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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to oximetry and, more particularly, to automatic
calibration techniques employed in oximetry.
The arterial oxygen saturation and pulse rate of an individual may be of
interest for a variety of reasons. For example, in the operating room
up-to-date information regarding oxygen saturation can be used to signal
changing physiological factors, the malfunction of anaesthesia equipment,
or physician error. Similarly, in the intensive care unit, oxygen
saturation information can be used to confirm the provision of proper
patient ventilation and allow the patient to be withdrawn from a
ventilator at an optimal rate.
In many applications, particularly including the operating room and
intensive care unit, continual information regarding pulse rate and oxygen
saturation is important if the presence of harmful physiological
conditions is to be detected before a substantial risk to the patient is
presented. A noninvasive technique is also desirable in many applications,
for example, when a home health care nurse is performing a routine
check-up, because it increases both operator convenience and patient
comfort. Pulse transmittance oximetry is addressed to these problems and
provides noninvasive, continual information about pulse rate and oxygen
saturation. The information produced, however, is only useful when the
operator can depend on its accuracy. The method and apparatus of the
present invention are, therefore, directed to the improved accuracy of
such information without undue cost.
As will be discussed in greater detail below, pulse transmittance oximetry
basically involves measurement of the effect arterial blood in tissue has
on the intensity of light passing therethrough. More particularly, the
volume of blood in the tissue is a function of the arterial pulse, with a
greater volume present at systole and a lesser volume present at diastole.
Because blood absorbs some of the light passing through the tissue, the
intensity of the light emerging from the tissue is inversely proportional
to the volume of blood in the tissue. Thus, the emergent light intensity
will vary with the arterial pulse and can be used to indicate a patient's
pulse rate. In addition, the absorption coefficient of oxyhemoglobin
(hemoglobin combined with oxygen, HbO.sub.2) is different from that of
unoxygenated hemoglobin (Hb) for most wavelengths of light. For that
reason, differences in the amount of light absorbed by the blood at two
different wavelengths can be used to indicate the hemoglobin oxygen
saturation, % SaO.sub.2 (OS), which equals ([HbO.sub.2 ]/([Hb]+[HbO.sub.2
])).times.100%. Thus, measurement of the amount of light transmitted
through, for example, a finger can be used to determine both the patient's
pulse rate and hemoglobin oxygen saturation.
As will be appreciated, the intensity of light transmitted through a finger
is a function of the absorption coefficient of both "fixed" components,
such as bone, tissue, skin, and hair, as well as "variable" components,
such as the volume of blood in the tissue. The intensity of light
transmitted through the tissue, when expressed as a function of time, is
often said to include a baseline component, which varies slowly with time
and represents the effect of the fixed components on the light, as well as
a periodic pulsatile component, which varies more rapidly with time and
represents the effect that changing tissue blood volume has on the light.
Because the attenuation produced by the fixed tissue components does not
contain information about pulse rate and arterial oxygen saturation, the
pulsatile signal is of primary interest. In that regard, many of the prior
art transmittance oximetry techniques eliminate the so-called "DC"
baseline component from the signal analyzed.
For example, in U.S. Pat. No. 2,706,927 (Wood) measurements of light
absorption at two wavelengths are taken under a "bloodless" condition and
a "normal" condition. In the bloodless condition, as much blood as
possible is squeezed from the tissue being analyzed. Then, light at both
wavelengths is transmitted through the tissue and absorption measurements
made. These measurements indicate the effect that all nonblood tissue
components have on the light. When normal blood flow has been restored to
the tissue, a second set of measurements is made that indicates the
influence of both the blood and nonblood components. The difference in
light absorption between the two conditions is then used to determine the
average oxygen saturation of the tissue, including the effects of both
arterial and venous blood. As will be readily apparent, this process
basically eliminates the DC, nonblood component from the signal that the
oxygen saturation is extracted from.
For a number of reasons, however, the Wood method fails to provide the
necessary accuracy. For example, a true bloodless condition is not
practical to obtain. In addition, efforts to obtain a bloodless condition,
such as by squeezing the tissue, may result in a different light
transmission path for the two conditions. In addition to problems with
accuracy, the Wood approach is both inconvenient and time consuming.
