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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a noninvasive device for photoelectrically
measuring a property of arterial blood, such as an oximeter or a
densitometer for measurement of a pigment in blood.
2. Description of the Prior Art
The behavior of light in materials has been a topic of study in various
theoretical works such as disclosed in "New Contributions to the Optics of
Intensely Light-Scattering Materials. Part 1," by Paul Kubelka, Journal of
the Optical Society of America, Volume 38, No. 5, May 1948; "Optical
Transmission and Reflection by Blood," by R. J. Zdrojkowski and N. R.
Pisharoty, IEEE Transactions on Bio-Medical Engineering, Vol. BME-17, No.
2, April, 1970, and "Optical Diffusion in Blood," by Curtis C. Johnson,
IEEE Transactions on Bio-Medical Engineering, Vol. BME-17, No. 2, April,
1970.
On the other hand, various practical devices or methods for measuring blood
property have been disclosed in the patent literature such as U.S. Pat.
Nos. 3,368,640, 3,998,550 and 4,086,915, and Japanese Patent Publication
No. 53-26437.
In the specific field of the medical-optical art noninvasive measurements
relating to the amount of a pigment in the blood, such as hemoglobin,
hemoglobin oxide, bilirubin or an artificially injected pigment, have
generally taken the following form. The oximeter usually comprises means
for providing a source light; means for photoelectrically measuring the
intensity of the source light after contact with a living tissue
containing the arterial blood at a first wavelength, at which the light
absorption coefficients for hemoglobin and hemoglobin oxide are equal, and
a second wavelength, at which the two light absorption coefficients
greatly differ from each other, to produce a pair of electric signals,
respectively, the signals each include an alternating-current (AC)
component and a direct-current (DC) component; means for calculating
information representative of the amplitude of the alternating-current
component relative to the direct-current component with respect to the
first and second wavelengths to produce a first and second calculated
output, respectively; means for presenting a final output indicative of
the oxygen saturation, and means for relating the final output with the
first and second calculated outputs so that the final output is a linear
function of a ratio between the first and second calculating outputs.
However, clinical experiences have recently reported that an oximeter of
the above type was apt to show some aberration or error of measurement in
the relatively lower oxygen saturation range although the measurements are
quite accurate in the higher oxygen saturation range. Thus there is still
a need in the prior art to provide improved electro-optical measuring
devices for medical use.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a non-invasive device for
photoelectrically measuring a property of arterial blood with high
accuracy throughout a wide measurement range.
Another object of the present invention is to provide a novel oximeter
capable of measuring the oxygen saturation with an improved accuracy
throughout a wide oxygen saturation range.
According to the present invention the relation of the final output, such
as the oxygen saturation or a density of the pigment in the blood, with
the above-mentioned first and second calculating outputs is determined so
that the final output is a square function of a ratio between the first
and second calculating outputs, or a joint combination of a plurality of
linear functions which correspond to an approximation of the square
function.
The objects and features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The present
invention, both as to its organization and manner of operation, together
with further objects and advantages thereof, may best be understood by
reference to the following description, taken in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a block diagram of a first embodiment of the present
invention;
FIG. 2 represents graphic plots of output voltages of the photoelectric
device of FIG. 1;
FIG. 3 represents a block diagram of a first type of the first calculation
circuit in FIG. 1;
FIG. 4 represents a block diagram of a second type of the first calculation
circuit in FIG. 1;
FIG. 5 represents a block diagram of a third type of the first calculation
circuit in FIG. 1;
FIG. 6 represents a circuit diagram of the details of the square circuit in
FIG. 1;
FIG. 7 represents a circuit diagrm of the details of the linear function
circuit in FIG. 1;
FIG. 8 represents a block diagram of a second embodiment of the present
invention;
FIG. 9 represents a circuit diagram showing a modification of the square
circuit and dividing circuit in either FIG. 1 or FIG. 8;
FIG. 10 represents a block diagram of a third embodiment of the present
invention;
FIG. 11 represents a circuit diagram of a detail of the linear function
circuit of FIG. 10;
FIG. 12 represents a block diagram of a fourth embodiment of the present
invention; and
FIG. 13 represents a block diagram of a fifth embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is provided to enable any person skilled in the
electro-optical art to make and use the present invention and sets forth
the best modes contemplated by the inventor of carrying out his invention.
