CA1304503C - Method and apparatus for indicating perfusion and oxygen saturation trends in oximetry - Google Patents

Abstract

METHOD AND APPARATUS FOR INDICATING PERFUSION AND
OXYGEN SATURATION TRENDS IN OXIMETRY
Abstract of the Invention The present invention discloses a method and apparatus for indicating perfusion and oxygen saturation treands in oximetry. In transmittanceand reflectance oximetry, LEDs (40, 42) are typically employed to expose tissue to light at two different wavelengths. The light transmitted through, or reflected by, the tissue is received by a detector (38) where signals proportional to the intensity of light are produced. These signals are then processed by oximeter circuitry (14, 16) to determine oxygen saturation, pulse rate, and per usiom Displays (20) are provided including a display (132, 134) of the change in the oxygen saturation during a specified interval. This display may include first (132) and second (134) trend indication displays that indicate when the oxygen saturation has either been increasing or decreasing at a rate in excess of some predetermined level. Preferably, these displays are triangular, upwardly and downwardly directed light-emitting diodes. A digital display (138) of the changein oxygen saturation may also be provided. A second type of display included provldes pulse and perfusion information, with the perfusion being displayed as a logarithmic function of the actual perfusion. This display comprises an aligned array of light-emitting diodes (136) with the number of light-emitting diodes lit at any one tlme being logarithmically proportional to the magnitude of the perfus{on. The display is automatically scaled to produce a full-scale display when the peak perfusion exceeds some predetermined level.

Description

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MI~THOD AND APPARATUS YOR INDICATINa PER~llSlON AND
OXYGRN SATURATION TR13NDS IN O~IMI~TRY
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 informaffon 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 paffent ventilation and aUow the paffentto be witMrawn from a ventilator at an optimal rate.
In many applications, particularly including the operating room and intensive care unit, conffnual 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 noninvasivetechnique is also desirable in many applicaffons, 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 noninvaslve, conffnual information about pulse rate and oxygen saturaffon. The information produced, however, is o~y useflil 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 wiU be discussed in greater detail below, pulse transmittance oximetry basically involves measurement of the effect arterial blood in tissue ' h 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 paæsing through the tiæue, the intensity of the light emerging :
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~,;}f~S~ 3 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, HbO2) is different from thst of deoxygenated hemoglobin (Hb) for most wavelengths of light. For that reason, differences in the amount of light absorbed by the blood at two dlfferent wavelengths can be used to indicate the hemoglobln oxygen saturation, % SaO2 (OS), which equals ([HbO2]/([Hb] ~ [HbO2]))x100%. 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 s 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 variesmore 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 ofthe prior art transmittance oximetry techniques eliminate the so-called "DC"
baseline component from the signal analyzed.
For example, in U.S. Patent No. 2,706,927 (Wood) measurements of 2 5 ligSht absorption at two wavelengths are taken under a "bloodlessShS condition and a "normal" condition. In the bloodless condition, as much Mood as possible is squeezed from the tissue being analyzed. Then, light at both wavelengths is ¦ transmitted thsrough the ffssue and absorption measurements made. These f measurements indicate the effect that aU nonblood tissue components hf~ve on the light. When normal blood flow has been restored to the tissue, a second set f of messurements is made that indicates the influence of both the blood and nonblood components. The difference in ligm absorption between the two conditions is then used to determine the average oxygen saturation of the tissue, including the effects of both arterisl and venous blofcfd. As will be readily apffparent, this proce$ffs basically eliminates the DC, nonblofcfd component from the signal that the oxygen saturation is extracted from.
For 8 number of reasons, however, the Wood method fails to provide the necessary accuracy. For example, a true bloodless condition is not J.;~ S(~3 prQctical to obtQln. In addition, etforts to obtain a bloodless condition, such as by squeezing the tissue, may result in a different light transmlssion path for the two conditions. In addition to problems with accuracy, the Wood approach i8 both inconvenient and time consuming.
A more refined approach to pulse trsnsmittance oximetry i9 disclosed in U.S. Patent No. 4,167,331 (Nielson). The disclosed oximeter is based upon the principle that the absorption of light by a materi01 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 sign~l 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 ofthe fixed absorpffve 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 u~ed to determine the pulse rate.
Another reference addressed to pulse transmittance oximetry is U.S. Patent 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 eQrlobe, producing a signQlfor each wavelength having a DC and AC component resulting from the presence of constant and pulsaffle absorptive components in the eQrlobe. A normalization circuit employs feedback to scale both signals so that the DC nonpulsaffle 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 mulffplexer prior to analo~to-digital (A/D) ' ~ 3 5 conversion. Oxygen saturation is determined by a digital processor in accordance with the following relationship:
X1R( A 1) + X2R( ~ 2) OS X3R~ A1) + X4R( A 2~

