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Claims  |
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What is claimed is:
1. A monitoring device for photoelectrically obtaining information relating
to a characteristic of the presence of arterial blood in living tissue
comprising:
a light source capable of providing a light signal;
means for photoelectrically converting light from the light source, under
the optical influence of living tissue, to a first output signal;
means for separating a direct current component from said first output
signal; and
means for producing an output signal representative of a logarithm of a
quotient of said first output signal divided by said separated direct
current component.
2. The device of claim 1, wherein said producing means comprises:
means for dividing said first output signal by said separated direct
current component to produce a second output signal; and
means for converting said second output signal into a logarithm thereof to
produce a third output signal whereby said third output signal is
representative of a logarithm of a quotient of said first output signal
divided by said separated direct current component.
3. An oximeter for obtaining an indication of the degree of oxygen
saturation in arterial blood, comprising:
a light source capable of providing a light signal;
a pair of means for photoelectrically converting light from said light
source, under the optical influence of a living body, to a pair of first
output signals, respectively, said pair of photoelectrical converting
means being adapted to respond to light signals of different wavelengths;
a pair of means for respectively computing the corresponding first output
signals of said pair of photoelectrical converting means, each of said
computing means including:
means for separating a direct current component from said first output
signal;
means for producing an output signal representative of a logarithm of a
quotient of said first output signal divided by said separated direct
current component;
and
final dividing means for dividing the output signal of one of said
producing means by the output signal of the other of said producing means.
4. The oximeter of claim 3, further comprising:
a first differentiation circuit connected between one of said pair of
computing means and said dividing means; and
a second differentiation circuit connected between the other of said
computing means and said dividing means.
5. The oximeter of claim 3, wherein each of said pair of producing means,
comprises:
means for dividing said first output signal by said separated direct
current component to produce a second output signal; and
means for converting said second output signal into a logarithm thereof to
produce a third output signal whereby said third output signal is
representative of a logarithm of a quotient of said first output signal
divided by said separated direct current component.
6. The oximeter of claim 5, further comprising:
a first differentiation circuit connected between one of said pair of
computing means and said dividing means; and
a second differentiation circuit connected between the other of said
computing means and said dividing means.
7. The oximeter of claim 6, further comprising:
a first low-pass filter, a first rectification circuit connected to said
filter and a first integration circuit connected to said rectification
circuit, the combination connected between said first differentiation
circuit and said final dividing means; and
a second low-pass filter, a second rectification circuit connected to said
filter, and a second integration circuit connected to said rectification
circuit, the combination connected between said second differentiation
circuit and said final dividing means.
8. The oximeter of claim 7, further comprising:
means for calculating the oxygen saturation from the output of said
dividing means; and
means for indicating the oxygen saturation calculated by said calculating
means.
9. The oximeter of claim 8 wherein said oxygen saturation calculating
means, comprises means for executing the equation:
##EQU8##
wherein, S is the oxygen saturation of the blood; .beta..sub.R and
.beta..sub.IR are the light absorption coefficients of the blood in terms
of light having a first wavelength (R) and light having a second
wavelength (IR), respectively;
A.sub.r (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a first wavelength;
A.sub.r (hb) represents the light absorption coefficient of Hb in terms of
light having a first wavelength;
A.sub.ir (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a second wavelength, and
A.sub.ir (hb) represents the light absorption coefficient of Hb in terms of
light having a second wavelength.
10. The oximeter of claim 8 wherein said oxygen saturation calculating
means, comprises means for executing the equation:
##EQU9##
wherein, S is the oxygen saturation of the blood; .beta..sub.R and
.beta..sub.IR are the light absorption coefficients of the blood in terms
of light having a first wavelength (R) and light having a second
wavelength (IR), respectively;
A.sub.r (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a first wavelength;
A.sub.r (hb) represents the light absorption coefficient of Hb in terms of
light having a first wavelength;
A.sub.ir (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a second wavelength, and
A.sub.ir (hb) represents the light absorption coefficient of Hb in terms of
light having a second wavelength.
11. A non-invasive apparatus for obtaining information relating to a
characteristic of arterial blood in living tissue comprising:
a source of white light, directed at said living tissue;
means for receiving light transmitted through said living tissue and
generating an electrical signal in response thereto;
means for separating a direct current component from said electrical
signal; and
means for producing a signal representative of the logarithm of the
quotient of the electrical signal divided by the direct current component
from said separating means.
