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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to oximetry and, more particularly, to
signal-processing 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patient ventilation and allow the patient to be withdrawn from a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
deoxygenated 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. 4,086,915 (Kofsky et al.). The Kofsky et al. reference is of
interest for two reasons. First, the technique employed automatically
eliminates the effect that fixed components in the tissue have on the
light transmitted therethrough, avoiding the need to produce bloodless
tissue. More particularly, as developed in the Kofsky et al. reference
from the Beer-Lambert law of absorption, the derivatives of the intensity
of the light transmitted through the tissue at two different wavelengths,
when multiplied by predetermined pseudocoefficients, can be used to
determine oxygen saturation. Basic mathematics indicate that such
derivatives are substantially independent of the DC component of the
intensity. The pseudocoefficients are determined through measurements
taken during a calibration procedure in which a patient first respires air
having a normal oxygen content and, later, respires air of a reduced
oxygen content. As will be appreciated, this calibration process is at
best cumbersome.
The second feature of the Kofsky et al. arrangement that is of interest is
its removal of the DC component of the signal prior to being amplified for
subsequent processing. More particularly, the signal is amplified to allow
its slope (i.e., the derivative) to be more accurately determined. To
avoid amplifier saturation, a portion of the relatively large DC component
of the signal is removed prior to amplification. To accomplish this
removal, the signal from the light detector is applied to the two inputs
of a differential amplifier as follows. The signal is directly input to
the positive terminal of the amplifier. The signal is also passed through
a low-resolution A/D converter, followed by a D/A converter, before being
input to the negative terminal of the amplifier. The A/D converter has a
resolution of approximately 1/10 that of the input signal. For example, if
the signal is at 6.3 volts, the output of the A/D converter would be 6
volts. Therefore, the output of the converter represents a substantial
portion of the signal, which typically can be used to approximate the DC
signal level. Combination of that signal with the directly applied
detector signal at the amplifier produces an output that can be used to
approximate the AC signal. As will be readily appreciated, however, the
process may be relatively inaccurate because the output of the A/D
converter is often a poor indicator of the DC signal.
U.S. Pat. No. 4,167,331 (Nielson) discloses another pulse transmittance
oximeter. 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 and attenuated by the tissue,
producing 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 from each is removed. 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:
##EQU1##
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. 83304939.8 (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 a pulsatile 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 pulsatile 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
patients' fingers, an adjustable drive source for the LEDs is provided. In
addition, an apparatus for automatically caibrating the device is
disclosed.
Prior art oximeters have, however, not always employed signal-processing
techniques that are adequate to provide maximum resolution of the signal
received for analysis. As a result, the accuracy of oxygen saturation and
pulse rate determinations made by the oximeter may suffer. The disclosed
invention addresses this problem and improves the accuracy previously
attainable in the art of oximetry.
SUMMARY OF THE INVENTION
The present invention discloses an apparatus for processing signals
produced by a sensor that contain information about the oxygen saturation
of arterial blood flowing in tissue. The apparatus includes an offset
subtractor for subtracting a controlled portion of the sensor signal from
that signal. The offset subtractor produces an output substantially equal
to the portion of the sensor signal remaining after the controlled portion
has been subtracted therefrom. The system also includes a controller,
coupled to the offset subtractor, which receives the output of the offset
subtractor and produces a subtraction control signal dependent upon that
output. The subtraction control signal is transferred to the offset
subtractor and determines the magnitude of the controlled portion of the
signal subtracted thereby. An analyzer receives the output of the offset
subtractor and produces an indication of the oxygen saturation of the
arterial blood.
In accordance with a particular aspect of the invention, the controlled
portion of the detector signal subtracted is held constant when the
absolute value of the offset subtractor output is less than a first
predetermined level. When the absolute value of the offset subtractor
output falls within a predetermined range above that level, however, a
subtraction control signal is produced indicating that the offset
subtractor is to adjust the magnitude of the controlled portion by an
amount proportional to the magnitude of the offset subtractor ouput.
When the absolute value of the offset subtractor output exceeds a second
predetermined level, a subtraction control signal is produced indicating
that the offset subtractor is no longer able to adjust the controlled
portion of the signal to be subtracted. Preferably, the controlled portion
subtracted from the detector signal by the offset subtractor is
initialized at a predetermined value.
In accordance with another aspect of the invention, the system further
includes a controllable gain amplifier for amplifying the output of the
offset subtractor by a controlled gain. The amplifier produces an output
that is substantially equal to the product of the offset subtractor output
and the gain. The controller produces an amplifier control signal that is
received by the amplifier, which adjusts the controlled gain in response
thereto.
In accordance with a further aspect of the invention, the controller
produces a sensor control signal to which said sensor responds. The
controller establishes the sensor control signal at a level sufficient to
cause the sensor signal to fall within a predetrmined sensor signal range.
