 |
Claims  |
|
|
We claim:
1. A blood oxygen saturation monitoring system comprising:
a first source of electromagnetic radiation at a first wavelength;
a second source of electromagnetic radiation at a second wavelength;
means for positioning said first and second sources of electromagnetic
radiation to illuminate a sample of blood;
sensing means for receiving electromagnetic radiation reflected by said
sample of blood, said reflected electromagnetic radiation comprising an AC
component and a DC component, said sensing means producing an output
signal corresponding only to the AC components of the reflected portions
of said first and second electromagnetic radiation;
means for producing a quotient of squared values of said AC voltage
components;
means for calculating blood oxygen saturation by correlating said quotient
of said squared AC components with an oxygen saturation reference curve,
said reference curve being uniquely representative of the blood oxygen
saturation characteristics of a particular individual.
2. The monitoring system according to claim 1, wherein said first and
second sources of electromagnetic radiation comprise first and second
light emitting diodes.
3. The monitoring system according to claim 2, wherein said first light
emitting diode provides light having a wavelength corresponding to red and
said second light emitting diode provides light having a wavelength
corresponding to infrared.
4. The monitoring system according to claim 3, wherein said first reference
curve is defined by first and second data points, said first data point
comprising a quotient calculated using optical properties of blood for
said first wavelength, said second data point being a unique oxygen
saturation point for said particular individual.
5. The monitoring system according to claim 4, wherein said first data
point is defined by a quotient comprising values for the absorption
coefficient for reduced hemoglobin for red light and the absorption
coefficient for oxygenated hemoglobin for red light.
6. The monitoring system according to claim 5, wherein said second data
point is defined by a unique blood oxygen saturation point for said
particular individual.
7. A blood oxygen saturation monitoring system comprising:
a first light emitting diode for producing light at a first wavelength;
a second light emitting diode for producing light at a second wavelength;
means for positioning said first and second light emitting diodes to
illuminate a sample of blood;
sensing means for receiving light reflected by said sample of blood, said
reflected light comprising an AC component and a DC component, said
sensing means producing an output signal corresponding only to the AC
components of said reflected light at said first and second wavelengths;
means for calculating a first reference curve comprising a linear
relationship between the ratio of said AC voltage components, said
reference curve being defined by first and second data points, said first
data point corresponding to a quotient of absorption coefficients of light
at said first wavelength, said second data point being a unique data point
corresponding to oxygen saturation characteristics of a particular
individual;
means for calculating a second reference curve defined a linear
relationship between squared values of said AC voltage components, said
second reference curve being defined by third and fourth data points, said
third data point being a data point calculated using said first reference
curve, said fourth data point being defined by a quotient of absorption
coefficients of light at said first wavelength; and
means for calculating blood oxygen saturation by correlating said quotient
of said squared values of said AC voltage components with said second
oxygen saturation reference curve.
8. The monitoring system according to claim 7, wherein said third data
point is in the range of 88 to 92 percent oxygen saturation.
9. The monitoring system according to claim 8, wherein said first light
emitting diode provides light having a wavelength corresponding to red and
said second light emitting diode provides light having a wavelength
corresponding to infrared.
10. The monitoring system according to claim 9, wherein said first
wavelength of light is approximately 660 nm and said second wavelength of
light is approximately 900 nm.
11. The monitoring system according to claim 9, wherein said first data
point is defined by a quotient comprising values for the absorption
coefficient for reduced hemoglobin for red light and the absorption
coefficient for oxygenated hemoglobin for red light.
12. The monitoring system according to claim 11, wherein said second data
point is defined by a unique 98% blood oxygen saturation point for said
particular individual.
13. A method for determining the oxygen saturation of arterial blood,
comprising the steps of:
illuminating a sample of said blood with electromagnetic radiation at a
first wavelength;
illuminating said sample of blood with electromagnetic radiation at a
second wavelength;
collecting electromagnetic radiation reflected by said sample of blood,
said radiation comprising an AC and a DC component, and developing
therefrom an electrical signal representing only the alternating current
components of said reflected radiation at said first and second
wavelengths;
calculating a first reference curve comprising a linear relationship
between the ratio of said AC voltage components, said reference curve
being defined by first and second data points, said first data point
correponding to a quotient of absorption coefficients of light at said
first wavelength, said second data point being a unique data point
corresponding to oxygen saturation characteristics of a particular
individual;
calculating a second reference curve defined a linear relationship between
squared values of said AC voltage components, said second reference curve
being defined by third and fourth data points, said third data point being
a data point calculated using said first reference curve, said fourth data
point being defined by a quotient of absorption coefficients of light at
said first wavelength; and
calculating blood oxygen saturation by correlating said quotient of said
squared values of said AC voltage components with said second oxygen
saturation reference curve.
