as set forth above, the majority of the
currrent flux passed through the lungs, such that the major
observed impedance characteristics were controlled by the pulsating
pulmonary volume changes. Furthermore, the relatively accurate
cardiac output determinations made by these impedance measurements
were apparently based on an indirect indication of right
ventricular stroke volume as reflected by the pulmonary vascular
bed.
Among the objects of the invention is the provision of a method and
apparatus for sensing and recording cardiac output.
Another object of the invention is the provision of a
plethysmographic system for obtaining a record of cardiac output
wherein a minimum of subject restraint is required and wherein no
surgical procedure is necessary.
Another object of the invention is the provision of an improved
plethysmographic process and apparatus suitable for use in routine
physical testing to obtain a record from which cardiac output can
be derived.
Another object of the invention is the provision of an improved
plethysmographic process and apparatus including a reliable means
for eliminating errors caused by skin impedance at the stimulating
electrodes.
Yet another object of the invention is the provision of an improved
process and apparatus for measuring and recording cardiac output
wherein a single manual adjustment is required to set the
electrically sensing and recording devices of the invention in
condition for making a recording.
Yet another object of the invention is the provision of an improved
means for sensing and recording impedance changes in an organ
wherein the reactive component of the impedance of the organ will
not prevent making reliable impedance readings.
Yet another object of the invention is the provision of an improved
means for measuring cardiac output wherein a first set of
electrodes are placed respectively at the upper and lower ends of
the thorax to provide electrical stimulation and a second pair of
measuring electrodes are attached to the thorax intermediate the
excitation electrodes.
A still further object of the invention is the provision of an
improved plethysmographic apparatus and process for measuring
cardiac output including a means for generating a constant
fluctuating current to excite the tissue, a means for applying the
current thus generated to the thorax of an animal, a sensing means
connected to the thorax for receiving a signal from which the
impedance between two spaced apart points can be measured, a
balancing means connected to the current generating means for
matching the current produced by the current generating means with
the signal thus sensed and a means for rectifying both the
excitation signal and the sensed signal and for comparing the
signal thus sensed with the excitation signal.
Other objects of the invention will become apparent as the
description proceeds.
To the accomplishment of the foregoing and related ends, this
invention then comprises the features hereinafter fully described
and particularly pointed out in the claims, the following
description setting forth in detail certain illustrative
embodiments of the invention, these being indicative, however, of
but a few of the various ways in which the principles of the
invention may be employed.
The invention is illustrated by the accompanying drawings in which
the same numerals refer to corresponding parts and in which:
FIG. 1 is a partial front elevational view of a human subject
showing the position of the excitation and measuring electrodes
when placed in position for use;
FIG. 2 is a diagram representing a portion of a strip chart record
prepared in accordance with the system of the present invention and
including a record of audible heart sounds at the top thereof and
at the bottom a record of the impedance changes in the thorax as
measured by the use of the system of the invention;
FIG. 3 is a schematic block diagram illustrating the means used for
connecting the current excitation and recording electrodes in
accordance with the system of the invention;
FIG. 4 is a schematic circuit diagram of the current excitation
oscillator and recording means of the system of the invention;
FIG. 5 is a partial diagrammatic view of a mammalian body
illustrating typical equipotential surfaces as determined in the
evaluation of the system of the invention; and
FIG. 6 is a diagram illustrating the position and direction of
current flux lines between the stimulating electrodes of the system
of the invention.
Referring now to the figures of the drawings which illustrate by
way of example a preferred form of practicing the invention, there
is shown in FIGS. 1 and 3 a pair of current excitational electrodes
A and D each desirably formed from a braided copper wire. The
electrodes A and D are positioned, respectively, at the upper
portion of the neck and around the lower abdomen. Positioned
intermediate the electrodes A and D is a pair of measuring
electrodes B and C, the electrode B being positioned at the base of
the neck and the electrode C approximately 2 cm. below the
xiphisternal joint with the skin of the thorax. The electrodes B
and C are also desirably constructed of braided copper wire.
As shown in FIG. 3, there is provided a constant current oscillator
10, the construction of which will be described more fully
hereinbelow, and including an output lead 12 which is connected to
the electrode A and an output lead 14 connected to the electrode D.
The oscillator 10 is of the type adapted to produce a constant
fluctuating current, such as a 100 kc. alternating current. The
oscillator 10 passes the same current applied through the
conductors 12 and 14 through a set of conductors 16 and 18 which
are connected to the ends of the balancing potentiometer 20.