In accordance with further aspects of this invention, a differential
current-to-voltage amplifier amplifies the sensor signal before it is
received by the offset subtractor. An analog-to-digital converter also
converts the output of the controllable-gain amplifier into a digital
format for analysis. The analyzer removes the gain and adds the controlled
portion back to the amplifier output before producing the indication of
oxygen saturation.
As will be appreciated, the disclosed invention also includes an oximeter
employing the apparatus described above in conjunction with a sensor. The
sensor includes a light source that responds to a control signal from the
controller and illuminates the tissue. The intensity of the illumination
is determined by the control signal. A detector included in the sensor
responds to the illumination of the tissue by producing a signal that
contains information about the oxygen saturation of the arterial blood. A
red optical filter may be included to filter the light received by the
detector.
As will also be appreciated, the disclosed invention includes the method of
processing signals employed by the apparatus discussed above to determine
the oxygen saturation of arterial blood flowing in tissue. In a basic
form, the method includes the steps of subtracting from the sensor signal
a controlled portion of the signal in response to a subtraction control
signal. A subtraction output is produced that substantially equals the
portion of the sensor signal remaining after the controlled portion has
been subtracted therefrom. A subtraction control signal is also produced,
dependent on the subtraction output in a manner indicating the desired
adjustment in the controlled portion subtracted from the sensor signal.
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 empirically derived oxygen saturation
measurement with a variable that is measurable 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 a more detailed schematic of the I/O circuit illustrated in the
system of FIG. 1;
FIG. 12 is a schematic diagram of a conventional current-to-voltage
amplifier circuit;
FIG. 13 is a schematic diagram of a differential current-to-voltage
preamplifier circuit included in the I/O circuit of FIG. 1;
FIG. 14 is a functional block diagram illustrating the basic operation of
the feedback control system constructed in accordance with this invention;
FIG. 15 is a graphical representation of the possible ranges of I/O circuit
output, showing the desired response to the I/O ciruit and microcomputer
at each of the various possible ranges;
FIG. 16 is a block diagram of a portion of an interrupt level software
routine included in the microcomputer illustrated in FIG. 1;
FIGS. 17 through 20 are more detailed block diagrams of the interrupt level
routine depicted in FIG. 16;
FIG. 21 is a graphical representation of the possible ranges of current
supplied to the sensor, showing the desired response of the I/O circuit
and microcomputer at each of the various possible ranges as a function of
sensor output;
FIGS. 22 through 24 are further detailed block diagrams of the interrupt
level routine depicted in FIG. 16;
FIG. 25 is a block diagram of reconstruction software included in the
microcomputer illustrated in FIG. 1;
FIG. 26 illustrates a calibrated offset table stored in the microcomputer
for use in adjusting the operation of the I/O circuit; and
FIG. 27 is a more complete schematic diagram of the microcomputer
illustrated in FIG. 1.
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 relative 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
(where .alpha.=.alpha..sub.1) as I.sub.0 10.sup.-.alpha..sbsp.1.sup.n, or
in base e (where .alpha.=.alpha..sub.2) as I.sub.0
e.sup.-.alpha..sbsp.2.sup.n. 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 (1)
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, d is
the thickness of the medium per unit length 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 an isobestic wavelength. 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
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 arterial blood 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..sbsp.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..sbsp.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:
##EQU2##
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..sbsp.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:
ln T.sub..DELTA.A =ln (e.sup.-.alpha..sbsp.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 wavelenths.
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 substantially unequal at all
light wavelengths in the red and near-infrared wavelength regions except
for an isobestic wavelength occurring at approximately 805 nanometers.
With an arterial oxygen saturation OS of approximately 90 percent, it will
be appareciated from equation (9) that the arterial absorbance
.alpha..sub.A is 90 percent attributable to the oxygenated arterial
absorbance .alpha..sub.OA and 10 percent attributable to the deoxygenated
arterial absorbance .alpha..sub.DA. At the isobestic wavelength, the
relative contribution of these two coefficients to the arterial absorbance
.alpha..sub.A is of minimal significance in that both .alpha..sub.OA and
.alpha..sub.DA are equal. Thus, a wavelength roughly approximating the
isobestic wavelength of the curves illustrated in FIG. 5 is a convenient
one for use in eliminating the change in finger thickness .DELTA.d
attributable to arterial blood flow.
A second wavelength is selected at a distance from the approximately
isobestic wavelength that is sufficient to allow the two signals to be
easily distinguished. In addition, the relative difference of the
oxygenated and deoxygenated arterial absorbances at this wavelength is
more pronounced. In light of the foregoing considerations, it is generally
preferred that the two wavelengths selected fall within the red and
infrared regions of the electromagnetic spectrum.
The foregoing information, when combined with equation (8) is used to
produce the following ratio:
##EQU3##
where T.sub..DELTA.AR equals the change in arterial transmission of light
at the red wavelength .lambda..sub.R and T.sub..DELTA.AIR is the change in
arterial transmission at the infrared wavelength .lambd | | |
