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Claims  |
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We claim:
1. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means, the
wavelengths of the radiation from said source means being such that the
first ratio is independent of physiological changes of the sample blood
other than oxygen saturation at a first oxygen saturation level and the
second ratio is independent of physiological changes other than oxygen
saturation at a second oxygen saturation level; and
first means coupled to the circuit means for receiving the first and second
outputs of said circuit means and applying selected coefficients thereto
to minimize the effect of varying physiological characteristics of the
blood under test other than oxygen saturation, said first means producing
an output representative of the algebraic combination of selectively
weighted first and second outputs of said circuit means; and
second means coupled to the circuit means for receiving the first and
second outputs of said circuit means and applying selected coefficients
thereto to minimize the effect of varying physiological characteristics of
the blood under test other than oxygen saturation, said second means
producing an output representative of the algebraic combination of
selectively weighted first and second outputs of said circuit means; and
means coupled to the first and second means for receiving the outputs
therefrom and producing an output manifestation of the ratio of the
outputs from the first and second means.
2. Apparatus as in claim 1 wherein said first and second means produce said
outputs representing the polynomials A.sub.0 + A.sub.1 (I.sub.1 /I.sub.2)
+ A.sub.2 (I.sub.3 /I.sub.2) and B.sub.0 + B.sub.1 (I.sub.1 /I.sub.2) +
B.sub.2 (I.sub.3 /I.sub.2) respectively where A and B coefficients are
selectively weighted.
3. Apparatus as in claim 1 wherein the first wavelength is approximately
670 nanometers, the second wavelength is approximately 700 nanometers and
the third wavelength is approximately 800 nanometers.
4. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means; and
first means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of the first and second outputs;
second means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of said first and second outputs; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
5. Apparatus as in claim 4 wherein said first and second means produce said
outputs representative additionally of algebraic combinations with
selectively weighted reference outputs.
6. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at a plurality of
radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a plurality of
outputs each representative of the ratio of a respective pair of the
signals from said detector means;
first means coupled to the circuit means for producing corresponding
resultant signals from at least one output multiplied in value by an
output a selected number of times and for producing an output
representative of the sum of all selectively weighted resultant signals;
and
means coupled to the first means and responsive to said output therefrom
for producing an output manifestation of oxygen saturation.
7. Apparatus as in claiam 6 wherein the output of said first means its
representative additionally of algebraic combinations with selectively
weighted reference outputs.
8. A method of determining oxygen saturation of blood comprising the steps
of:
producing electro-magnetic radiation at three different wavebands;
coupling the radiation at each of the wavebands to blood under test;
detecting radiation at each of the wavebands received back from blood under
test for producing a corresponding electrical signal representative of the
intensity of the radiation received back from the blood under test at the
respective waveband;
producing a first output representative of the ratio of one pair of
electrical signals;
producing a second output representative of the ratio of another pair of
said electrical signals;
producing third and fourth outputs representative of the algebraic
combination of selectively weighted first and second outputs; and
producing an output manifestation of oxygen saturation as the ratio of said
third output and said fourth output.
9. A method of determining oxygen saturation of blood comprising the steps
of:
producing electro-magnetic radiation at three different wavebands;
coupling the radiation at each of the wavebands to blood under test;
detecting radiation at each of the wavebands received back from blood under
test for producing a corresponding electrical signed representative of the
intensity of the radiation received back from the blood under test at the
respective waveband;
producing a first output representative of the ratio of one pair of said
electrical signals;
producing a second output representative of the ratio of another pair of
said electrical signals;
producing a third output representative of the algebraic combination of
selectively weighted first output and second output and square of one of
the first and second outputs;
producing a fourth output representative of the algebraic combination of
selectively weighted first output and second output and square of one of
the first and second outputs; and
producing an output manifestation of oxygen saturation as the ratio of said
third output and said fourth output.
10. A method of determining oxygen saturation of blood comprising the steps
of:
producing electro-magnetic radiation at a plurality of different radiation
wavelengths;
coupling the radiation at each of the respective wavelengths to the blood
under test;
detecting the radiation at each of the wavelengths received back from the
blood under test for producing signals representative of the intensities
of the radiation received back from the blood under test at the respective
wavelengths;
producing a plurality of outputs each representative of the ratio of a
respective pair of said signals;
producing corresponding resultant signals from each output multiplied in
value by an output a selected number of times;
producing an output representative of the sum of all selectively weighted
resultant signals; and
producing an output manifestation of oxygen saturation as the ratio of
selected sums of resultant signals.
11. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means;
first means coupled to the circuit means for receiving the outputs
therefrom and for weighting said outputs by applying coefficients thereto,
wherein the algebraic sum of said coefficients substantially equals zero,
to produce an output representative of the algebraic combination of
selectively weighted first and second outputs from said circuit means; and
second means coupled to the circuit means for receiving the outputs
therefrom and for weighting said outputs by applying coefficients thereto,
wherein the algebraic sum of said coefficients substantially equals zero,
to produce an output representative of the algebraic combination of
selectively weighted first and second outputs from said circuit means; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
12. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means;
first means coupled to the circuit means for receiving the outputs
therefrom and for weighting said outputs by applying coefficients thereto,
wherein the algebraic sum of said coefficients substantially equals zero,
to produce an output representative of the algebraic combination of
selectively weighted first and second outputs and square of one of said
first and second outputs from said circuit means;
second means coupled to the circuit means for receiving the outputs
therefrom and for weighting said outputs by applying coefficients thereto,
wherein the algebraic sum of said coefficients substantially equals zero,
to produce an output representative of the algebraic combination of
selectively weighted first and second outputs and square of one of said
first and second outputs from said circuit means; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
13. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at a plurality of
radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a plurality of
outputs each respective of the ratio of a respective pair of the signals
from said detector means;
first means coupled to the circuit means for receiving the outputs
therefrom, raising at least some of said outputs to selected powers of
said outputs, and for weighting said outputs by applying coefficients
thereto, wherein the algebraic sum of said coefficients substantially
equals zero, to produce an output representative of the algebraic
combination thereof;
second means coupled to the circuit means for receiving the outputs
therefrom, raising at least some of said outputs to selected powers of
said outputs, and for weighting said outputs by applying coefficients
thereto, wherein the algebraic sum of said coefficients substantially
equals zero, to produce an output representative of the algebraic
combination thereof; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
14. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means; and
first means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs;
second means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs;
output means coupled to the first and second means and responsive to said
outputs therefrom for producing an output manifestation of the ratio of
said outputs of the first and second means; and
the outputs of said circuit means being selectively weighted by amplifier
gain and circuit component values of said first and second means to cause
the partial derivative of said output manifestation of said output means
with respect to the first output of said circuit means to be approximately
zero near one extreme of the range of oxygen saturation to be measured,
and to cause the partial derivative of the output manifestation of said
output means with respect to the second output of said circuit means to be
approximately zero near the other extreme of the range of values of oxygen
saturation to be measured.
15. Apparatus for determining oxygen saturation of blood comprising:
source means producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means;
the first, second and third wavelengths of the source means having values
to cause the cumulative dependence of the ratio of resultant signals of a
first pair of wavelengths as represented by the first output of said
circuit means upon variations in physiologic parameters other than oxygen
saturation to be minimized near one extreme of the range of values of
oxygen saturation to be measured and to cause the cumulative dependence of
the ratio of resultant signals at a second pair of wavelengths as
representated by the second output of said circuit means upon variations
in physiologic parameters other than oxygen saturation to be minimized
near the other extreme of the range of values of oxygen saturation to be
measured;
first means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs;
second means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
16. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detecor means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means; and
first means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of the first and second outputs;
second means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of said first and second outputs;
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs; and
the outputs of said circuit means being selectively weighted by amplifier
gain and circuit component values of said first and second means to cause
the partial derivative of said output manifestation of said output means
with respect to the first output of said circuit means to be approximately
zero near one extreme of the range of oxygen saturation to be measured,
and to cause the partial derivative of the output manifestation of said
output means with respect to the second output of said circuit means to be
approximately zero near the other extreme of the range of values of oxygen
saturation to be measured.
17. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at first, second and
third radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a first output
representative of the ratio of a first pair of the signals from said
detector means, and for producing a second output representative of the
ratio of a second pair of the signals from said detector means;
the first, second and third wavelengths of the souce means having values to
cause the cumulative dependence of the ratio of resultant signals of a
first pair of wavelengths as represented by the first output of said
circuit means upon variations in physiologic parameters other than oxygen
saturation to be minimized near one extreme of the range of values of
oxygen saturation to be measured and to cause the cumulative dependence of
the ratio of resultant signals at a second pair of wavelengths as
represented by the second output of said circuit means upon variations in
physiologic parameters other than oxygen saturation to be minimized near
the other extreme of the range of values of oxygen saturation to be
measured;
first means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of the first and second outputs;
second means coupled to the circuit means for producing an output
representative of the algebraic combination of selectively weighted first
and second outputs and square of one of said first and second outputs; and
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs.
18. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at a plurality of
radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities and the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a plurality of
outputs each representative of the ratio of a respective pair of the
signals from said detector means;
first means coupled to the circuit means for producing corresponding
resultant signals from at least one output multiplied in value by an
output a selected number of times and for producing an output
representative of the sum of all selectively weighted resultant signals;
and
second means coupled to the circuit means for producing corresponding
resultant signals for at least one output multiplied in value by an output
a selected number of times and for producing an output representative of
the sum of all selectively weighted resultant signals;
means coupled to the first and second means and responsive to said outputs
therefrom for producing an output manifestation of the ratio of said
outputs; and
the outputs of said circuit means being selectively weighted by amplifier
gain and circuit component values of said first and second means to cause
the partial derivative of said output manifestation of said output means
with respect to the first output of said circuit means to be approximately
zero near one extreme of the range of oxygen saturation to be measured,
and to cause the partial derivative of the output manifestation of said
output means with respect to the second output of said circuit means to be
approximately zero near the other extreme of the range of values of oxygen
saturation to be measured.
19. Apparatus for determining oxygen saturation of blood comprising:
source means for producing electro-magnetic radiation at a plurality of
radiation wavelengths;
means coupled to the source means for supplying the radiation therefrom to
blood under test with incident intensities at the respective wavelengths;
detector means disposed to receive radiation from the blood under test
altered from the incident intensities of the respective wavelengths by the
optical properties of the blood under test for producing signals
representative of the intensities of the radiation received thereby at the
respective wavelengths;
circuit means coupled to the detector means for producing a plurality of
outputs each representative of the ratio of a respective pair of the
signals from said detector means;
the radiation wavelengths of said source means being selected to cause the
cumulative dependence of the ratios of resultant signals of respective
pairs of wavelengths, represented by respective outputs of said circuit
means, upon variations in physiologic parameters other than oxygen
saturation to be minimized near respective different values in the range
of values of oxygen saturation to be measured;
first means coupled to the circuit means for producing corresponding
resultant signals from at least one output multiplied in value by an
output a selected number of times and for producing an output
representative of the sum of all selectively weighted resultant signals;
and
means coupled to the first means and responsive to said output therefrom
for producing an output manifestation of oxygen saturation. |
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Claims |
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Description |
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RELATED APPLICATIONS
The subject matter of this application is related to the subject matter of
application Ser. No. 733,279 entitled "Improved Optical Catheter Not
Requiring Individual Calibration" filed on Oct. 18, 1976 and application
Ser. No. 733,280 entitled "Sterilizable Disposable Optical Scattering
Reference Medium" filed Oct. 18, 1976 on the disclosures of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
Blood oxygen saturation is the relative amount of oxygenated hemoglobin in
all of the hemoglobin present in the blood stream. This hemoglobin is
packaged in biconcave discs of approximately 10 micrometers diameter which
commonly occur with a density of approximately five million red blood
cells per cubic millimeter. The red blood cells both scatter and transmit
the incident radiant energy. The differential absorption by oxygenated and
non-oxygenated hemoglobin of the radiant energy transmitted through the
red blood cells furnishes the basis for the oxygen saturation measurement.
In conventional catheter oximetry, the radiation wavebands being used to
measure the blood at a measurement site in vivo are conducted from the
oximeter device to the position of interest within the flowing blood
stream by means of an optical catheter including light-transmitting and
light-receiving fiberoptic light guides. The receiving fiberoptic light
guide for conducting light from the blood stream back to a photodetector
in the oximeter device commonly has its inlet aperture coplanar with the
outlet aperture of the transmitting fiberoptic light guide. Thus, only
back-scattered light is available for measurement, and this represents
only a very small fraction of the total light transmitted to the
measurement site. The light scatterers present about the measurement site
thus act as sources of light for the receiving optical light guide.