A more refined approach to pulse transmittance oximetry is disclosed in
U.S. Pat. No. 4,167,331 (Nielsen). The disclosed oximeter is based upon
the principle that the absorption of light by a material is directly
proportional to the logarithm of the light intensity after having been
attenuated by the absorber, as derived from the Beer-Lambert law. The
oximeter employs light-emitting diodes (LEDs) to produce light at red and
infrared wavelengths for transmission through tissue. A photosensitive
device responds to the light produced by the LEDs, after it has been
attenuated by the tissue, and produces an output current. That output
current is amplified by a logarithmic amplifier to produce a signal having
AC and DC components and containing information about the intensity of
light transmitted at both wavelengths. Sample-and-hold circuits demodulate
the red and infrared wavelength signals. The DC components of each signal
are then blocked by a series bandpass amplifier and capacitors,
eliminating the effect of the fixed absorptive components from the signal.
The resultant AC signal components are unaffected by fixed absorption
components, such as hair, bone, tissue, skin. An average value of each AC
signal is then produced. The ratio of the two averages is then used to
determine the oxygen saturation from empirically determined values
associated with the ratio. The AC components are also used to determine
the pulse rate.
Another reference addressed to pulse transmittance oximetry is U.S. Pat.
No. 4,407,290 (Wilber). In that reference, light pulses produced by LEDs
at two different wavelengths are applied to, for example, an earlobe. A
sensor responds to the light transmitted through the earlobe, producing a
signal for each wavelength having a DC and AC component resulting from the
presence of constant and pulsatile absorptive components in the earlobe. A
normalization circuit employs feedback to scale both signals so that the
DC nonpulsatile components of each are equal and the offset voltages
removed. Decoders separate the two signals, so controlled, into channels A
and B where the DC component is removed from each. The remaining AC
components of the signals are amplified and combined at a multiplexer
prior to analog-to-digital (A/D) conversion. Oxygen saturation is
determined by a digital processor in accordance with the following
relationship:
OS=X.sub.1 R(.lambda..sub.1)+X.sub.2 R(.lambda..sub.2)/(X.sub.3
R(.lambda..sub.1)+X.sub.4 R(.lambda.2) (1)
wherein empirically derived data for the constants X.sub.1, X.sub.2,
X.sub.3 and X.sub.4 is stored in the processor.
European Patent Application No. 83,304,939.8 (New, Jr. et al.) discloses an
additional pulse transmittance oximeter. Two LEDs expose a body member,
for example, a finger, to light having red and infrared wavelengths, with
each LED having a one-in-four duty cycle. A detector produces a signal in
response that is then split into two channels. The one-in-four duty cycle
allows negatively amplified noise signals to be integrated with positively
amplified signals including the detector response and noise, thereby
eliminating the effect of noise on the signal produced. The resultant
signals include a substantially constant DC component and an AC component.
To improve the accuracy of a subsequent analog-to-digital (A/D)
conversion, a fixed DC value is subtracted from the signal prior to the
conversion. This level is then added back in by a microprocessor after the
conversion. Logarithmic analysis is avoided by the microprocessor in the
following manner. For each wavelength of light transmitted through the
finger, a quotient of the AC component over the constant component is
determined. The ratio of the two quotients is then determined and fitted
to a curve of independently derived oxygen saturations. To compensate for
the different transmission characteristics of different patient's fingers,
an adjustable drive source for the LEDs is provided.
In European Patent Application No. 83,304,940.6 (New et al.) a calibrated
oximeter probe is disclosed. That probe includes a coding resistor or
coding connector used to identify the particular combination of
wavelengths of light emitted by the two LEDs contained thereon. Oximeter
circuitry then senses the code of the resistor or connector to determine
the wavelengths of light emitted by the LEDs. In this manner, the effect
that different wavelengths have on the oxygen saturation computations can
be compensated for. The basis upon which oxygen saturation is measured
involves the determination of the quotient of the pulsatile component over
the constant component of light transmitted at each wavelength. The ratio
of the quotients for the two wavelengths is then fitted to a curve of
independently derived oxygen saturations. Outputs include pulse rate and
oxygen saturation.
Even with the calibration technique of New, Jr. et al. employed, however,
the wavelengths of light emitted by the LEDs may change in a manner that
the oximeter circuitry is unable to detect. As will be appreciated, such
variations can significantly affect the accuracy of the oxygen saturation
measurements. The disclosed invention is directed to the provision of more
complete information about the actual wavelengths of the light emitted
and, hence, the production of more accurate oxygen saturation
measurements.
SUMMARY OF THE INVENTION
The present invention discloses a method of determining the oxygen
saturation of arterial blood flowing in tissue. The method includes an
initial step in which the tissue is exposed to light from two sources at
separate temperature-dependent wavelengths. An indication of the
temperature of the sources is produced, as are signals produced in
response to the exposure of the tissue to the light at the separate
temperature-dependent wavelengths. A preliminary indication of the oxygen
saturation is then produced from the signals. A comparison of
independently derived oxygen saturations with a continuum of such
preliminary indications of oxygen saturation is then selected in
accordance with the indication of the temperature of the sources earlier
produced. From this comparison, the actual oxygen saturation corresponding
to the preliminary indication previously obtained is produced.