12. An oximeter for obtaining an indication of the degree of oxygen
saturation in arterial blood, comprising:
a source of light directed at said arterial blood;
first means for responding to light having a first wavelength transmitted
through said arterial blood by generating a first signal;
second means for responding to light having a second wavelength transmitted
through said arterial blood by generating a second signal;
first means for computing the logarithm of the quotient of the first signal
divided by the DC component thereof;
second means for computing the logarithm of the quotient of the second
signal divided by the DC component thereof; and
final dividing means for dividing the output of one of said computing means
by the output of the other of said computing means.
13. The oximeter of claim 12, wherein said first and second computing
means, comprise:
means for separating a DC component from the received signal;
means for dividing the received signal by the DC component; and
means for forming the logarithm of the signal from said dividing means.
14. The oximeter of claim 13, further comprising:
first differentiation means connected between said first computing means
and said final dividing means for differentiating the first signal; and
second differentiation means connected between said second computing means
and said final dividing means for differentiating the second signal.
15. The oximeter of claim 14, further comprising;
a first low-pass filter, a first rectification circuit connected to said
filter and a first integration circuit connected to said rectification
circuit, the combination connected between said first differentiation
circuit and said final dividing means; and
a second low-pass filter, a second rectification circuit connected to said
filter, and a second integration circuit connected to said rectification
circuit, the combination connected between said second differentiation
circuit and said final dividing means.
16. The oximeter of claim 12, further commprising:
a first differentiation means connected between said first computing means
and said final dividing means for differentiating the first signal; and
a second differentiation means connected between said second computing
means and said final dividing means for differentiating the second signal.
17. The oximeter of claim 16, further comprising:
a first low-pass filter, a first rectification circuit connected to said
filter and a first integration circuit connected to said rectification
circuit, the combination connected between said first differentiation
circuit and said final dividing means; and
a second low-pass filter, a second rectification circuit connected to said
filter, and a second integration circuit connected to said rectification
circuit, the combination connected between said second differentiation
circuit and said final dividing means.
18. The oximeter of claim 12, further comprising;
means for calculating the oxygen saturation from the output of said final
dividing means by executing the following equation:
##EQU10##
wherein, S is the oxygen saturation of the blood; .beta..sub.R are
.beta..sub.IR are the light absorption coefficients of the blood in terms
of light having a first wavelength (R) and light having a second
wavelength (IR), respectively;
A.sub.r (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a first wavelength;
A.sub.r (hb) represents the light absorption coefficient of Hb in terms of
light having a first wavelength;
A.sub.ir (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a second wavelength, and
A.sub.ir (hb) represents the light absorption coefficient of Hb in terms of
light having a second wavelength.
19. The oximeter of claim 12, further comprising:
means for calculating the oxygen saturation from the output of said final
dividing means by executing the following equation:
##EQU11##
wherein, S is the oxygen saturation of the blood; .beta..sub.R and
.beta..sub.IR are the light absorption coefficients of the blood in terms
of light having a first wavelength (R) and light having a second
wavelength (IR), respectively;
A.sub.r (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a first wavelength;
A.sub.r (hb) represents the light absorption coefficient of Hb in terms of
light having a first wavelength;
A.sub.ir (hbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a second wavelength, and
A.sub.ir (hb) represents the light absorption coefficient of Hb in terms of
light having a second wavelength. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a non-invasive oximeter capable
of measuring arterial oxygen saturation, and more particularly, to an
optical oximeter which analyzes light wave signals transmitted through a
living body to detect the oxygen saturation in the arterial blood
contained therein.
2. Description of the Prior Art
In general, methods for measuring oxygen saturation in arterial blood
without penetrating body tissue utilize the relative difference between
the light absorption coefficient of hemoglobin (Hb) and that of the
hemoglobin oxide (HbO.sub.2). The light absorption coefficient for Hb and
HbO.sub.2 is characteristically tied to the wavelength of the light
traveling through them. Both Hb and HbO.sub.2 transmit light having a
wavelength in the infrared region to approximately the same degree.
However, in the visible region, the light absorption coefficient for Hb is
quite different from the light absorption coefficient of HbO.sub.2.
One example of a non-invasive oximeter is described in an article titled
"Photoelectric Determination of Arterial Oxygen Saturation in Man" by Wood
and Geraci, in the Journal of Laboratory and Clinical Medicine, Volume 34,
1949. The oximeter described therein utilizes a light source that
generates light in the infrared region and in the red region. Both light
wave signals are transmitted through body tissue. The respective light
wave signals leaving the body tissue are photoelectrically converted into
a first and second output signal. Ultimately, these signals are analyzed
to get an indication of the oxygen saturation in the arterial blood.