Various modifications, however, will remain apparent to those skilled in
the art, since the generic principles of the present invention have been
defined herein specifically to provide a noninvasive device for
photoelectrically measuring a property of blood.
FIG. 1 represents a block diagram of a first embodiment of the present
invention in the form of an oximeter, in which a source light having a
wide wavelength band emerges from a light source 1 to enter interference
filters 3 and 4, respectively, by way of transmission through the living
tissue 2 being monitored. The peak of transmission of interference filter
3 is at a wavelength, .lambda..sub.1 at which the light absorption
coefficient for the hemoglobin is equal to that for the hemoglobin oxide,
while the peak of transmission of intereference filter 4 is at a
wavelength, .lambda..sub.2 wherein the two light absorption coefficients
are greatly different from each other. Thus, photoelectric devices 5 and 6
are responsive to the intensities of light at wavlengths, .lambda..sub.1
and .lambda..sub.2, respectively. The changes depending on the lapse of
time in output voltages of photoelectric devices 5 and 6 are shown in FIG.
2, wherein an alternating-current (AC) component is added to a
direct-current (DC) component since the light transmitted through living
tissue is generally absorbed by muscle, bone, venous blood and arterial
blood, and the quantity of arterial blood periodically changes in response
to the pulsation of the heart in contrast to other unchanged factors.
A pair of first calculation circuits 7 and 8 each calculate information
representative of the amplitude of the alternating-current component
relative to the direct-current component which provides a reference level
with respect to wavelengths, .lambda..sub.1 and .lambda..sub.2,
respectively. The pair of outputs, which are derived from the first
calculation circuits 7 and 8, are squared by a pair of square circuits 9
and 10, respectively. Dividing circuit 11 is for obtaining the ratio
between the outputs from the pair of square circuits 9 and 10. The output,
X of dividing circuit 11, which corresponds to the square of the ratio
between the outputs of the pair of first calculation circuits 7 and 8, is
transmitted to a linear function circuit 12 to be multiplied by a first
constant, A.sub.1 and added to a second constant, B.sub.1. The results of
the calculation by the linear function circuit 12 is representative of the
oxygen saturation of blood and can be displayed by some indicator 50 such
as a meter or a digital display circuit.
In more detail, according to the present invention, the output voltages,
E.sub.1 and E.sub.2 of the pair of photoelectric devices 5 and 6 are
defined as follows, respectively:
##EQU1##
wherein: K.sub.1 and K.sub.2 represent a pair of constants determined by
photosensitive elements in the photoelectric devices 5 and 6,
respectively; I.sub.01 and I.sub.02 represent the intensities of source
light at wavelengths, .lambda..sub.1 and .lambda..sub.2, respectively;
F.sub.T1 and F.sub.T2 represent the light absorption coefficients of
materials other than the arterial blood at wavelengths, .lambda..sub.1 and
.lambda..sub.2, respectively; D.sub.1 and D.sub.2 represents a pair of
constants depending on the scattering coefficient and the absorption
coefficient of the arterial blood at wavelengths, .lambda..sub.1 and
.lambda..sub.2, respectively; g.sub.1 and g.sub.2 represent a pair of
constants depending on the scattering coefficient of the arterial blood at
wavelengths .lambda..sub.1 and .lambda..sub.2 and the total density of the
hemoglobin and the hemoglobin oxide, respectively; .beta..sub.1 and
.beta..sub.2 represent the light absorption coefficients of the arterial
blood at wavelengths, .lambda..sub.1 and .lambda..sub.2 ; d represents an
average thickness of the arterial blood; and .DELTA.d represents the
change depending on the lapse of time in the thickness of the arterial
blood.