In accordance with a particular aspect of the invention, an indication of
the separate temperature-dependent wavelengths of light emitted by the
sources at a reference temperature is produced. This indication is used to
further aid in the selection of the appropriate comparison of
independently derived oxygen saturations to the preliminary indications of
oxygen saturation.
In accordance with a further aspect of the invention, an oximeter is
disclosed that employs the foregoing method to determine the oxygen
saturation of arterial blood flowing in tissue. The oximeter includes
first and second light sources that illuminate the tissue with light at
separate temperature-dependent wavelengths. The oximeter also includes a
temperature detector that produces an indication of the temperature of the
light sources. Signals that are proportional to the intensity of light
received from the tissue at each of the temperature-dependent wavelengths
are produced by a light detector and a processor analyzes the signals to
produce a preliminary indication of the oxygen saturation of the blood. A
selection circuit selects a particular comparison of oxygen saturations
with the continuum of preliminary indications of the oxygen saturation in
accordance with the indication of temperature received. Finally, a
converter converts the preliminary indication of oxygen saturation into an
oxygen saturation determination by reference to the comparison selected.
In accordance with additional aspects of the invention, a red optical
filter filters the light received by the light detector. The signals
produced by the light detector can, similarly, be amplified by a
differential current-to-voltage amplifier before being analyzed by the
processor. A sensor housing, having first and second elements, is employed
to receive the tissue being analyzed and to define a light path between
the light sources and the detector. A mirror, attached to the housing, is
positioned between the light sources and detector and breaks the lightpath
up into first and second segments at a predetermined angle with respect to
each other. The two elements of the housing may pivot and be closably
biased. In another arrangement, an apparatus is constructed in accordance
with this invention independently of the light sources and light detector.
In accordance with another aspect of the invention, a sensor is disclosed
for use with an oximeter to determine the oxygen saturation of arterial
blood flowing in tissue. The sensor includes first and second light
sources for illuminating the tissue with light at separate
temperature-dependent wavelengths. A temperature indicator is also
included to produce an indication of the temperature of the light sources.
Signals are produced in response to the illumination of the tissue at each
of the temperature-dependent wavelengths by a light detector.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can best be understood by reference to the following portion
of the specification, taken in conjunction with the accompanying drawings
in which:
FIG. 1 is a block diagram of an oximeter including a sensor, input/output
(I/O) circuit, microcomputer, alarm, displays, power supply, and keyboard;
FIG. 2 is a block diagram illustrating the transmission of light through an
absorptive medium;
FIG. 3 is a block diagram illustrating the transmission of light through
the absorptive medium of FIG. 2, wherein the medium is broken up into
elemental components;
FIG. 4 is a graphical comparison of the incident light intensity to the
emergent light intensity as modeled in FIG. 2;
FIG. 5 is a graphical comparison of the specific absorption coefficients
for oxygenated hemoglobin and deoxygenated hemoglobin as a function of the
wavelength of light transmitted therethrough;
FIG. 6 is a block diagram illustrating the transmission of light through a
block model of the components of a finger;
FIG. 7 is a graphical comparison of independently derived oxygen saturation
measurements with a variable that is measured by the oximeter;
FIG. 8 is a schematic illustration of the transmission of light at two
wavelengths through a finger in accordance with the invention;
FIG. 9 is a graphical plot as a function of time of the transmittance of
light at the red wavelength through the finger;
FIG. 10 is a graphical plot as a function of time of the transmission of
infrared light through the finger;
FIG. 11 is an exploded view showing the sensor of FIG. 1 in greater detail;
FIG. 12 is a more detailed schematic of the I/O circuit illustrated in the
system of FIG. 1;
FIG. 13 is a schematic diagram of a conventional current-to-voltage
amplifier circuit;
FIG. 14 is a schematic diagram of a differential current-to-voltage
preamplifier circuit included in the I/O circuit of FIG. 1;
FIG. 15 is a graphical representation of the possible ranges of I/O circuit
output, showing the desired response of the I/O circuit and microcomputer
at each of the various possible ranges;
FIG. 16 is a more complete schematic diagram of the microcomputer
illustrated in FIG. 1; and
FIG. 17 is a family of curves similar to the one illustrated in FIG. 7.