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5~:j3 6283g-1007 whereln empirically derlved data for the constants X1, X2, X3 and X4 is stored ln the processor.
European Patent Publlcatlon No. 0102816 published March 14, 1984 (New, Jr. et al.) discloses an additlonal pulse transmittance oximeter. Two LEDs expose a body member, for example, a finger, to llght havlng red and infrared wavelengths, with each LED having a one-in-four duty cycle. A detector produces a signal in response that ls then split into two channels. The one-ln-four duty cycle allows negatively amplified nolse 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 lnclude 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 i8 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 mlcroprocessor in the following manner. For each wavelength of llght 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 ad~ustable drlve source for the LEDs is provided. In addition, an apparatus for automatically calibrating the device is dlsclosed.
European Patent Publication No. 0104771 published April 4, 19~4 ~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 lndicator having a plurality of LEDs is provided wherein the number of LEDs strobed is proportional to the magnltude 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. Ad~ustable alarm l'mits are provided .,, ~ .

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, , 1.3r,l~503 for high and low pulse rates as well as oxygen saturation levels.
Separate selector swltches 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 llght lndlcates that a pulse has not been established.
While the displays disclosed by New, Jr. et al. provlde information to the oximeter operator, additional information may be advantageously extracted by the oxlmeter. It is the display of certain types of this additional information to which the present invention i5 directed.

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Summary of the Inventlon According to the present invention, an apparatus is disclosed for processing signals containing information about the oxygen saturation of arterial blood flowing in tissue. The apparatus includes a processor that determines the oxygen saturation of the arterial blood nowlng in the tissue from the slgnals and 5 a display that produoes an output indicative or the change In the oxygen saturation during a specifled interval.
In accordance with a partioular 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 10 by a first predetermined amount during the specified interval. Similarly, thesecond 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 15 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 oxygen20 saturation fails to decrease by a fourth predetermined amount over the secondpredetermined interval (e.g., 2.5% over 2 minutes). In one embodiment, the firstand second trend indicaffon displays are upwardly and downwardly directed ¦ triangular light-emitffng diodes. As an alternative to, or for use in conjunction with, the flrst and second trend indicaffon displays, the display msy provide a 2 5 numeric representation of the change in oxygen saturation.
In accordance with another aspect Or the invention, an apparatus is discl~ed for processing signals containing informaffon about the pulse rate and perfusion of arteriat blood nowing therein. A detection means produces signals that are proporffonal to the intensity of light received from the tissue in response to the illumination. Processing means then determine the oxygen saturation, pulse rate, and perfusio~ 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.
3t/5 In accordance with further aspects of the invenffon, 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.
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~1.3~ 5l~3 The display means may conveniently comprlse an atigned array of light-emittlng diodes, with the number of light-emitting diodes lit at any one time imaging plethysmographic waveform, peak-to-peak scaling is employed which i8 indlca-tive of signal level and perfusion.
S Brief Description of the Drawing~
The invention can best be understood by reference to the foltowing portion of the specification, taken in con~unction with the accompanying drawings in which:
PIGURE 1 is block diagram of an oximeter including a sensor, input/output ~I/O) circuit, microcomputer, alarm, displays, power supply, and keyboard;
FIGURE 2 is a block diagram illustrating the transmission of light through an absorptive medium;
FIGURE 3 is a block diagram iltustrating the transmission of light through the absorptive medium of FIGURE 2, wherein the medium is broken up into elemental components;
FIGURE 4 is a graphical comparison of the incident light intensity to the emergent light intensity as modeled in FIGURE 2;
FIGURE 5 is a graphicat comparison of the specific absorption 2 coefficients for oxygenated hemoglobin and deoxygenated hemoglobin as a funcffon o the wavelength of tight transmitted therethrough;
FIGURE 6 is a block diagram ittustrating the transmission of tight through a block modet of the components of a finger;
FIGURE 7 is a graphicat comparison o empiricatty derived oxygen 2 5 saturation measurements related to a measurable vatue determined by thej o~dmeter;
FIGURE 8 is a schematic iltustration of the transmission of light at two wavetengths t~ough a finger in accordance with the invenffon;
FIGURE 9 is a graphical plot as a function of time of the t ~ 30 transmittance of tight at the red wavetength through the inger;
FIGURE lO is a graphicat plot as a function of time of the transmission of infrared tight through the finger;
FIGURE ll is a more detailed schematic of the I/O circuit ,, iltustrated in the system of FIGURE l;
FIGURE 12 is a schematic diagram of a conventional current-to-, ~ voltage ampli-ier circuit;
FIGURE 13 is a schematic diagram of a differential, current-to-voltage preamplifier circuit included in the l/O circuit of FIGURE l;