14. The method according to claim 13, wherein said third data point is in
the range of 88 to 92 percent oxygen saturation.
15. The method according to claim 14, wherein said radiation at said first
and second wavelengths is produced by first and second light emitting
diodes, respectively.
16. The method according to claim 15, wherein said first and second
wavelengths of light are approximately 660 nm and 900 nm, respectively.
17. The method according to claim 16, wherein said first data point is
defined by a quotient comprising values for the absorption coefficient for
reduced hemoglobin for red light and the absorption coefficient for
oxygenated hemoglobin for red light.
18. The monitoring system according to claim 17, wherein said second data
point is defined by a unique 98% blood oxygen saturation point for said
particular individual. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
FIELD OF THE INVENTION
The present invention relates generally to monitoring equipment which can
be used to estimate the degree of oxygen saturation of arterial blood.
More specifically, the present invention provides an effective noninvasive
reflectance oximeter capable of providing accurate readings at lower
levels of oxygen saturation.
BACKGROUND
In many clinical situations, it is extremely desirable to be able to obtain
continuous measurements of tissue oxygenation. While it is desirable to
have an absolute measure of OS, it is often sufficient to measure relative
changes in the blood oxygen saturation. For example, in the operating
room, the physician is typically concerned only with significant changes
in the patient's OS, and is less concerned with the measurement of
absolute OS. In this situation, a noninvasive oximeter which is capable of
detecting significant changes in the blood oxygen content would be
especially useful.
It is well known that hemoglobin and oxyhemoglobin have different optical
absorption spectra and that this difference in absorption spectra can be
used as a basis for an optical oximeter. Most of the currently available
oximeters using optical methods to determine blood oxygen saturation are
based on transmission oximetry. These devices operate by transmitting
light through an appendage such as a finger or an earlobe. By comparing
the characteristics of the light transmitted into one side of the
appendage with that detected on the opposite side, it is possible to
compute oxygen concentrations. The main disadvantage of transmission
oximetry is that it can only be used on portions of the body which are
thin enough to allow passage of light. There has been considerable
interest in recent years in the development of an oximeter which is
capable of using reflected light to measure blood oxygen saturation. A
reflectance oximeter would be especially useful for measuring blood oxygen
saturation in portions of the patient's body which are not well suited to
transmission measurements.
Various methods and apparati for utilizing the optical properties of blood
to measure blood oxygen saturation have been shown in the patent
literature. Representative devices for utilizing the transmission method
of oximetry have been disclosed in U.S. Pat. Nos. 4,586,513; 4,446,871;
4,407,290; 4,226,554; 4,167,331; and 3,998,550. In addition, reflectance
oximetry devices and techniques are shown generally in U.S. Pat. Nos.
4,447,150; 4,086,915; and 3,825,342.
A theoretical discussion of a basis for the design of a reflectance
oximeter is contained in "Theory and Development of a Transcutaneous
Reflectance Oximeter System for Noninvasive Measurements of Arterial
Oxygen Saturation," by Yitzhak Mendelson (Published Doctoral
Dissertation), No. 8329355, University Microfilms, Ann Arbor, Mich.
(1983). A theoretical discussion of the optical properties of blood is
found in "Optical Scattering in Blood," by Narayanan R. Pisharoty,
(Published Doctoral Dissertation), No. 7124861, University Microfilms, Ann
Arbor, Mich. (1971).
Numerous other works have disclosed theoretical approaches for analyzing
the behavior of light in blood and other materials. The following is a
brief list of some of the most relevant of these references: "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 Biomedical
Engineering, Vol. BME-17, No. 2, April 1970; and "Optical Diffusion in
Blood," by Curtis C. Johnson, IEEE Transactions on Biomedical Engineering,
Vol. BME-17, No. 2, April 1970.
One of the difficulties which has been encountered in the use of optical
oximeters is the calibration of such devices to provide accurate readings
at lower levels of oxygen saturation. In particular, difficulties have
been encountered in the use of optical oximeters to measure oxygen
saturations below 90%. The noninvasive reflectance oximeter provided by
the present invention overcomes these difficulties, as described in
greater detail below.
