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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patient ventilation and allow the patient to be withdrawn from a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. 4,167,331 (Nielson). 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 ane 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 calibrating the device is
disclosed.
European patent application No. 83304938.0 (New, Jr. et al.) discloses a
pulse oximeter monitor having a variety of displays. For example, digital
displays of oxygen saturation and pulse rate are provided. In addition, an
indicator having a plurality of LEDs is provided wherein the number of
LEDs strobed is proportional to the magnitude of the pulse and the strobe
rate is proportional to the pulse. An audible tone signal is provided
having a pitch that is proportional to the oxygen saturation and a
repetition rate that is proportional to pulse. Adjustable alarm limits are
provided for high and low pulse rates as well as oxygen saturation levels.
Separate selector switches indicate the alarm limit to be adjusted and a
limit knob is used to set the level. Default limits are initially assigned
to these values and in the event an alarm limit is exceeded, a
constant-pitch, continuous audible tone is produced. Upon start-up, a sync
status light indicates that a pulse has not been established.
While the displays disclosed by New, Jr. et al. provide information to the
oximeter operator, additional information may be advantageously extracted
by the oximeter. It is the display of certain types of this additional
information to which the present invention is directed.
SUMMARY OF THE INVENTION
According to the present invention, an apparatus is disclosed for
processing signals containing information about the oxygen saturation of
arteria blood flowing in tissue. The apparatus includes a processor that
determines the oxygen saturation of the arterial blood flowing in the
tissue from the signals and a display that produces an output indicative
of the change in the oxygen saturation during a specified interval.
In accordance with a particular aspect of the invention, the display means
includes first and second trend indication displays. The first trend
indication display produces an output when the oxygen saturation has
increased by a first predetermined amount during the specified interval.
Similarly, the second trend indication display produces an output when the
oxygen saturation has decreased by a second predetermined amount during
the specified interval. The first predetermined amount, second
predetermined amount, and specified interval can be selectively
controlled. With, for example, the first and second predetermined amounts
being set at a three percent change in oxygen saturation and the specified
interval being set at two minutes. The first trend indication display may
be extinguished when the oxygen saturation fails to increase by a third
predetermined amount over a second specified interval (e.g., 2.5% over 2
minutes), and the second trend indication display extinguished when the
oxygen saturation fails to decrease by a fourth predetermined amount over
the second predetermined interval (e.g., 2.5% over 2 minutes). In one
embodiment, the first and second trend indication displays are upwardy and
downwardly directed triangular light-emitting diodes. As an alternative
to, or for use in conjunction with, the first and second trend indication
displays, the display may provide a numeric representation of the change
in oxygen saturation.
In accordance with another aspect of the invention, an apparatus is
disclosed for processing signals containing information about the pulse
rate and perfusion of arterial blood flowing therein. A detection means
produces signals that are proportional to the intensity of light received
from the tissue in response to the illumination. Processing means then
determine the oxygen saturation, pulse rate, and perfusion of the arterial
blood from the signals produced by the detection means. An output
indicative of the pulse rate and perfusion is produced by a display means,
with the perfusion being displayed as a logarithmic function of the
perfusion determined by the processing means.
In accordance with further aspects of the invention, the display means may
automatically scale the perfusion displayed to produce a full-scale
display at peak perfusion when the signal level exceeds a predetermined
level. The display means may conveniently comprise an aligned array of
light-emitting diodes, with the number of light-emitting diodes lit at any
one time imaging pleth. Waveform, peak to peak scaling is employed which
is indicative of signal level and perfusion.
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 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
measurements related to a measurable value determined 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 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. 15 is a more complete schematic diagram of the microcomputer
illustrated in FIG. 1;
FIG. 16 is a more complete schematic diagram of the power source
illustrated in FIG. 1;
FIG. 17 is a detailed view of the front panel of an oximeter constructed in
accordance with the present invention illustrating some of the displays
employed; and
FIG. 18 is an alternative display for use on the front panel shown in FIG.
17 to indicate oxygen saturation trends.
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 relevent 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. Simlarly, a transmission factor T for the medium is
defined as the reciprocal of the absorbance factor, I/A. The intensiy 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 appreicated, 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 appreicated 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. 1.sup.n, or in
base e (where .alpha.=.alpha..sub.2) as I.sub.0 e.sup.-.alpha. 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 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 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. A.sup..DELTA.d (3)
Substitution of the expression for I.sub.1 developed in equation (2) for
that used in equation (3), where 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 obtaind in equations
(2) and (3), respectively, equation (5) | | |