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~3r3451)3 FlGURE 14 is 8 graphical representation of the possible ranges o~
I/O circuit output, showing the des~red response ot the l/O ¢trcuit and mloro-computer at each of the various posslble ranges;
FIGURE 15 is a more complete schematic diagram of the micro-5 computer illustrated in FIGURE 1;
FIGURE 16 is a more complete schematic diagram of the powersource illustrated in FIGURE 1;
FIGURE 17 is a detailed view of the front panel of an oximeter constructed in accordance with the present invention illustrating some of the 10 displays employed; and FIGURE 18 is an alternative display for use on the front panel shown in FIGURE 17 to indicate oxygen saturation trends.
- Detailed Description Referring to the overall system block diagram shown in FIGURE 1, 5 a puise transmittance oximeter 10 employing this invention includes a sensor 12, input/output ~/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 practicedby the oximeter of FIGURE 1 is provided.
- 20 An understanding of the relevant theory begins with a discussion of the Beer-Lambert law. This law governs the absorpUon of opUcal radiation by homogeneous absorbing media and can best be understood with reference to PIGURES 2 and 3 in the ollowing manner.
- ~ As shown in FIGURE 2, incident light having an intensity lo25 impinges upon an absorpUve medium 26. Medium 26 has a characteristic absorbance factor A that indicates the attenuating affect mediwn 26 has on the incident light. Simlarly, a transmission actor T for the medium is defined as the reciprocal of the absorbance actor, UA. The intensity o the light I1 emerging from medium 2B is less than Io and can be expressed functionally as the product 30 Tlo. With medium 26 divided into a number of idenUcal components, each of unit thickness (in the direction of light transmission) and the same transmission factorT, the effect of medium 26 on the incident light Io is as sho~Yn~in j FIGURE 3.
There, medium 26 is illustrated as consisting of three com~
35 onents 28, 30, and 32. As will be appreciated, the intensity Il of the light emerg~ng from component 28 is equal to the incident light intensity Io multiplied by the transmission factor T. Component 30 has a similar effect on light passingtherethrough. Thus, because the light incident upon component 30 is equal to the i.3~.14S('~3 product Tlo, the emergent light Intensity 12 is equal to the product Tll or T21o.
Component 32 has the same effect on llght and, as shown in FIGURE 3, the intensity of the emergent light 13 for the entire medium 26 80 modeled is equal to the product T12 or T310. If the thickness d of medium 26 is n unit lengths, it S can be modeled as inc~uding n identical components of unit thiakness. It will then be appreciated that the intensity of light emerging from medium 26 can be designated In and the product is equal to Tnlo. Expressed as a function of the absorbance constant A, In can also be written as the product (l/An) 10.
From the preceding discussion, it will be readily appreciated that 0 the absorptive effect of medium 26 on the intensity of the incident light 10 is one of exponential decay. Because A may be an inconvenient base to work with, In can be rewritten as a funcffon of a more convenient base, b, by recognizing thatAn is equal to ban, where o~ is the absorbance of medium 26 per unit length.
The term a is frequently referred to as the relative extinction coefficient and is equal to log bA.
Given the preceding discussion, it will be appreciated that the intensity of the light In emerging from medium 26 can be expressed in base 10 (where = al) as IolO ~ln, or in base e (where = a~2) as IOe ~2n. The enect that the thickness of medium 26 has on the emergent light intensity In is graphically depicted in PIGURE 4. If the light incident upon medium 26 is established as having unit intensity, FIGURE 4 also represents the transmission factor T of the entire medium as a function of thickness.
The discussion above can be spplied generally to the medium 26 shown in ~IGURE 2 to produce:
' 25 ~ I e~d (1) where 11 is the emergent light intensity, 10 is the incident light intensity, ~ 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 Or base e. Equation (1) is commor~y referredto as the Beer-Lambert law of exponenffal light decay through a homogeneous absorbing medium.
With this basic understanding of the Beer-Lambert 18w, a discus-sion of its application to the problems of pulse rate and hemoglobin oxygen saturation measurement is now presented. As shown in FIGURE 5, the absorp-tion coemcients for oxygenated and deoxygenated hemoglobin are different at every wavelength, except an isobestic wavelength. Thus, it will be appreciated ~;:
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5(~3 g that If a person's finger ~9 exposed to ina~dent 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 5 informaffon i9 extracted from the Beer-Lambert law ~8 discussed below. In addition, it wlll 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 varles slightly with each pulse, creating a changing path length forlight transmitted through the finger. Because a longer lightpath allows 10 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 putse. 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 in-cident 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. Eor example, the precise intensity of the incident light appied to thefinger is not easily determined. Thus, it may be necessary to extract the 2 0 required information independently of the intensity of the incident light.
Eurther, because the changing volume of blood in the finger and, hence, thiokness 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 FIGURE 6, a human finger is modeled by two components 34 and 36, in a manner similar to that shown in FIGURE 3. Baseline component 34 models the unchanging absorptive elements of the fingeh This component includes, for example, bone, tissue, skin, hair, and baseline venous and arteri~l blood and has a ; ~ thickness designated d and an absorbance n~.
P~satile component 36 represents the changing absorptive portion of the finger, the arterial blood volume. As shown, the thickness of this ' component is designated ~ d, representing the variable nature of the thickness, 3 5 and the absorbance of this arterial blood component is designated A
representing the srterial blood absorbance.