SUMMARY OF THE INVENTION
The present invention provides a noninvasive reflectance oximeter which is
capable of providing accurate indications of a patient's blood oxygen
saturation. In the preferred embodiment of the present invention, the
blood oxygen saturation of a patient's arterial blood is determined by a
noninvasive optical technique which takes advantage of differences in the
absorption spectra of hemoglobin and oxyhemoglobin. In its simplest form,
the invention comprises means for illuminating the patient's arterial
blood with light at two different wavelengths, means for measuring the
intensity of the reflected light after contact with the blood and means
for correlating the intensity of the reflected light with an oxygen
saturation reference curve to determine the oxygen saturation of the
patient's blood. One of the sources of light is at a wavelength for which
the absorption coefficients of hemoglobin and oxyhemoglobin differ from
one another. The reflected light signal detected by the system comprises
an alternating-current (AC) component and a direct-current (DC) component
for each of the respective light sources. The AC components of each of the
reflected signals is filtered from the output of the sensor and a voltage
amplitude ratio is calculated. This ratio is then correlated with an
oxygen saturation reference curve to obtain an indication of the oxygen
saturation of the patient's arterial blood.
The reference curve used in the preferred embodiment of the invention is
calibrated in a two-step process which minimizes the effects of
calibration errors. A first oxygen saturation reference curve is
calculated which is based on a linear relationship between the ratio of
the AC components of the reflected light. This curve is then used to
calibrate a second reference curve based on a linear ratio of the squared
values of the AC components of the reflected light. Once the second
reference curve has been properly calibrated, it is used for all
subsequent measurements of oxygen saturation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a simplified embodiment of the
noninvasive blood oxygen saturation monitoring system of the present
invention.
FIG. 2 is a graphical representation of a linear oxygen saturation curve
provided by the monitoring system shown in FIG. 1.
FIG. 3 is a graphical representation of an oxygen saturation reference
curves formed by extrapolation techniques using the mathematical
relationships provided by the method of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings in more detail, and to FIG. 1 in particular, the
noninvasive monitoring system 10 of the present invention is shown in its
preferred embodiment. A monitoring probe 12 is positioned over a portion
of the patient's tissue 14 such that light produced by two light emitting
diodes (LED) 16 and 18 will be reflected by arterial blood in the tissue
and detected by a photodetector 20. In the preferred embodiment, the LED
16 emits light having a wavelength of 660 nm (red) and the LED 18 emits
light having a wavelength of 900 nm (infrared). However, the invention is
not intended to be limited to any specific wavelength of light produced by
the above-mentioned LEDs. Proper operation of the invention requires only
that one of the sources of light have a wavelength at which the absorption
coefficients of hemoglobin and oxyhemoglobin differ from one another. The
output of the photodetector 20 will be an electrical signal representing a
combination of direct-current (DC) and alternating-current (AC) components
of the light reflected by the arterial blood in the tissue 14. This output
signal is processed by an appropriate filter 22 to produce signals
corresponding to the AC voltage components of each of the wavelengths of
incident light. These AC voltage signals are then processed by a voltage
amplitude ratio circuit 24 to provide an output signal corresponding to
the ratio of the AC portions of the reflected signals. The voltage
amplitude ratio output signal is provided to a microprocessor 26 which
calculates the oxygen saturation using a Linear Model Reference Curve and
a Square Model Reference Curve described in greater detail below. The
calculated oxygen saturation is then displayed on an appropriate display
device 28.
The use of the AC component of the reflected signal offers significant
advantages for correlating the signals with blood oxygen saturation. As
blood volume increases during systole, more light is absorbed by the blood
and a decrease in tissue reflectance can be observed. During diastole,
tissue blood volume decreases and an increase in the reflected light
intensity can be observed. In general, the amplitude ratio of the AC
components of the reflected signals will not be significantly affected by
fixed light absorbers, such as bone, hair and skin pigmentation.
The techniques used to calculate oxygen saturation in the invention system
can be understood from the following discussion of the relationships
between the reflected light signals detected by the system. The following
equation describes light reflection from a turbid, both absorbing and
scattering, medium:
##EQU1##
R.sub.d --reflection of light from a medium. d--thickness of the medium.
s--scattering coefficient of medium.
k--absorption coefficient of medium.