DETAILED DESCRIPTION
Referring to the overall system block diagram shown in FIG. 1, a pulse
transmittance oximeter 10 employing this invention includes a sensor 12,
input/output (I/O) circuit 14, microcomputer 16, power source 18, display
20, keyboard 22 and alarm 24. Before discussing these elements in detail,
however, an outline of the theoretical basis of pulse transmittance
oximetry as practiced by the oximeter of FIG. 1 is provided.
An understanding of the relevant theory begins with a discussion of the
Beer-Lambert law. This law governs the absorption of optical radiation by
homogeneous absorbing media and can best be understood with reference to
FIGS. 2 and 3 in the following manner.
As shown in FIG. 2, incident light having an intensity I.sub.0 impinges
upon an absorptive medium 26. Medium 26 has a characteristic absorbance
factor A that indicates the attenuating affect medium 26 has on the
incident light. Similarly, a transmission factor T for the medium is
defined as the reciprocal of the absorbance factor, I/A. The intensity of
the light I.sub.1 emerging from medium 26 is less than I.sub.0 and can be
expressed functionally as the product TI.sub.0. With medium 26 divided
into a number of identical components, each of unit thickness (in the
direction of light transmission) and the same transmission factor T, the
effect of medium 26 on the incident light I.sub.0 is as shown in FIG. 3.
There, medium 26 is illustrated as consisting of three components 28, 30,
and 32. As will be appreciated, the intensity I.sub.1 of the light
emerging from component 28 is equal to the incident light intensity
I.sub.0 multiplied by the transmission factor T. Component 30 has a
similar effect on light passing therethrough. Thus, because the light
incident upon component 30 is equal to the product TI.sub.0, the emergent
light intensity I.sub.2 is equal to the product TI.sub.1 or T.sup.2
I.sub.0. Component 32 has the same effect on light and, as shown in FIG.
3, the intensity of the emergent light I.sub.3 for the entire medium 26 so
modeled is equal to the product TI.sub.2 or T.sup.3 I.sub.0. If the
thickness d of medium 26 is n unit lengths, it can be modeled as including
n identical components of unit thickness. It will then be appreciated that
the intensity of light emerging from medium 26 can be designated I.sub.n
and the product is equal to T.sup.n I.sub.0. Expressed as a function of
the absorbance constant A, I.sub.n can also be written as the product
(1/A.sup.n) I.sub.0.
From the preceding discussion, it will be readily appreciated that the
absorptive effect of medium 26 on the intensity of the incident light
I.sub.0 is one of exponential decay. Because A may be an inconvenient base
to work with, I.sub.n can be rewritten as a function of a more convenient
base, b, by recognizing that A.sup.n is equal to b.sup..alpha.n, where
.alpha. is the absorbance of medium 26 per unit length. The term .alpha.
is frequently referred to as the extinction coefficient and is equal to
log .sub.b A.
Given the preceding discussion, it will be appreciated that the intensity
of the light I.sub.n emerging from medium 26 can be expressed in base 10
as I.sub.0 10.sup.-.alpha. 1.sup.n, or in base e as I.sub.0 e.sup.-.alpha.
2.sup.n, where .alpha..sub.1 and .alpha..sub.2 are the appropriate
relative extinction coefficients for base 10 and base e respectively. The
effect that the thickness of medium 26 has on the emergent light intensity
I.sub.n is graphically depicted in FIG. 4. If the light incident upon
medium 26 is established as having unit intensity, FIG. 4 also represents
the transmission factor T of the entire medium as a function of thickness.
The discussion above can be applied generally to the medium 26 shown in
FIG. 2 to produce:
I.sub.1 =I.sub.0 e.sup.-.alpha.d
where I.sub.1 is the emergent light intensity, I.sub.0 is the incident
light intensity, .alpha. is the absorbance coefficient of the medium per
unit length, d is the thickness of the medium in unit lengths, and the
exponential nature of the relationship has arbitrarily been expressed in
terms of base e. Equation (1) is commonly referred to as the Beer-Lambert
law of exponential light decay through a homogeneous absorbing medium.
With this basic understanding of the Beer-Lambert law, a discussion of its
application to the problems of pulse rate and hemoglobin oxygen saturation
measurement is now presented. As shown in FIG. 5, the absorption
coefficients for oxygenated and deoxygenated hemoglobin are different at
every wavelength, except isobestic wavelengths. Thus, it will be
appreciated that if a person's finger is exposed to incident light and the
emergent light intensity measured, the difference in intensity between the
two, which is the amount of light absorbed, contains information relating
to the oxygenated hemoglobin content of the blood in the finger. The
manner in which this information is extracted from the Beer-Lambert law is
discussed below. In addition, it will be appreciated that the volume of
blood contained within an individual's finger varies with the individual's
arterial pulse. Thus, the thickness of the finger also varies slightly
with each pulse, creating a changing path length for light transmitted
through the finger. Because a longer lightpath allows additional light to
be absorbed, time-dependent information relating to the difference between
the incident and emergent light intensities can be used to determine the
individual's pulse. The manner in which this information is extracted from
the Beer-Lambert law is also discussed below.