In determining the above equations (1) and (2), the present invention
regards the light measured by way of transmission through the living
tissue as determined under the conditions that:
(i) the influence of scattering by the arterial blood is not negligible;
(ii) the optical path in the living tissue is sufficiently long; and
(iii) the scattering coefficient is sufficiently great relative to the
absorption coefficient.
The pair of first calculation circuits 7 and 8 each may be practically
designed in accordance with any one of FIGS. 3, 4 and 5. Specifically, the
circuit in FIG. 3 comprises a first logarithmic conversion circuit 13 to
obtain a logarithm of the output from the photoelectric device 5 or 6, a
low-pass filter 14, a second logarithmic conversion circuit 15 to obtain a
logarithm of the direct-current component of the output from the
photoelectric device 5 or 6, and a differential amplifier 16 to subtract
the output of circuit 15 from the output of circuit 13, for calculating a
logarithm of a ratio of the whole output of photoelectric device 5 or 6 to
the direct-current component thereof. On the other hand, the circuit in
FIG. 4 comprises a high-pass filter 17 to obtain the alternating-current
component of the output from photoelectric device 5 or 6, and a dividing
circuit 18, for calculating a ratio of the alternating-current component
of the output of photoelectric device 5 or 6 to the whole output thereof.
Further, the circuit in FIG. 5 comprises a logarithmic conversion circuit
19 to obtain the logarithm of the output of photoelectric device 5 or 6
and a high-pass filter 20 to obtain the alternating-current component of
the output of circuit 19.
In designing the oximeter, the pair of first calculating circuits 7 and 8
should adopt the same type of circuit, although the type may be selectable
among FIGS. 3 to 5. Any one of the circuits in FIGS. 3 to 5 is
substantially capable of calculating information representative of the
relative amplitude of the alternating-current component of the output of
photoelectric device 5 or 6, although the degree of approximation is
individually different.
Thus, the output voltages, E.sub.3 and E.sub.4 of the pair of first
calculating circuits 7 and 8 in FIG. 1 are given in accordance with the
following equations, respectively:
##EQU2##
FIG. 6 represents an example of a detailed circuit applicable for square
circuits 9 and 10, and its function is self-evident to a person skilled in
the optical electrical art without any further explanation. However, it is
pointed out that block 21 represents a rectifier and smoothing circuit
connected to the output of the first calculation circuit 7 or 8.
The output voltages, E.sub.5 and E.sub.6 of the pair of square circuits 9
and 10 in FIG. 1 is as follows:
E.sub.5 =(g.sub.1 .DELTA.d).sup.2 .beta..sub.1 (5)
E.sub.6 =(g.sub.2 .DELTA.d).sup.2 .beta..sub.2 (6)
Further, the output voltage, E.sub.7 of the dividing circuit 11 is as
follows:
##EQU3##
Here, it should be noted that the light absorption coefficients,
.beta..sub.1 and .beta..sub.2 are generally defined by the following
equations:
.beta..sub.1 =C{S[a.sub.1 (HbO.sub.2)-a.sub.1 (Hb)]+a.sub.1 (Hb)}(8)
.beta..sub.2 =C{S[a.sub.2 (HbO.sub.2)-a.sub.2 (Hb)]+a.sub.2 (Hb)}(9)
wherein: a.sub.1 (HbO.sub.2) and a.sub.2 (HbO.sub.2) represent the light
absorption coefficients of hemoglobin oxide, HbO.sub.2 at wavelengths,
.lambda..sub.1 and .lambda..sub.2, respectively; a.sub.1 (Hb) and a.sub.2
(Hb) represent the light absorption coefficients of hemoglobin, Hb at
wavelengths, .lambda..sub.1 and .lambda..sub.2 ; C represents the total
density of the hemoglobin and the hemoglobin oxide in the arterial blood;
and S represents the oxygen saturation in the arterial blood.