Consequently, the intensity of the light scattered back to the receiving
optical light guide is influenced by variations in the number of
scatterers, their location, size, shape and orientation as well as by the
differential absorption by oxyhemoglobin and hemoglobin.
The blood under test flows within a vessel of interest in a pulsatile
manner, and the catheter tip thus moves in an uncontrolled manner with
respect to the blood vessel walls. Whenever a blood vessel wall appears in
the near field of the catheter tip, this has the effect of introducing a
very large array of tightly packed backscatterers into the measurement
system. This introduces a significant change in the distribution and
number of scatterers, which has a substantial and wavelength-dependent
effect upon intensities of light received by the receiving fiber as a
function of transmission through hemoglobin and oxyhemoglobin (which have
wavelength-dependent radiation absorption characteristics).
Certain known catheter-type oximeter devices respond to the intensities of
such back-scattered radiation at only two different wavelengths. Oximeter
devices of this type are disclosed in the literature (see, for example,
U.S. Pat. No. 3,847,483 issued to R. F. Shaw, et al, on Nov. 12, 1974). In
these known devices, the radiation intensities measured at two wavelengths
provide an indication of oxygen saturation according to the relationship:
##EQU1##
where I.sub.1 and I.sub.2 are the light intensities measured at
wavelengths .lambda..sub.1 and .lambda..sub.2, respectively.
It should be noted that if both the numerator and denominator of equation 1
are divided by one of the light intensity measurements, i.e., I.sub.1, the
resultant expression is
##EQU2##
Because the OS measurement thus made according to the prior art remains a
function not only of the ratio of light intensities but of individual
light intensities as well, variations in such phenomena as blood flow
velocity, hematocrit, pH, pCO.sub.2, and the like (which are
multiplicative and wavelength dependent), can introduce errors into the
oxygen saturation measurement thus obtained.
The apparatus of the aforecited patent exhibits greater immunity to such
sources of error as variations in blood flow velocities, hematocrits, and
hemoglobin concentrations than apparatus previously known. However, even
greater immunity is desirable to such sources of error in applications
requiring high-accuracy in vivo measurements of oxygen saturation. In
particular, less sensitivity to proximity of the catheter tip to blood
vessel walls is preferable. In addition, a detectable influence upon
measurement accuracy due to variations in hematocrit, flow velocity, pH,
pCO.sub.2, osmolarity, and variations in the transmissivity of the optical
fibers is also present, and, to some extent, error also can result from a
linear characterization of nonlinear phenomena in the prior art.
SUMMARY OF THE INVENTION
In accordance with the present invention, oxygen saturation is measured as
a function only of the ratios of light intensities at selected wavebands,
and thus, multiplicative wavelength-independent variations do not degrade
the measurement accuracies.
Since the relationship between oxygen saturation and the ratio of light
intensities is not quite linear, the apparatus of the present invention
uses piecewise linear relationships or nonlinear relationships to measure
oxygen saturation over a wide dynamic range of values.
Also, since many of the above noted phenomena which are present at the
measurement site within a blood vessel may vary, both multiplicative and
additive aspects of the optical measurement are considered in the present
invention which provides an oxygen saturation measurement in accordance
with one of the following equations:
##EQU3##
where A.sub.0, A.sub.1, A.sub.2, A.sub.3, and A.sub.i are weighting
factors or coefficients, B.sub.0, B.sub.1, B.sub.2 and B.sub.3, and
B.sub.i are weighting factors or coefficients, and I.sub.1, I.sub.2, and
I.sub.3 are light intensities measured at wavelengths .lambda..sub.1,
.lambda..sub.2 and .lambda..sub.3, respectively, each normalized with
respect to a reference light intensity measurement, and R.sub.i is the
ratio of the normalized light intensities measured at the three different
wavelengths.
It should be noted that oxygen saturation measured in accordance with
Equation 3 is a function of the ratios of light intensity measurements
which is useful for determining oxygen saturation over a narrow range of
values. However, to compensate for the non-linearities of the underlying
phenomena which have significant effect over a wide dynamic range of
values, Equation 3 can be augmented by adding terms proportional to the
square of a ratio of light intensities, as indicated in Equation 4. In
addition, these equations can be further extended to the general
expression indicated in Equation 5.