Before these first and second signals are generated, the body tissue is
compressed to occlusive pressure (200 mm. of mercury), squeezing blood
from the tissue under test. This is done to obtain reference output
signals which have information regarding light absorption by the tissue
itself, i.e., muscles, bone or skin, without the blood. These reference
signals are required in order to separate the information regarding light
absorption by blood alone from the above mentioned first and second output
signals, which includes information regarding light absorption by the
blood and tissue together.
As a result of this tissue compression set up, it is impossible to
continuously detect the oxygen saturation in the blood, because a
measurement under compression interrupts the normal flow of blood. As a
result of the necessity of providing a compression pressure of 200 mm. of
mercury, this prior art oximeter is quite bulky and cumbersome, and may
even introduce a certain amount of discomfort to the subject.
SUMMARY AND OBJECTS OF THE INVENTION
An object of the present invention is to provide means in an oximeter for
obtaining information regarding light absorption by blood alone without
compressing the tissue under test.
Another object of the present invention is to provide an oximeter which
does not compress the tissue under test.
Still another object of the present invention is to provide an oximeter
circuit in which the fluctuations of the reference level of the pulsating
output signal is removed to increase the reliability of the measurement.
The above objects as well as the general purpose of the present invention,
is accomplished by utilizing a pair of computing circuits, each computing
circuit receiving signals from its respective photoelectric device. Each
computing circuit includes a circuit for separating the direct current
component from the received photoelectrically converted signal. A circuit
combination in the computing circuit takes the received photoelectrically
converted signal and the separated out direct current component and
provides an output signal representative of the logarithm of the quotient
of the received photoelectric signal divided by the direct current
component. The respective signals from the computing circuits are
differentiated and then divided, one by the other, to provide an
indication of oxygen saturation in the blood.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and many of the attendant advantages of this invention will
be readily appreciated as the same becomes better understood by reference
to the following detailed description when considered in conjunction with
the accompanying drawings in which like reference numerals designated like
parts throughout the figures thereof and wherein:
FIG. 1 is a block diagram of a preferred embodiment of a portion of the
present invention.
FIG. 2 is a block diagram of a preferred embodiment of an oximeter
according to the present invention.
FIG. 3 is a graph representing variation in the light absorption
coefficient of hemoglobin and hemoglobin oxide versus the wavelength of
light.
FIG. 4 is a graph illustrating the final output signal from the circuit of
FIG. 1.
FIG. 5 is a graph representing the output signals from each of the
differentiation circuits of FIG. 2.
FIG. 6A is a graph showing the variation in the quantity of blood in an
artery, in terms of time.
FIG. 6B is a graph showing the variation in the ratio between light
absorption coefficients of the blood at first and second wavelengths in
terms of time.
FIG. 7 is a circuit diagram of a photoelectric device that may be used with
the present invention.
FIG. 8 is a circuit diagram of the various circuits used in the embodiment
of FIG. 2.
FIG. 9 is a circuit diagram of the rectification and integration circuits
of the embodiment of FIG. 2.
FIG. 10 is a circuit diagram of the division circuit of the embodiment of
FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a light source 1, such as a light emitting diode or
the like, emits a quantity of light I.sub.o. A great majority of the
quantity of light I.sub.o being emitted is passed to a living body 2, such
as a finger tip, where a portion thereof is absorbed. The portion not
absorbed becomes a final quantity of light I.sub.AC.sub.+DC which contains
an AC component relating to absorption by the pulsating blood in the
arteries, and a DC component relating to absorption by the tissue in the
body 2, I.sub.AC.sub.+DC can be represented by the following equation:
I.sub.AC.sub.+DC = I.sub.o F.sub.T e.sup.-.sup..beta.(d.sub.+l) 1.
wherein:
.beta. is the absorption coefficient of blood
F.sub.t is a light-quantity attenuation index for the absorption of light
by the body tissue;
d is a quantity of blood that remains in the tissue on a steady-state
basis; and
l is the quantity of blood which varies in terms of time due to pulsation.
A photoelectric device 3, including a CDS, CDSe, or silicon photodiode,
produces an output signal:
E = A I.sup..gamma.
for a quantity I of light incident thereupon. The output (E.sub.AC.sub.+DC)
quantity of light I.sub.AC.sub.+DC is represented by the expression:
E.sub.AC.sub.+DC = A I.sub. AC.sub.+DC
substituting for I.sub.AC.sub.+DC from equation 1:
E.sub.AC.sub.+DC = A [I.sub.o F.sub.T
e.sup.-.sup..beta.(d.sub.+1)].sup..gamma. = A I.sub. o F.sub. T
e.sup.-.sup..beta..sup..gamma.e.sup.-.sup..beta..sup..gamma..sup.l 2
wherein A and .gamma. are constants.