Equation (8) is simplified as follows since a.sub.1 (HbO.sub.2)=a.sub.1
(Hb) at wavelength .lambda..sub.1 :
.beta..sub.1 =C a.sub.1 (Hb) (10)
From equations (7), (9) and (10), the following equation (11) results:
##EQU4##
Therefore,
##EQU5##
The constants A.sub.1 and B.sub.1 are defined by the following equations:
##EQU6##
The above equation (12) can be further summarized as follows:
##EQU7##
This means that the oxygen saturation S is calculated as a linear function
of (E.sub.4 /E.sub.3).sup.2, which represents a square of the ratio
between the outputs of the pair of first calculation circuits 7 and 8.
Linear function circuit 12 performs the calculation in accordance with the
linear function defined by equation (12). FIG. 7 represents an example of
a detailed circuit applicable to a linear function circuit and its
function will be self-evident with no additional explanation. However, it
should be noted that the circuit constants R.sub.1 to R.sub.3 and V.sub.3
have to fulfill the following equations:
##EQU8##
The values for A.sub.1 and B.sub.1 can be determined in accordance with
equations (13) and (14), respectively.
FIG. 8 represents a second embodiment of the present invention, in which
the same elements as those in FIG. 1 are indicated by the same symbols and
explanations thereof are accordingly omitted.
In summary, the same embodiment in FIG. 8 is substantially identical with
the first embodiment in FIG. 1 except that the outputs of the pair of
first calculation circuits 7 and 8 are subjected to division in dividing
circuit 22 and, in turn, squared by a single square circuit 23, in place
of each being squared by a pair of square circuits 9 and 10 prior to being
divided by dividing circuit 11 in FIG. 1. The circuit in FIG. 6 is also
applicable as square circuit 23 in FIG. 8. The output voltage, E.sub.8 of
dividing circuit 22 is given as follows:
##EQU9##
Further, the output voltage, E.sub.9 of square circuit 23 is given as
follows:
##EQU10##
which is identical with equation (7).
FIG. 9 provides a modification of the embodiments in FIGS. 1 and 8. Namely,
the circuits 9, 10 and 11 in FIG. 1 or the equivalent circuits 22 and 23
in FIG. 8 can be alternatively constructed as a composite circuit in FIG.
9 capable of both the squaring and dividing functions, and its operation
will be self-evident without additional explanation except that the
rectifier and smoothing circuits 24 and 25 are connected to the pair of
first calculation circuits 7 and 8, respectively, and that the gain of the
differential amplifier OP in FIG. 9 should be set at twice the value for
the purpose of obtaining the following output at terminal 26:
##EQU11##
FIG. 10 represents a third embodiment of the present invention, in which
the same elements as those in FIG. 8 are indicated by the same symbols to
avoid any redundant explanation. The third embodiment is designed in
accordance with the findings that equation (15), which is a square
function of E.sub.4 /E.sub.3, can be approximately substituted by a joint
combination of the following linear functions of E.sub.4 /E.sub.3,
provided that the oxygen saturation is greater than 50 percent:
##EQU12##
wherein, A.sub.2, B.sub.2, B.sub.3 and M are given constants,
respectively. In FIG. 10, linear function circuit 26 is capable of
determining whether or not E.sub.4 /E.sub.3 is greater than M in addition
to calculating the oxygen saturation S in accordance with equation (16) or
(17) selected in response to such a determination.
FIG. 11 represents an example of a circuit applicable to such a linear
function circuit 26. In FIG. 11, the output, E.sub.4 /E.sub.3 of dividing
circuit 22 is transmitted to terminal 27 as a positive voltage. At
terminal 28, a negative voltage V.sub.5, which fulfills M=-V.sub.5, is
transmitted.
When E.sub.4 /E.sub.3 <-V.sub.5, the output of differential amplifier
OP.sub.2 is zero to allow only the output of differential amplifier
OP.sub.1 to be transmitted to differential amplifier OP.sub.3. The output
voltage of differential amplifier OP.sub.1 is designed to be equal to the
following value for the input, E.sub.4 /E.sub.3 by means of adjusting the
variable resistors VR.sub.1 and VR.sub.2 :
##EQU13##
Thus, the output voltage of differential amplifier OP.sub.3 connected to
terminal 29 is as follows when E.sub.4 /E.sub.3 <-V.sub.5 :
##EQU14##
which is identical with equation (16).