In the present invention, at least three wavebands illuminate the blood at
the measurement site in vivo and furnish the two ratios of intensities
required to determine oxygen saturation at the measurement site. These
wavebands have been selected to minimize errors introduced into the oxygen
saturation measurement by wavelength-dependent variations in the phenomena
noted above. The coefficients of the terms in Equations 3 and 4 are
selected such that the partial derivative of the calculated oxygen
saturation with respect to one of the ratios is approximately zero near
the lower extreme of the range of oxygen saturation values of physiologic
interest, and the partial derivative of calculated oxygen saturation with
respect to the other ratio in Equations 3 and 4 is approximately zero near
the high extreme of the range of oxygen saturation values of physiologic
interest. Also, the coefficients of the terms in each of these equations
may be selected to satisfy the constraint that the sum of all numerator
coefficients is approximately zero and the sum of all denominator
coefficients is approximately zero.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pictorial diagram of one embodiment of the present invention;
and
FIG. 2 is a schematic diagram of one embodiment of a circuit for producing
the oxygen saturation output as a function of the ratios of radiation
intensities at three wavelengths; and
FIG. 3 is a schematic diagram of another embodiment of a circuit for
producing the oxygen saturation output as a function of the ratios of
radiation intensities at three wavelengths; and
FIG. 4 is a graph showing the effect of a variation in a typical
wavelength-dependent characteristic of the blood on the relationship
between oxygen saturation values and the ratios of intensities of detected
radiation at three different wavelengths.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is a shown a portion of a schematic diagram
including radiation source means, optical combiner and integrator means,
catheter, detector, and signal processing means according to one
embodiment of the present invention. The radiation source means includes
three light emitting diodes 11, 12, and 4 which are arranged to
alternately irradiate three branches 13, 14 and 3 of a fiber optic guide.
The light emitting diodes 11, 12, and 4 are each alternately energized
typically for about 25 percent of the operating cycle, in non-overlapping
temporal relationship by the pulse generator 15. The operating cycle
computes three periods of sequential energizing of the light emitting
diodes followed by a period in which none of the light emitting diodes 11
or 12 or 4 is energized. Each operating cycle thus comprises four periods
and an example rate is two hundred and fifty such cycles per second.
Light emitting diodes 4, 11, and 12 emit radiation at wavelengths
.lambda..sub.1, .lambda..sub.2, and .lambda..sub.3, respectively. This
radiation is collected by fiber optic guides 13, 14, and 3 which may
contain one or more fibers that are physically combined into a bundle
having minimum end cross section 5 of about the same shape and size as the
optical integrator 6.
The optical integrator 6 is disposed continuous to the surface 5 and is a
single light guide of approximately the same size as end 5 and has a
square cross section and a large ratio of length-to-lateral dimension to
assure that spatially-separated radiation at surface 5 is uniformly
distributed over exit aperture 6A. Consequently, a single transmitting
fiber 9 or a bundle of transmitting fibers coupled to aperture 6A thus
receives an amount of radiation at each of the three wavelengths which is
not changed significantly by small transverse misalignments which might
occur between optical integrator 6 and the transmitting fiber 9.
Only one efferent fiber 9 of catheter 8 is required to transmit the
radiation at the three wavelengths to blood at the distal end of the
catheter 8. The transmitting light guide 9 and the receiving light guide
10 of catheter 8 may each consist of only a single optical fiber which
greatly simplifies the construction of the catheter and makes possible a
low-cost disposable catheter which can be coupled to the measuring
equipment at an interface connector 10A.
When the distal tip of the catheter 8 is immersed in blood in a blood
vessel or other blood-confining container, radiation from the transmitting
light guide 9 at each of the three wavelengths is selectively absorbed and
scattered by the red cells, and a portion of the back-scattered radiation
enters the aperture at the distal tip of the receiving light guide 10. At
the proximal end of the catheter 8, the aperture of light guide 10 is
optically coupled to the radiation detector 16 so that substantially all
of the radiation exiting from the light guide 10 impinges upon the active
area of the detector 16.