As noted in the preceding paragraph, information about the incident and
emergent intensities of light transmitted through a finger can be used to
determine oxygen saturation and pulse rate. The theoretical basis for
extracting the required information, however, is complicated by several
problems. For example, the precise intensity of the incident light applied
to the finger is not easily determined. Thus, it may be necessary to
extract the required information independently of the intensity of the
incident light. Further, because the changing volume of blood in the
finger and, hence, thickness of the lightpath therethrough, are not
exclusively dependent upon the individual's pulse, it is desirable to
eliminate the changing path length as a variable from the computations.
The manner in which the Beer-Lambert law is refined to eliminate the
incident intensity and path length as variables is as follows. With
reference to FIG. 6, a human finger is modeled by two components 34 and
36, in a manner similar to that shown in FIG. 3. Baseline component 34
models the unchanging absorptive elements of the finger. This component
includes, for example, bone, tissue, skin, hair, and baseline venous and
arterial blood and has a thickness designated d and an absorbance .alpha..
Pulsatile component 36 represents the changing absorptive portion of the
finger, the arterial blood volume. As shown, the thickness of this
component is designated .DELTA.d, representing the variable nature of the
thickness, and the absorbance of this component is designated
.alpha..sub.A representing the arterial blood absorbance.
As will be appreciated from the earlier analysis with respect to FIG. 3,
the light I.sub.1 emerging from component 34 can be written as a function
of the incident light intensity I.sub.0 as follows:
I.sub.1 =I.sub.0 e.sup.-.alpha.d (2)
Likewise, the intensity of light I.sub.2 emerging from component 36 is a
function of its incident light intensity I.sub.1, and:
I.sub.2 =I.sub.1 e.sup.-.alpha. A.sup..DELTA.d (3)
Substitution of the expression for I.sub.1 developed in equation (2) for
that used in equation (3), when simplified, results in the following
expression for the intensity I.sub.2 of light emerging from the finger as
a function of the intensity of light I.sub.0 incident upon the finger;
I.sub.2 =I.sub.0 e.sup.-[.alpha.d+.alpha. A.sup..DELTA.d] (4)
Because our interest lies in the effect on the light produced by the
arterial blood volume, the relationship between I.sub.2 and I.sub.1 is of
particular interest. Defining the change in transmission produced by the
arterial component 36 as T.sub..DELTA.A, we have:
T.sub..DELTA.A =I.sub.2 /I.sub.1 (5)
Substituting the expressions for I.sub.1 and I.sub.2 obtained in equations
(2) and (3), respectively, equation (5) becomes:
##EQU1##
It will be appreciated that the I.sub.0 term can be cancelled from both
the numerator and denominator of equation (6), thereby eliminating the
input light intensity as a variable in the equation. With equation (6)
fully simplified, the change in arterial transmission can be expressed as:
T.sub..DELTA.A =e.sup.-.alpha. A.sup..DELTA.d (7)
A device employing this principle of operation is effectively
self-calibrating, being independent of the incident light intensity
I.sub.0.
At this point, a consideration of equation (7) reveals that the changing
thickness of the finger, .DELTA.d, produced by the changing arterial blood
volume still remains as a variable. The .DELTA.d variable is eliminated in
the following manner. For convenience of expression, the logarithms of the
terms in equation (7) are produced with respect to the same base
originally employed in equation (1). Thus, equation (7) becomes:
lnT.sub..DELTA.A =ln(e.sup.-.alpha. A.sup..DELTA.d)=-.alpha..sub.A
.DELTA.d(8)
A preferred technique for eliminating the .DELTA.d variable utilizes
information drawn from the change in arterial transmission experienced at
two wavelengths.
The particular wavelengths selected are determined in part by consideration
of a more complete expression of the arterial absorbance .alpha..sub.A :
.alpha..sub.A =(.alpha..sub.OA)(OS)-(.alpha..sub.DA)(1-OS) (9)
where .alpha..sub.OA is the oxygenated arterial absorbance, .alpha..sub.DA
is the deoxygenated arterial absorbance, and OS is the hemoglobin oxygen
saturation of the arterial blood volume. As will be appreciated from FIG.
5, .alpha..sub.OA and .alpha..sub.DA are su | | |