A separation circuit 5 receives the E.sub.AC.sub.+DC from the input circuit
4 and provides the DC component E.sub.DC to one output terminal and the
received signal E.sub.AC.sub.+DC to the other output terminal. From
equation (2), it can be seen that only the variable l is changed in terms
of time, and E.sub.DC can be expressed as follows:
E.sub.DC = A I.sub.o F.sub.T e.sup.-.sup..beta..sup..gamma..sup.d 3.
Amplifiers 6 and 7 amplify the signals e.sub.AC.sub.+DC and E.sub.DC
respectively received from the separation circuit 5. The outputs of both
amplifiers are connected to a division circuit 8, in which a division
E.sub.AC.sub.+DC /E.sub.DC is effected. From equations 2 and 3, the output
signal from the dividing circuit 8 can be represented by the following
expression:
E.sub.AC.sub.+DC /E.sub.DC = e.sup.-.sup..beta..sup..gamma..sup.l 4.
A logarithmic conversion circuit 9 receives this signal from the division
circuit 8. From equation (4 ), the output signal Y from the logarithmic
conversion circuit 9 can be expressed as follows:
Y = log (E.sub.AC.sub.+DC /E.sub.DC) = -.beta..gamma.l log e 5.
From equation (5), since .gamma. log e is a constant having a predetermined
value, the output signal Y supplies information about .beta.l, the
absorption coefficient of blood times a certain quantity of blood. As a
result, the embodiment of FIG. 1 will supply a signal that indicates the
degree of absorption of the light generated by the light source 1, by the
blood in the arteries. As can be seen from FIG. 1, this is accomplished
without compression of the body tissue 2, and without impeding the flow of
blood in any way.
Since log (E.sub.AC.sub.+DC /E.sub.DC) is desired, a more convenient
equivalent for the embodiment of FIG. 1 may comprise a circuit, which is
arranged so that E.sub.AC.sub.+DC and E.sub.DC is converted into
logarithmic form first and the division circuit substituted by a
subtraction circuit for subtracting log E.sub.DC from log
E.sub.AC.sub.+DC. The end result is the same.
FIG. 2 illustrates a preferred embodiment of an oximeter which utilizes a
pair of signal generating circuits 0,0' of the type shown in FIG. 1. A
first photoelectric device 3, connected to a first computing section 0 is
arranged to receive light having first wavelength (R) that may be in the
red region. A second photoelectric device 3', which may be identical to
the first photoelectric device 3, except for its light filter, is
connected to a second computing section 0'. This photoelectric device is
arranged to receive light having a second wavelength (IR) that may be in
the infrared region.
Referring to FIG. 3, the characteristic change in the light absorption
coefficient of hemoglobin (Hb) and that of hemoglobin oxide (HbO.sub.2),
in relation to the wavelength of light is plotted. As can be seen from the
graph, the absorption coefficient 18 of hemoglobin is equal to the
absorption coefficient 19 of hemoglobin oxide for light having a
wavelength B, in the infrared region. When light having a wavelength A, in
the red region, or C, above infrared, is used, the absorption coefficients
for Hb and HbO.sub.2 differ considerably.
Referring again to FIG. 2, the photoelectric devices in the Y signal
generating circuits 0 and 0' are structured to receive light at different
wavelengths, for example, by the selection of appropriate filters for the
photoelectric devices 3 and 3'. As a result, the output signal from signal
generating circuit 0 will be different from the output signal from signal
generating circuit 0'. Equation (5) expresses the output Y for each of the
signal generating circuits as follows:
Y.sub.R = -.beta..sub.R .gamma. llog e 6.
Y.sub.IR = -.beta..sub.IR .gamma. llog e 7.
where:
Y.sub.R and Y.sub.IR are representative of the outputs from the respective
circuits 0 and 0' .beta..sub.R and .beta..sub.IR are the light absorption
coefficient of the blood in terms of light having a first wavelength (R)
and light having a second wavelength (IR), respectively
The absorption coefficients .beta..sub.R and .beta..sub.IR can be expressed
as follows:
.beta..sub.R = A.sub.R (HbO.sub.2 )C(HbO.sub. 2) + A.sub.R (Hb)C(Hb) 8.