For a reflectance oximeter the thickness of the medium can be considered to
be very large. Using this assumption, the above equation can be simplified
to yield the following relationship:
##EQU2##
By subtracting the signal at diastolic from the signal at systolic, and
using two different wavelengths, the following relationship can be seen:
##EQU3##
where: BS.sub.AC.sbsb.red --pulsatile component of backscatter signal for
red wavelength.
BS.sub.AC.sbsb.ired --pulsatile component of backscatter signal for ired
wavelength.
s.sub.d.sbsb.red --scattering coefficient of tissue at diastolic for red
wavelength.
s.sub.s.sbsb.red --scattering coefficient of tissue and arterial blood
mixture at systolic for red wavelength.
k.sub.d.sbsb.red --absorption coefficient of tissue at diastolic for red
wavelength.
k.sub.s.sbsb.red --absorption coefficient of tissue and arterial blood
mixture at systolic for red wavelength.
s.sub.d.sbsb.ired --scattering coefficient of tissue at diastolic for ired
wavelength.
s.sub.s.sbsb.ired --scattering coefficient of tissue and arterial blood
mixture at systolic for ired wavelength.
k.sub.d.sbsb.ired --absorption coefficient of tissue at diastolic for ired
wavelength.
k.sub.s.sbsb.ired --absorption coefficient of tissue and arterial blood
mixture at systolic for ired wavelength.
Dividing the red value by the infrared value gives the following equation
for the measured ratio, r:
##EQU4##
It has been determined that a linear relationship exists between the
measured ratio, r, and oxygen saturation. This relationship, hereafter
referred to as the Linear Model, can be expressed as follows:
OS=A.sub.lin +B.sub.lin r
where:
OS--Oxygen Saturation.
A.sub.lin --intercept of Linear Model regression line.
B.sub.lin --slope of Linear Model regression line.
The intercept of the Linear Model, A.sub.lin, can be calculated as follows:
##EQU5##
Simplification of the above equation yields the following equations:
##EQU6##
Therefore:
##EQU7##
where: k.sub.a.sbsb.red --absorption coefficient of arterial blood for red
wavelength.
s.sub.a.sbsb.red --scattering coefficient of arterial blood for red
wavelength.
v.sub.a --change in tissue volume due to arterial blood.
HbO2.sub.red --absorption coefficient of oxygenated hemoglobin for red
wavelength.
Hb.sub.red --absorption coefficient of reduced hemoglobin for red
wavelength.
C.sub.Hb --concentration of hemoglobin in the arterial blood.
The first term in the equation for the intercept, A.sub.lin, is very samll
in comparison to the second term. Thus the first term can be eliminated
with very little error. The equation can thus be simplified as follows:
##EQU8##
The term A.sub.lin is composed of the absorption coefficients for
oxygenated and reduced hemoglobin at a known wavelength, for example 660
nm (red). These values are known constants which are related to the
wavelength of light used to illuminate the blood. Therefore, the
Y-intercept can be calculated by substituting the values of these
absorption coefficients. As an example, for light at 660 nm, the
Y-intercept is calculated to be a hypothetical value of 113%. While this
point has no physical meaning, experimental data has shown that the
hypothetical Y-intercept, A.sub.lin tends to be fairly constant for
different individuals. The slope of the regression curve, however, tends
to vary for different individuals. Therefore, the slope of the Linear
Model, B.sub.lin, is determined by calibrating the system to a patient at
a known oxygen saturation value, e.g. 98%. Given a known oxygen saturation
and a corresponding measured voltage ratio, the slope, B.sub.lin, can be
calculated as follows:
##EQU9##
FIG. 2 is a graphical representation of an oxygen saturation reference
curve 30 obtained using the Linear Model described above. Empirical data
has shown that this model provides an accurate indication of oxygen
saturation in the range from 90% to 100%. However, in the lower range of
oxygen saturation the readings provided by the Linear Model tend to be
inaccurate. It has been determined that oxygen saturation at lower levels
can represented by a curve such as 30', which can be defined by the
following relationship.
OS=A.sub.sqr +B.sub.sqr r.sup.2
where:
A.sub.sqr --intercept of Square Model regression line.
B.sub.sqr --slope of Square Model regression line.