Connected to the electrode B by means of a conductor 22 is an
alternating current amplifier 24 which will be described more fully
hereinbelow. A similar amplifier 26 is connected to the slide arm
28 of the potentiometer 20. The outputs of the amplifiers 24 and 26
are fed through conductors 30 and 32, respectively, to rectifying
means, such as detectors 34 and 36, to a subtraction or comparing
means such as a DC differential amplifier 42. A conductor 44 is
connected between the electrode C and the differential amplifier
42. The output signal produced by the differential amplifier 42 is
fed through a pair of conductors 46 and 48 to a suitable measuring
and recording instrument such as a strip chart recorder 50, as seen
in FIG. 4.
The skin impedance is eliminated by introducing into the thorax a
current with the two outer electrodes using the constant current
source 10 and then measuring a voltage between the inner electrodes
that is proportional to the electrical impedance. Since the current
is constant, the voltage equals a constant times the impedance. If
the electrical input impedance of the instrument that measures the
voltage between the inner electrodes is very high compared with the
impedance between the inner electrodes, very little current will
flow to the electrodes and, therefore, very little skin impedance
will be measured.
In FIG. 2 there will be seen a strip of recording paper 51 upon
which heart sounds are recorded as a trace 53. Trace 53 includes a
plurality of peaks 55 and 57 which indicate, respectively, the
opening of the heart valve during systole and the closing of the
heart valve at the end of systole. At the lower portion of the
graph is recorded a trace 59 representing impedance measurements
obtained from the differential amplifier 42. From the peaks 55 and
57 are constructed vertical lines 61 and 63. A third line 65 is
constructed by drawing a straight line tangent to the maximum
decreasing impedance slope at 67 during systole and extending this
line to the first and second heart sounds or, in the alternative,
to any other means indicating the beginning and end of systole.
Where the extended line 65 intersects the lines 61 and 63,
horizontal lines 69 and 71 are constructed. The distance between
lines 71 and 69 is measured to find .DELTA.Z. Flow during each
stroke of the heart continues between the opening of the heart
valve as shown by the peak 55 and the closing of the valve at the
end of systole as shown by the peak 57. The flow rate is a function
of the rate of change of the impedance as shown by the slope at 67.
Thus, by constructing line 65 between the opening and closing of
the valves during systole a determination can be made of the
theoretical change in impedance (.DELTA.Z) which takes into account
all of the blood injected during that stroke into the thorax. The
impedance change .DELTA.Z (across the electrodes B and C) can be
determined from the distance between lines 69 and 71 by producing a
known impedance change and measuring the height of the deflection
peak caused by this change in the trace 59.
Referring now to FIG. 4, and particularly to the constant current
oscillator 10, it will be seen that current is provided from a 30
volt power supply (not shown) across lines 60 and 62. Connected to
line 62 is a line 64 which is coupled to the emitter of a
transistor Q1 through a variable resistance R6 in parallel with a
fixed resistance R11. Line 62 is also connected through the line 64
to a fixed resistance R4 in parallel with a capacitance C3.
Connected to line 60 in series with the resistance R4 and
capacitance C3 are two parallel connected resistances R2 and R3.
The base of Q1 is connected through a resistance R1 in series with
parallel connected capacitances C1 and C2 and a capacitance C5 to
the collector of a transistor Q2. The collector of Q1 is connected
through a resistance R5 to line 60 and through a capacitance C4 to
the base of Q2. Line 64 is also connected to line 60 through series
connected resistances R7 and R9.
The emitter of Q2 is connected to line 64 through a resistance R8
while the collector thereof is connected to line 60 through a
resistance R10 and to the base of a transistor Q3 through a
capacitance C6. The base of Q3 is also connected to line 60 through
resistance R12 which the emitter thereof is connected to line 62
through parallel connected resistances R13 and R16 in series with
resistance R14. The collector of transistor Q3 is connected
directly to line 60 by means of a conductor 65. The output of the
transistor Q3 is fed through a line 66 connected between R13 and
R16 and R14 and through capacitance C7 to the base of a transistor
Q4, said base also being connected to line 60 through a resistance
R15. The emitter of Q4 is wired through parallel connected
resistances R17 and R18 with the line 62. The collector of Q4 is
connected to the series coupled primary coils of transformers T1
and T2, the free terminal of the primary of T2 being connected to
line 60.