.beta..sub.IR = A.sub.IR (HbO.sub.2 )C(HbO.sub.2) + A.sub.IR (Hb)C(Hb) 9.
wherein:
A.sub.R (HbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a first wavelength,
A.sub.R (Hb) represents the light absorption coefficient of Hb in terms of
light having a first wavelength,
A.sub.IR HbO.sub.2) represents the light absorption coefficient of
HbO.sub.2 in terms of light having a second wavelength,
A.sub.IR (Hb) represents the light absorption coefficient of Hb in terms of
light having a second wavelength C(HbO.sub.2) is representative of the
density of HbO.sub.2 in the blood,
C(Hb) is representative of the density of Hb in the blood.
The quantity to be measured is oxygen saturation (S) of the blood. S is
defined according to the variables of equations (8) and (9), as follows:
S = C(HbO.sub. 2)/(C(HbO.sub. 2) + C(Hb)) 10.
From equations (8) and (9), equation (10) may be transformed in terms of
absorption coefficient as follows:
##EQU1##
All the terms of equation (11), other than .beta..sub.R /.beta..sub.IR are
constants which can be determined beforehand. As a result S (oxygen
saturation) can be calculated when the ratio .beta..sub.R /.beta..sub.IR
is known. If a lightwave having a length B in the infrared region is
selected as the IR signal, then
A.sub.IR (Hb) = A.sub.IR (HbO.sub.2), see FIG. 3, and therefore, the
equation (11) is represented in the form:
##EQU2##
By connecting a dividing circuit, such as division circuit 15, to the
output terminals P and P' of the Y signal generating circuits 0 and 0',
respectively, Y.sub. R Y.sub. IR is obtained. According to equations (6)
and (7), the following relationship exists:
Y.sub.R /Y.sub.IR = .beta..sub.R /.beta..sub.IR 13.
either equation (11) or (12) will supply a measure of oxygen saturation
(S), .beta..sub.R /.beta..sub.IR being the only variable therein.
The signal Y.sub.R /Y.sub.IR obtained from the divider circuit connected to
the output of the two Y signal generating circuits 0 and 0' is not in an
optimized condition. Due to instability, which is largely dependent on
respiration of the patient, the output signal fluctuates widely. Due to
the time constant of a smoothing circuit included in the separation
circuit 5 (FIG. 1), a reference level of the Y output signal from the Y
signal generating circuits fluctuates as represented by curve 20 in FIG.
4. The reference level is shown as fluctuating between a level L and L'.
This fluctuation affects the computation of the output signal Y, possibly
leading to error in the final indication.
To eliminate this possibility, the output signals Y.sub.R and Y.sub.IR,
from their respective circuits are fed as inputs to differentiation
circuits 10 and 10', respectively, shown in FIG. 2. The representative
output signal 21 from the differentiation circuits 10 and 10' has a
reference level L", as shown in FIG. 5. This reference level is constant,
and not under the influence of the fluctuation in the reference level of
the output signals Y.sub.R and Y.sub.IR , respectively. From equations (6)
and (7) the output signals from the differentiation circuits 10 and 10'
can be represented as follows:
##EQU3##
FIG. 6A plots the experimentally obtained variations of blood vs. time in a
human. FIG. 6B plots the corresponding variation of .beta..sub.R
/.beta..sub.IR vs. time. The variation of .beta..sub.R /.beta..sub.IR vs.
time (FIG. 6B) is extremely slow relative to the variation in the quantity
of blood (l) vs. time (FIG. 6A) for one pulse variation in the quantity of
blood. Therefore d.beta..sub.R /dt and d.beta..sub.IR /dt can be regarded
as zero for one pulse cycle, providing the following equalities:
d.beta..sub.R /dt = 0, d.beta..sub.IR /dt = 0 16.
If the output signals from the differentiation circuits 10 and 10' are fed
as inputs to a divising circuit for computation than from equations (15)
and (16), the following relationship is true:
##EQU4##
It will be remembered that the oxygen saturation factor S can be calculated
when .beta..sub.R /.beta..sub.IR is known. By differentiating the Y.sub.R
signals from the Y.sub.IR signal generating circuits 0 and 0', the
fluctuation is the reference level of the Y signals is circumvented,
thereby avoiding error in the final indication of oxygen saturation.