This relationship, which hereafter is referred to as the Square Model, can
also be used to measure oxygen saturation in the region between 90% and
100%. This is illustrated by the curves 30 and 32 shown in FIG. 3. As can
be seen the values of the two curves are approximately the same in the
region between 90% and 100%. The slope and intercept of the Square Model
can be calculated by incorporating the assumptions regarding the
similarities of the Models in this region into the equation for the Square
Model, as shown below:
OS.sub.1 =A.sub.lin +B.sub.lin r.sub.1
OS.sub.2 =A.sub.lin +B.sub.lin r.sub.2
OS.sub.1 =A.sub.sqr +B.sub.sqr r.sub.1.sup.2
OS.sub.2 =A.sub.sqr +B.sub.sqr r.sub.2.sup.2
where:
OS.sub.1 --98% oxygen saturation.
OS.sub.2 --92% oxygen saturation.
r.sub.1 --the value of the linear ratio at OS.sub.1.
r.sub.2 --the value of the linear ratio at OS.sub.2.
Using the above equations, the Square Model can be solved for the
intercept, A.sub.sqr, as follows:
##EQU10##
Using the above equations, the Square Model can be solved for the slope,
B.sub.sqr, as follows:
##EQU11##
Therefore:
##EQU12##
By substituting the appropriate known constants into the equation for the
Y-intercept for the Square Model, A.sub.sqr, can be calculated. For
example, the A.sub.sqr for light at a wavelength of 660 nm can be
calculated to be approximately 105%. The slope, B.sub.sqr, of the Square
Model curve can then be calculated by measuring an actual oxygen
saturation data point for a particular individual, e.g., 98%, and using
the mathematical relationships discussed above.
Referring again to FIG. 3, it can be seen that the Linear Model reference
curve 30 and the Square Model reference curve 32 will provide very similar
oxygen saturation readings in the range from 90% to 100%. Therefore,
either of these reference curves can be used to provide accurate readings
in this range. However, as discussed above, the Square Model reference
curve 32 will provide more accurate readings in the range below 90%. Since
the Square Model reference curve can be used to provide accurate readings
in both the upper and lower ranges of the curve, it would be desirable to
use this single reference curve for all oxygen saturation measurements.
One of the difficulties encountered in the use of the Square Model
reference curve is related to the calibration of the curve. In particular,
the hypothetical Y-intercept, A.sub.sqr, and the actual oxygen saturation
point used to construct the curve are typically very close together, e.g.,
105% and 98%. A small error in the determination of the voltage ratio
corresponding to the 98% point can lead to a significant error in the
lower ranges of the curve after extrapolating downward. This problem has
been overcome in the present invention by first calibrating the system
with the Linear Model reference curve, which is less susceptible to
propogation of error. Once the linear reference curve has been calculated,
the oxygen saturation for a point in the vicinity of 90% is calculated and
this data point is used to calibrate the Square Model reference curve.
Once the Square Model reference curve has been calibrated, it is used for
all subsequent calculations of oxygen saturation.
The two step calibration technique used in the preferred embodiment can be
summarized as follows: A first data point A.sub.lin is calculated using
absorption coefficients for oxygenated and reduced hemoglobin at a known
wavelength. For the wavelengths of light used in the preferred embodiment,
the value of A.sub.lin will be a hypothetical value of approximately 113%.
A second data point is then obtained by measuring an actual oxygen
saturation point for a particular individual. In the preferred embodiment,
the second data point is measured at approximately 98%, although other
saturation levels could be used without departing from the principles of
the present invention. Using these two data points, the slope, B.sub.lin,
of the Linear Model reference curve can be calculated and the linear
reference curve can be formed. This reference curve is then used to
calculate a third data point corresponding to an oxygen saturation in the
vicinity of 90% (e.g., between 88 and 92%). A fourth data point
corresponding to the Y-intercept, A.sub.sqr of the Square Model reference
curve is then calculated using absorption coefficients for oxygenated and
reduced hemoglobin at a known wavelength. In the preferred embodiment,
this fourth data point is a hypothetical value of approximately 105%.
Using these third and fourth data points, the slope, B.sub.sqr, of the
Square Model reference curve is calculated and the square reference curve
is formed. Once this curve has been formed, it is used for all subsequent
measurements of oxygen saturation.
While the invention method and apparatus for noninvasive monitoring of
arterial blood oxygen saturation has been described in connection with the
preferred embodiment, it is not intended to be limited to the specific
form set forth herein, but on the contrary, it is intended to cover such
alternatives, modifications and equivalents as may be reasonably included
within the spirit and scope of the invention as defined by the appended
claims.
* * * * *
|
|
|
|
|
|
|
Description |
|