The secondary of the transformer T2 is connected by means of the
conductors 12 and 14 to the electrodes A and D, while the secondary
of the transformer T1 is connected by means of the conductors 16
and 18 to the balancing potentiometer 20. In this manner, an
oscillating output signal is fed in phase and in equal strength to
both the excitation electrodes A and D and to the DC amplier 26
through the potentiometer 20.
The amplifier 24 includes a pair of transistors Q5 and Q7. The base
of Q5 is connected to the electrode B through a capacitance C8. The
collector of Q5 is connected to the base of Q7 through a
capacitance C10. Connected between C10 and Q5 and a power supply
line 70 is a resistance R24. Between the base of Q7, C10 and the
line 70 is a resistance R28. Line 70 is connected to a suitable
source of current, such as a 30 volt power supply, by means of a
terminal 72. Line 70 is connected directly to the collector of Q7
through a line 74. The base and emitter of Q5 are connected to
ground through resistances R21 and R25 respectively. The emitter of
Q7 is connected to ground through a resistance R30 and to the diode
34 through a capacitance C12. One side of diode 34 is connected to
ground through a resistance R32 and the other side is connected to
ground through a capacitance C14.
The amplifier 26 includes a pair of transistors Q6 and Q8. Power is
supplied through a line 80 to the base of Q6 through a resistance
R23 and to the collector thereof through a resistance R27. Line 80
is also connected to the base of Q8 through a resistance R29 and
directly to the collector through a conductor 82. The emitters of
Q6 and Q8 are grounded through resistances R26 and R31,
respectively. The base of Q6 is connected to ground through a
resistance R22 and to the slide arm of the potentiometer 20.
Between the collector of Q6 and the base of Q8 is a capacitance
C11. The emitter of Q8 is connected through a capacitance C13 with
the diode 36. The diode 36 is also connected to ground through a
resistance R33 and the other side thereof is connected to ground
through capacitance C15. In this manner, the output of the
amplifiers 24 and 26 is rectified by the detectors 34 and 36 and
the signals are subtracted by means of the differential amplifier
42. Any difference in the signals is either observed on the meter
(not shown) connected to the lines 46 and 48 at the output of the
differential amplifier 42 or recorded by means of recorder 50. It
should also be seen that through the use of the present invention
the capacitance component of the impedance will not interfere with
accurate readings since phase differences are eliminated by
rectifying the signals before they are compared and the difference
recorded.
By way of example, the following circuit constants can be employed.
The resistors have a 1/2 watt rating unless otherwise
specified.
______________________________________ Q1, 2, 3 T1495. Q4 T1487.
Q5, 6 2N930. Q7, 8 2N336. 34, 36 1N625. C6 .05 mfd. C7 .5 mfd. C8,
9 100 mfd. C10, 11 220 mfd. R1, 4, 14 2.2K.OMEGA.. R2 82K.OMEGA..
C1 45-25 mfd. C2 300 mfd. C3 270 mfd. C4, 12, 13 .001 mfd. C5, 14,
15 .01 mfd. R3, 5 10K.OMEGA.. R7 47K.OMEGA.. R8 1K.OMEGA.. R9, 28,
29 330K.OMEGA.. R10, 11 6.8K.OMEGA.. R12 1MEG.OMEGA.. R15
39K.OMEGA.. R16, 25, 26, 30, 31 4.7K.OMEGA.. R17 100.OMEGA., 1
watt. R18 47.OMEGA., 1 watt. R20, 23 3.9MEG.OMEGA.. R21, 22
680K.OMEGA.. R24, 27, 32, 33 27K.OMEGA.. R6 1MEG.OMEGA. trimpot.
R13 20K.OMEGA. trimpot. 20 100.OMEGA. trimpot. T1 and 2 5 : 1 turns
ratio. ______________________________________
In operation, the balancing potentiometer 20 is adjusted until
there is an approximate zero steady state voltage at the output of
the DC differential amplifier. Since an equal amount of current is
passed through the thorax and through the balancing potentiometer
20, the voltage across each of them is proportional to the
impedance of each. Therefore, when the plethysmograph is balanced,
the resistance of the balancing potentiometer will equal the
impedance magnitude between the inner electrodes. Since the
balancing signal is obtained from the excitation signal, any
variations in the excitation signal are balanced out, thereby
eliminating artifacts caused by variations in the excitation
signal. The small changing voltage from the DC differential
amplifier which is caused by the change of impedance between the
two inner electrodes is then fed to a graphic recorder. While the
record is made the subject is instructed to hold his breath.