Radiation signals detected by 16 are amplified by the detector amplifier
A1. During the times that none of the light emitting diodes 11 or 12 or 4
is emitting radiation, switch S1 is closed by a signal from the pulse
generator 15. This forms a closed loop servo system between amplifiers A1
and A2 which establishes a bias voltage on amplifier A1 that adjusts its
output voltage to zero. During the times that switch S1 is open this
zero-correcting bias voltage for amplifier A1 is maintained by the charge
stored in the operationally-connected feedback capacitor 17. This action
assures that the output voltage of the detector amplifier A1 will be zero
when the detector 16 is receiving no back-scattered radiation and thereby
compensates for amplifier drift and spurious outputs from the detector 16.
During the time that diode 4 is radiating, switch S2 is closed by a signal
from the pulse generator 15 and the signal voltage at the output of the
detector amplifier A1 (due to the received radiation from the light
emitting diodes that is back-scattered by the blood) is applied to the
filter consisting of resistor 18 and capacitor 19. The action of switch
S2, resistor 17 and capacitor 19, thus produces an average signal voltage
across capacitor 19 which is representative of the intensity of the
radiation at the wavelength produced by light emitting diode 4 and
backscattered from the blood under test. This average signal voltage is
amplified by amplifier 50 to provide a continuous output voltage that is
directly related to the intensity of radiation at the wavelength
.lambda..sub.1 produced by light emitting diode 4 and backscattered from
blood under test.
Similarly, switch S3, resistor 22, and capacitor 23 and the amplifier 52
operate in substantially the same manner during the portion of the cycle
while light emitting diode 11 is energized to produce a continuous voltage
at the output of amplifier 52 that is directly related to the intensity of
radiation at the wavelength .lambda..sub.2 produced by light emitting
diode 11 and backscattered from the blood under test. In the same manner,
switch S4 and the resistor 26 and capacitor 28 and amplifier 54 operate
during a portion of the cycle while light emitting diode 12 is energized
to produce a continuous voltage at the output of amplifier 54 that is
directly related to the intensity of radiation of the wavelength
.lambda..sub.3 produced by the light emitting diode 12 and backscattered
from the blood under test.
Referring now to FIG. 2, there is shown a block diagram of a signal
processor according to one embodiment of the present invention. The output
signals from amplifiers 50, 52 and 54 of FIG. 1 are applied to terminals
60, 61 and 62, respectively, of FIG. 2.
Signals appearing on terminals 60 and 61 are applied to a dividing circuit
66 which produces an output 70 that is equal to the ratio of I.sub.1
/I.sub.2. The signal at output 70 is applied to the inputs 72 and 74 of a
multiplying circuit 76 which produces an output 78 that is equal to
(I.sub.1 /I.sub.2).sup.2.
The signals appearing on terminals 61 and 62 are also applied to a dividing
circuit 68 which produces an output 80 that is equal to the ratio of
I.sub.3 /I.sub.2.
Amplifier 82 operates on the output signal 78 with the appropriate gain and
sign, B.sub.2, to produce a signal at output 94 which equals B.sub.2
(I.sub.1 /I.sub.2).sup.2. Similarly, amplifiers 84, 86, 88, 90, and 92
operate on their respective inputs with appropriate gain and sign to
produce output signals which are representative of weighted intensity
ratios, as shown.
The amplifier outputs 96, 98, and 102 are applied to a summing amplifier
circuit 116 which produces output 118 that is equal to:
A.sub.0 + A.sub.1 (I.sub.1 /I.sub.2) + A.sub.2 (I.sub.1 /I.sub.2).sup.2 +
A.sub.3 (I.sub.3 /I.sub.2) (Eq. 6)
The voltage source 112, along with the resistors 110 and 114, produce the
A.sub.0 term in the output 118.
Similarly, the summing amplifier circuit 132 produces an output 134 that is
equal to:
B.sub.0 + B.sub.1 (I.sub.1 /I.sub.2) + B.sub.2 (I.sub.1 /I.sub.2).sup.2 +
B.sub.3 (I.sub.3 /I.sub.2) (Eq. 7)
The voltage source 128 along with resistors 126 and 130, produce the
B.sub.0 term in the output 134. The resultant signals at 118 and 134 are
applied to circuit 140 which takes the ratio of the signal at the input
136 and 138 and produces output 142 which is indicative of the oxygen
saturation of the blood under test. T | | |