The system described to this point is effectively illustrated in FIG. 2 if
the division circuit 15 had its respective inputs connected to points q,
q', the output of the respective differentiation circuits 10 and 10'. A
calculation circuit 16 receives the output of division circuit 15 and uses
it to execute either equation (11) or (12). The output of the calculation
circuit is supplied to an indicator device 17 which provides an indication
of the degree of oxygen saturation in the blood.
To improve the reliability of the indication by indicator 17, supplementary
circuits such as amplifiers 11, 11', low-pass filters 12, 12', rectifier
circuits 13, 13' and integration circuits 14, 14' are utilized. The
filters 12, 12' reduce the noise content of the signals received from the
amplifier, 11, 11'. The rectification circuits 13, 13' and integration
circuits 14, 14' are used to improve the signal quality of the signals
supplied to the division circuit 15.
As an example of specific circuitry that may be utilized to perform the
functions of the labeled blocks of FIG. 2, reference may be had to FIGS. 7
- 10. These circuits are presented only as an example, other circuits
equivalent in function and seen as well within the purview of a person or
ordinary skill in the art. The specific function of the circuitry
illustrated is seen as obvious from the figures when taken in combination
with the general description of their function. The elements of these
circuits, such as operational amplifiers, diodes, capacitors, and
switching transistors are well known and understood.
FIG. 7 is a preferred circuit design of the photoelectric device 3 or 3'.
As can be seen from the figure, a photoelectric diode 22 varies the bias
on a transistor circuit 25 according to the intensity of light falling on
the diode. Amplifiers 23 and 24 provide an output signal voltage
E.sub.AC.sub.+DC that contains both an alternating and steady-state
voltage component.
FIG. 8 is an example of the Y signal generating circuit 0 or 0', the
differentiating circuit 10 or 10', the amplifier 11 or 11', and the
low-pass filter 12 or 12' of FIG. 2. As can be seen from the illustration
straightforward signal processing is utilized.
FIG. 9 is an example of the rectification circuit 13 or 13', and the
integration circuit 14 or 14' of FIG. 2. Here again standard signal
processing using diodes, operational amplifiers, etc. is employed.
FIG. 10 is an example of a division circuit 15 that may be used as part of
the embodiment of FIG. 2. The two input terminals 26, 27 receive signals
from integrator circuits 14, 14', respectively.
The calculation circuit 16 of FIG. 2 as was noted, receives the output
signals from division circuit 16 and calculates the oxygen saturation (S)
in the blood, according to equation (11) or (12). The light absorption
coefficient terms of either equation are constants. The operation
performed by the calculation circuit amounts to multiplying, dividing,
adding, and subtracting utilizing the received variable and the stored
constants. The variable is .beta..sub.R /.beta..sub.IR. If the
photoelectric devices 3 and 3' of FIG. 2 are responding to light of 2
different wavelengths when neither is in the infrared region, the
calculation circuit must execute equation (11). This amounts to:
1. Multiplying the received variable .beta..sub.R /.beta..sub.IR times a
constant A.sub.IR (Hb) and a constant A.sub.IR (Hb) - A.sub.IR (HbO.sub.2)
to get a first product and second product, respectively.
2. Subtracting a constant A.sub.R (Hb) from the first product to get a
difference.
3. Adding a constant A.sub.R (HbO.sub.2) - A.sub.R (Hb) to the second
product to get a sum.
4. Dividing the sum into the difference.
If the photoelectric devices 3 and 3' of FIG. 2 are responding to light of
2 different wavelengths when one is in the infrared region, a calculation
circuit that executes equation (12) may be utilized. This amounts to:
1. Multiplying the received variable .beta..sub.R /.beta..sub.IR by a
constant
##EQU5##
to get a product.
2. subtracting a constant
##EQU6##
from that product. Since
##EQU7##
the calculation circuit 16 takes the variable signal from the division
circuit 15 and operates on it, as above noted to obtain oxygen saturation.
The exact circuitry for accomplishing the execution of either equation
(11) or (12) is seen as well within the purview of a person of ordinary
skill in the art. The indicator 17 receives a signal from the calculation
circuit 16 that is representative of the amount of oxygen saturation in
the arterial blood of a patient being tested. The indicator can either
generate a visual or audible indicator of the quantity of oxygen in the
arterial blood in any well known manner.
What has been described is a means in an oximeter for obtaining information
regarding light absorption by blood in living tissue without compressing
the tissue under test and a means for obtaining more reliable and stable
readings.
Obviously many modifications and variations of the present invention are
possible in light of the above teachings. It is therefore to be understood
that within the scope of the appended claims the invention may be
practiced otherwise than as specifically described.
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