Refer now to FIG. 5 which illustrates the equipotential planes
constructed in evaluating the use of the invention. To compile this
data, a pair of excitation electrodes 90 and 92 were placed around
the neck and lower torso, respectively. A third electrode 94 was
used as a movable probe to record potential at any location on the
surface of the body or within the body. To this end, the electrode
94 was in some instances swallowed or inserted into other body
cavities such as bronchi. A 100 kc. electrical excitation current
of about 5 ma. was applied to the electrodes 90 and 92.
The electrodes 90 and 92 were connected in place of electrodes A
and D of FIGS. 1, 3 and 4 and the electrode 94 was connected to the
conductor 44. The band electrodes 90 and 92 of FIG. 5 were made
from tinned copper braid shielding. The braid was stretched to a
width of about 1 cm. and coated on the inner side with electrode
paste to provide a low impedance skin contact. The electrodes were
applied approximately 5 min. before data were taken to allow the
skin and paste to reach an equilibrium condition. For the two band
electrode placement shown in FIG. 5, and the 100 kc. excitation
frequency used throughout this investigation, the thorax presented
an average impedance of 37 ohms with a phase angle of
15.degree..
Several probe configurations were used in conjunction with the
impedance plethysmograph to measure potential points on the surface
and at various internal points of the thorax of human subjects and
dogs. The external thorax surface probe was a 3/4 in. circular
brass disc. The probe used for potential measurements in the human
esophagus was machined from stainless steel and attached to a
nasogastric tube. A probe similar to the human esophagus probe but
of larger dimension was used for measuring potentials in the
trachea and large bronchi of dogs. To obtain measurements in the
lower esophagus, an electrode was constructed with the metal
contact from the lung probe mounted at the end of a 1/4 inch rigid
Plexiglas tube. A 0.003 inch stainless steel wire inserted in a
saline irrigated catheter was used to determine potentials in the
aorta, heart, and carotid artery. The esophagus electrode was
swallowed by the subject and positioned by X-ray. The other
electrodes were inserted orally or surgically into dogs
anesthetized with sodium pentobarbitol, and positioned by X-ray and
catheter length measurements.
The waveforms obtained while attempting to direct the current flux
through particular portions of the thorax by placing electrodes A
and D in selected locations showed there was a dominant
characteristic of decreasing impedance during systole. The
impedance decrease during systole exhibited by the waveform of the
two excitation band electrode configuration of FIGS. 1 and 3
suggested that the dominant characteristic being observed is the
movement of low resistivity blood into the thoracic regions
carrying the majority of the current flux, as described in
connection with FIG. 6. The two possible regions in which this can
occur are the pulmonary vascular bed, and the heart and arterial
system. As the voltage drop between the neck electrode and position
1 was due to a constant current source, the effect of blood
pulsations in the arteries above the rib cage appeared to produce
minor contributions, if any, to the observed two band waveform.
Although current flux densities cannot be readily determined from
equipotential surfaces, the direction of current flow can be
established from a knowledge of these surfaces. With the direction
of current flow normal to the equipotential surfaces, it is seen
from FIG. 5 that the current flux for the two band electrode
configuration appeared to be moving out from the center of the
thorax volume. It should be noted that the equipotential planes of
FIG. 5 suggest movement of flux away from the central regions.
The smallest electrode current density measured with the segmented
electrode was found for the segment located nearest the heart apex.
If the aorta and superior vena cava carried the majority of the
current flux in the upper thorax, the flux would be expected to
continue into the lower resistivity blood volume of the heart. The
contrary observation indicated that relatively little current flux
passed through the heart for the two band electrode configuration
of FIG. 5.
In preparing the diagram illustrating the equipotentials, the
following potential surfaces were designated: 40 mv., 35 mv., 30
mv., 25 mv., 24 mv., 20 mv., 18 mv., 15 mv. and 10 mv., as can be
seen in FIG. 5. These surfaces extend downwardly at their center to
a greater extent at the upper part of the thorax than at the lower
part of the thorax. The numerical values are proportional to
impedance values because the current is constant.
These studies indicate the position of flux lines at 100 as seen in
FIG. 6 and pass almost entirely through the lung tissue 102 rather
than through the heart, which is indicated diagrammatically at 104.
As can also be seen in FIG. 6, the blood leaving the right
ventricle 106 through the pulmonary artery 108 passes through the
lungs to the left atrium through the pulmonary vein 110. It is thus
concluded that the changes in impedance of the lungs are determined
by the changes of the voltage between the electrodes B and C as a
function of the stroke volume.