On the other hand, when E.sub.4 /E.sub.3 .gtoreq.-V.sub.5, the output
voltage or differential amplifier OP.sub.2 is as follows, provided that
R.sub.5 represents the resistance of variable resistor VR.sub.2 :
##EQU15##
The output voltage at terminal 29 in this case is as follows since the
voltages of above values (18) and (20) are both transmitted to
differential amplifier OP.sub.3 :
##EQU16##
which is identical with equation (17) provided that:
##EQU17##
Since the third embodiment is only an example of approximately substituting
the square function (15) by a combination of a plurality of linear
functions, it is needless to say that any other approximation by means of
utilizing another combination of a plurality of linear functions, e.g.,
three or more linear functions, can be possible within the scope of the
present invention.
FIG. 12 represents a fourth embodiment of the present invention, in which
the analog outputs E.sub.3 and E.sub.4 of the pair of first calculation
circuits 7 and 8 are converted into digital signals P.sub.1 and P.sub.2 by
means of a pair of rectifiers and smoothing circuits 30 and 31 and A-D
converters 32 and 33, respectively. The digital signals P.sub.1 and
P.sub.2 can be processed by microprocessor 34 with random access memory
(RAM) 35 and read only memory (ROM) 36 to indicate the oxygen saturation
by means of a digital display 37.
For example, microprocessor 34 is programmed to carry out the following
calculation, as in the first and second embodiments:
##EQU18##
Or, alternatively, microprocessor 34 is programmed to discriminate between
the following cases (i) and (ii) to select one of them, and carry out the
calculation in accordance with the selected case, as in the third
embodiment:
##EQU19##
The above constants A.sub.1, B.sub.1, A.sub.2, B.sub.2, A.sub.3 and M may
be stored in RAM 35 or ROM 36, or alternatively be inputted by means of a
combination of a plurality of switches representative of a digital code.
Another example of the function of microprocessor 34, RAM 35 and ROM 36 in
the fourth embodiment in FIG. 12 is as follows. Namely, various oxygen
saturation values, S.sub.0, S.sub.1, S.sub.2, . . . , S.sub.i, . . . ,
S.sub.n-1 and S.sub.n have been previously calculated in accordance with
equation (21) for various possible values (P.sub.2 /P.sub.1).sub.0,
(P.sub.2 P.sub.1).sub.1, (P.sub.2 P.sub.1).sub.2, . . . , (P.sub.2
P.sub.1).sub.i, . . . , (P.sub.2 /P.sub.1).sub.n-1 and (P.sub.2
/P.sub.1).sub.n and stored in ROM 36 at addresses K, K+1, K+2, . . . ,
K+i, . . . K+n, respectively. And (P.sub.2 /P.sub.1).sub.i fulfilling the
following condition is searched with respect to the actually obtained
ratio P.sub.2 /P.sub.1 :
(P.sub.2 /P.sub.1).sub.i .ltoreq.P.sub.2 /P.sub.1 <P.sub.2
/P.sub.1).sub.i+1
to determine address K+i at which the desired oxygen saturation S.sub.i is
stored. The oxygen saturation S.sub.i is read out from ROM 36 and
displayed at digital display 37.
Strictly speaking, such an oxygen saturation S.sub.i is not accurately
equal to the oxygen saturation S which would be directly calculated in
accordance with equation (21). However, S.sub.i is practically regarded as
S if the number, n is sufficiently great.
Or, if n is desired to be not so great, than a further modification is
possible such that S is calculated by microprocessor 34 in accordance with
a suitable interpolation such as:
##EQU20##
Although FIG. 12 discloses that the device includes two A-D converters 32
and 33, such a modification is possible that only one A-D converter is
alternatively utilized which is connected to both circuits 30 and 31 by
way of a suitable multiplexer controlled through microprocessor 34.