Cardiac output is calculated by multiplying the stroke volume,
.DELTA.V by the pulse rate. .DELTA.V is calculated as follows:
##EQU1## P=the resistivity of blood L=the distance in centimeters
between the stimulating electrodes A and D
Z=the base or static impedance between the electrodes C and B
.DELTA.z=measured change in impedance between the electrodes as
calculated in the manner described in connection with FIG. 2.
In normal subjects this is the right cardiac output and in patients
with congenital heart defects the cardiac output value will include
blood flow shunted from the left heart to the pulmonary
circulation.
The invention will be best understood by reference to the following
examples. In calculating the cardiac output in the Examples 1
through 4, L was 27 centimeters, Z was 31.9 ohms and P was 150
ohms/cm.
TABLE I ______________________________________ Example 1 Example 2
Example 3 ______________________________________ .DELTA.Z 85.OMEGA.
92.OMEGA. 90.OMEGA. .DELTA.V 91.4 cc 99 cc 96.8 cc. Pulse rate 76
beats/min 75.5 beats/min 74 beats/min C.O 6.98 liters/min 7.48
liters/min 7.15 liters/min.
______________________________________
Additional examples are given below in Table II, each example being
taken from a different human subject using the braided
copper-electrodes as described hereinabove and with a conductive
material such as electrode paste to provide a low impedance
electrical connection to the skin and to allow some movement
between the skin and the electrodes. The electrodes were allowed to
stand for a period of about five minutes in order that equilibrium
is attained. Each human subject represented in Table II was a
hospital patient having a heart defect. In some cases the tests
were run after the defect had been repaired. A comparison is made
in Table II between the cardiac output as determined by the system
of the present invention and that obtained with the well known Fick
pulmonary flow measurement on the same subject.
TABLE II ______________________________________ CARDIAC OUTPUT-FICK
COMPARISON Percent Differ- ence between Fick pulmo- Fick nary flow
Ex. Systemic Pulmonary, Impedance and imped- No. flow,l./min.
l./min. C.O.,l./min. ance C.O.
______________________________________ 4 6.3 7.86 5.21 -31.3 5 2.5
4.8 5.5 +14.6 6 5.0 5.2 5.22 0 7 3.6 5.0 4.43 -11.4 8 3.1 5.9 6.29
+6.6 9 4.65 5.0 4.81 -3.8 10 4.0 4.0 4.13 +3.2 11 6.0 6.0 6.66
+11.0 12 4.7 4.7 4.24 -9.6 13 5.4 5.4 6.22 +15.2 14 5.9 5.9 6.2
+5.1 15 5.9 5.9 6.34 +7.4 16 6.1 6.1 6.62 +8.5 17 6.8 6.8 6.71 -1.3
18 -- 5.3 5.42 -6.5 19 2.7 2.7 2.9 +7.4 20 7.3 7.3 7.1 -2.7 21 5.65
5.65 6.19 +9.5 22 4.85 4.75 4.15 -12.6 23 -- 4.3 4.87 +13.2 24 7.7
7.7 8.08 +4.9 25 7.2 7.2 7.19 -1.4 26 4.0 4.9 5.09 +3.9 27 5.3 5.3
5.9 +11.3 .sup.1 .+-.8 ______________________________________
.sup.1 Average difference.
For convenience, the invention has been described with reference to
electrodes of particular configuration, number and location,
excitation current of particular value, and the like. It will be
readily understood, however, that wide variations are possible
without materially altering the usefulness of the data obtained.
For example, while band electrodes encircling the neck and lower
thorax are described, the electrodes need not be in the form of
bands, nor is it necessary that the body portions be encircled. The
precise location and separation of the electrodes is not critical.
The excitation electrodes must be positioned above and below the
measuring electrodes. The upper excitation electrode is to be
positioned at or above the upper border of the lungs and the lower
excitation electrode is to be positioned at or below the lower
border of the heart and lungs. Within these limits numerous
variations are possible.
While the use of a 100 kc. excitation current has been described,
it will be apparent that the plethysmograph system is not so
limited. However, the desireability of using a standardized
excitation current source so that the data obtained at different
times and places, etc., can be compared and correlated will be
readily appreciated.
It is apparent that many modifications and variations of this
invention as hereinbefore set forth may be made without departing
from the spirit and scope thereof. The specific embodiments
described are given by way of example only and the invention is
limited only by the terms of the appended claims.
* * * * *