FIG. 13 represents a fifth embodiment of the present invention constructed
as a densitometer for a desired pigment, such as bilirubin or an
artificially injected pigment, in the arterial blood in contrast to the
foregoing embodiments which are constructed as an oximeter. In FIG. 13,
the same elements as those in FIG. 1 are represented by the same symbols
without additional explanation. Since the fifth embodiment is a
densitometer, the peak of transmission of interference filter 38 is at
wavelength, .lambda..sub.3 at which the light absorption by the pigment
does not occur, while the peak of transmission of interference filter 39
is at a wavelength, .lambda..sub.4 at which the light absorption by the
pigment effectively occurs. Therefore, the output voltages E.sub.10 and
E.sub.11 of the pair of photoelectric devices 5 and 6 are as follows,
respectively, which are substantially similar to equations (1) and (2):
##EQU21##
Wherein: I.sub.03 and I.sub.04 represent the intensities of source light
at wavelengths .lambda..sub.3 and .lambda..sub.4, respectively; F.sub.T3
and F.sub.T4 represent the light absorption coefficients of materials
other than the arterial blood at wavelengths .lambda..sub.3 and
.lambda..sub.4, respectively; D.sub.3 and D.sub.4 represents a pair of
constants depending on the scattering coefficient and the absorption
coefficient of the arterial blood at wavelengths .lambda..sub.3 and
.lambda..sub.4, respectively; g.sub.3 and g.sub.4 represent a pair of
constants depending on the scattering coefficients of the arterial blood
at wavelengths .lambda..sub.3 and .lambda..sub.4 and the total density of
the hemoglobin and the hemoglobin oxide, respectively; and .beta..sub.3
and .beta..sub.4 represent the light absorption coefficient of the
arterial blood at wavelengths .lambda..sub.3 and .lambda..sub.4.
The output voltages, E.sub.12 and E.sub.13 of the pair of first calculation
circuits 7 and 8 are as follows as in the first embodiment:
##EQU22##
Further, the output voltage, E.sub.14 of dividing circuit 11 is as
follows:
##EQU23##
Here, it should be noted that the light absorption coefficients,
.beta..sub.3 and .beta..sub.4 are generally defined by the following
equations, provided that .mu..sub.1 and .mu..sub.2 represent the light
absorption coefficients of the total of hemoglobin and hemoglobin oxide at
wavelengths .lambda..sub.3 and .lambda..sub.4, respectively, C' represents
the density of the pigment in the blood and .mu..sub.2 ' represent the
light absorption coefficient of the pigment at wavelength .lambda..sub.4 :
.beta..sub.3 =C.mu..sub.1 (26)
.beta..sub.4 =C.mu..sub.2 +C'.mu.'.sub.2 (27)
From equations (25) to (27), the following equation results:
##EQU24##
Therefore,
##EQU25##
The constants A.sub.4 and B.sub.3 in circuit 40 of FIG. 13 are defined as
follows:
##EQU26##
The equation (29) can be summarized as follows:
##EQU27##
Circuit 40 in FIG. 13 carries out a calculation in accordance with the
linear function (32) with respect to (E.sub.13 /E.sub.12).sup.2, which is
the output of dividing circuit 11. As the value for C in equations (30)
and (31), an average value of various healthy men is applicable, or
alternatively a personal value may be previously measured and applied,
although the value for C does not particularly depend on the individual
persons, but is rather common. Thus, the fifth embodiment in FIG. 13
functions as a densitometer for a desired pigment in the blood.
Clinical experiments in utilizing the device according to the present
invention have shown that the measurement result is quite accurate
throughout a wide range of variation in the oxygen saturation or the
density of desired pigment in the blood in comparison with the prior art
noninvasive oximeter using a pair of wavelengths.
While the above embodiments have been disclosed as the best modes presently
contemplated by the inventor, it should be realized that these examples
should not be interpreted as limiting, because artisans skilled in the
field, once given the present teachings can vary from these specific
embodiments. Accordingly, the scope of the present invention should be
determined solely from the following claims.
* * * * *
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