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' 1.3Q4S03 As will be appreciated from the earlier an~ysi3 with respect to FIGURE 3, the light Il emerging from component 34 can be written as a function of the incident ligm intensity Io as follows:

I =I e d ~ (2) Likewise, the intensity of light I2 emerging from component 36 is a function of its incident light intensity Il, and I2 = Ile YA ~ d (3) Substitution of the expression for Il developed in equation (2) for that used in equation (3), when simplified, results in the following expression for the intensity I2 of light emerging from the finger as a function of the intensity of light lo 15 incident upon the finger;
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: 2 0 (4) Because our interest lies in the effect on the light produced by the arterial blood 20 volume, the relatlonsbip between I2 and Il is of particular interest. Defining the chenge in transmission produced by the arterial component 36 as T ~A~ we have: I
T ~A Il (5) ;~ 1 -. j Substituting the expressions for Il and I2 obtained in equations (a) and (3), , respectively, equation (5) beoomes:
'~, : ~ T = IOe [ ad ~A I (6) ~l 30 s It will be appreoiated that the Io term o~n be cancelled from both the numerator ` ~ and denominator of equation (6), thereby eliminating the input light intensity as . a variable in the equation. With equaffon (6) fully simplified, the change in .,-,; ~
. ~ arterial transmission can be expressed asx T = e~ aAQd (7) 3~
A devioe employing this prinoiple d operation is effeoffvely self-oalibraffng, being independent of the inoident Iight intensity Io~

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1.3~45()3 At ehis point, a oonslderatlon of equatlon ~q) reveals that the changing thickness o~ the ~inger, ~d, produced by the changing arteri~ blood volume still remains as a variable. The ~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 ~A = ln (e A ~d) = _ aA ;~d (8) A preferred technique for eliminating the ~d variable utilizes information drawn from the change in arterial transmission experienced at two wavelengths.
The particular wavelengths selected are determined in pa~t by consideration of a more complete expression of the arterial absorbance aA:

Xos) ( XI-OS) (g) where OA is the oxygenated arterial absorbaoce, aDA is the deoxygenated arterial absorbance, and OS is the hemoglobin oxygen saturation of the arterial blood volume. As will be appreciated rrom FIGURE S, aOA and aDA ~are J 2 0 substantially unequal at all light wavelengths in the red and near-infrared i wavelength regions except for an isobestic wavelength occurring at awroxi-i mately 805 nanometers. With an arterial oxygen saturation OS of approximately 90 percent, it win be awreciated from equation (9) that the arterial ab-sorban¢e A is 90 percent attributable to the oxygenated arterial ab-sorbance OA and 10 percent attributable to the deoxygenated arterial ab-sorbance aDA At the isobestic wavelength, the relative contribution of these two coefficients to the arteriat absorbance A is of minimal signiffcance in that both aOA and DA are equal. Thus, a wavelength roughly approximating the isobestic wavelength of the curves mustrated in PlGURE 5 is a convenient one for use in eliminating the change in finger thickness ~d attributable to arterial blood flow.
A second wavelength is selected at a distance from the approxi-t mately isobestic wavelength that is sufficient to anow the two signals to be easily distinguished. In addition, the relaffve difference of the oxygenated anddeoxygenated arterial absorbances at this wavelength is more pronounced. In light of the foregoing considerations, it is generally preferred that the two wavelengths selected fan within the red and infrared regions of the electro-magneUc spectrum.

-12- 13~4S03 The forego1ng Wormation, when combined with equatlon (8) i9 wed to produce the tollowing ratio:
lnT ~AR ~ ~A~d 61 ~R (lO) ln T ~AIR - ~LA ~d @ AIR
where T ~AR equals the change in arterial transmission of light at the red wavelength AR and T ~AIR i9 the~change in arterial transmtssion at the infrared wsvelength AIR. It will be apprecisted that lf two sources are positioned at substantially the same location on the finger, the length of the llghtpath through 10 the finger is substantially the same for the iight emitted by each. Thus, thechange in the lightpath resulting from arterial blood flow, ~d, is substanffallythe same for both the red and infrared wavelength sources. For that reason, the ~d term in the numerator and denominator of the right-hand side of equation (10) canoel, prodùcing:
ln T ~AR ~A (3~ R - (11) ln T ~AIR ~A ~! AIR
j As will be appreciated, equaffon (l1) is independent of both the i incident light intensity Io and the change in finger thickness ~d attributable to arterial blood flow. The foregoing derivaffons form the theoretical basis of pulse oximetry measurement. Because of the complexity of the physiological process, however, the raffo indicated in eguaffon (11) does not directly provide an accurate measurement of oxygen saturation. The correlation between the ratio d equaffon (ll) and actusl srterial blood gas measurements is, therefore, relied2 5 on to produce an indicaffon Or the oxygen saturstion. Thus, if the raffo of the arterial absorbance at the red and infrared wavelengths can be determined, the oxygen saturation of the arterial blood flow can be extracted from independentlyderived, empirical calibraffon curves in a manner independent of Io and A d.
~or simplicity, a measured ratio RoS is defined from eguation (11) as~ aA~ AR (12) It is this measured value for RoS that is plotted on the x-axis of independentlyderived oxygen saturation curves, as shown in ~IGURE 7 and discussed in greater ' ~ ~ 35 detail below, with the hemoglobin oxygen saturation being referenced on the i I y-axia Measurement of the arterial absorbances at both wavelengths is ; ¦ performed in the following manner. As shown in PIGURE 8, a detector 38 placed on the side of a finger opposite red and infrared wavelength light sources 40 and ~, ,i ~;
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~.3~ S03 42 reoeives light at both wavelengths tr~nsmitted through the finger. As shown in FIGURE 9, the received red wavelength light intensity, plotted as a function of time, varies with each pulse, and has high and low values RH and RL, respectively. RL occurs substantially at systole, when arterial Mood volume is 5 at its greatest; wMle RH occurs substantially at diastole, when the arterial Mood volume is lowest. From the earlier discusslon of the exponential lIght decay through homogeneous media, it will be appreciated that:

R = IOe~ t a d A ~ ~ R (13) ' 10 Similarly:

RH =IOe @ ~R (14) 5 TaWng the ratio of equations (13) and (14) and simplifying, we have:
RL = I0e lad A @AR=e aA~ @ R (15) TaWng the logarithm of both sides of equation (15) producès:

2 0 ln (RL/RH) = ln (e A ~d)~ AR = _ aA ~d @ ~ R (16~

As will be readily appreclated, similar expression can be produced for the SigN~S
- ( representative of the infrared wavelength light received by detector 38. Thus, the minimum light intensity passing through the finger at the infrared wave-2 5 length can be written:

IRL = I0e- [ a d A ~ d] ~ A IR (17) Similarly, the maximum light intensity emerging from the finger at the infrared 30 wavelength c~n be expressed as:

IRH = I0e d ~ ~IR (18) The ratio of the terms in equaUons (17) and (18) can be expressed as:
IRL = IOe @ AIR = e~ aA ~d G~ A IR (19) Use of logarithms simplirles equation (19) to:

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~.3~4503 ln (IRL/IRH) - ln (e aA ~ d)(3 AIR = - aA ~ d@ AIR (20) The ratiometric combination of equations (16) and (20) yields.

ln (RL/RH) - aA ~d @ AR (21) ln (IRL/IRH) ~ A QdQ IR
Because the L d term in the numerator and denominator of the ri~ht-hand side of equation (21) c~ncel, as do the negative signs before each term, it will be appreciated that equation (21) when combined with equation (12) yields:
aA~ A R ln(RL/RH) ln(RH/RL) Ratio = R = = ~ /1 ) = 1 ~IR /lR ) (22) Thus, by measuring the minimum and màximuli~ emergent light intensities at both the red and infrared wavelengths (RL, RH, IRL, IRH), a value for the`term RoS can be computed. From this, empirically derived caIibraffon curves similar to that shown in FIGURE 7 can be used to determine the oxygen saturation as described in greater detail in conjuction with the discussion of the various components of oximeter 10 that follows. As will be appreciated, the determina-tion of oxygen saturaffon in this manner differs from prior art techniques, suchas that disclosed by Wilber, by performing measurements based upon both the baseline and pulsatile components of the signals.
The first component of oximeter 10 to be discussed is sensor 12.
The function of sensor 12 is substantia~y to provide the desired orientaffon of t light sources 40 and 42, for example, light-emitffng diodes (LEDs), and light detector 38 with respect to a suitable porffon of a patient's body. For example,the sensor must align LEDs 40 and 42 with detector 38 in a manner such that the path of light from each LED to the detector 38 is substantially the same ¦ distance. In addiffon, the path must traverse a portion of the patient's body through which a usable amount of light is paæed, for example, a ffnger, toe, earlobe, or the nasal septum. Because changes in the lightpatll can significantly affect the readings taken, as noted above, the sensor must maintain the positionof LEDs 40 and 42 and detector 38 with respect to the transmission path through the patient's skin at all times. Otherwise, signal fluctuations known as moffon-arfffact may be produced. In addition, the sensor should apply or~y insubstantial pressure to the patient's skin and underlying tiæue. Otherwise, normal arterial blood flow upon which the pulse oximeter relies for accurate operation, may be disrupted. Finally, the sensor should be quickly attachable to the paffent and should cause no discomfort.
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~ ' ~.3~503 LEDs 40 and 42 are supplled with current by transistor drivers 44 located in the l/O circuit 14, as shown in ~IGURE 11. Drivers 44 are controlled by microcomputer 16 to produce current pulses at a 960Hz repetition rate. The duration of each pulse is 70 microseconds snd a pulse is supplied to the red 5 wavelength LED 40 first and then to the infrared wavelength LED 42. The voltage drop across scsling resistors in the drivers 44 allows the msgnitude of the current p~ses to be determined and, thus, maintsined in a manner described in greater detail below. LEDs 40 snd 42 respond to the current pulses by producing corresponding light pulses transmitted through the finger to de-10 tector 38. Detector 38, in turn, produces a signal thst includes informationsbout the pulsatile response of the finger to the red and infrared wavelength light, intermixed at the 960Hz LED pulse repeffffon rate.
In a preferred embodiment of the invention, a red optical filter 45 interrupts the lightpsth between the LEDs 40 and 42 and the detector 38, ss 1 5 shown in FIGURE 8. Preferably, filter 45 is a~Kodak No. 29 wratten gel filter.
Its funcffon is to eliminate the innuence of fluorescent light flicker on the oxygen saturstion determination made. As will be appreciated, although the body of sensor 12 may be made of an opaque material that blocks a significant porffon of the ambient light, some ambient light may sffll reach detector 38.
20 Light from the sun and incandescent lamps is substanffally conffnuous. Fluo-rescent lighting, on the other hand, includes alternating energized and de-energized intervals that form a visually imperceptible nicker. The frequency of the fluorescent light flicker is such that it might influence the signal produced ', by detector 38 in response to light received from LED 40 at the red wavelength.
25 Thu~, the red optical filter 45 is placed over the detector 38 and filters out any fluorescent light present, eliminating the effect its flicker might have on the oxygen saturation determination made.
At the l/O circuit 14, the signal from detector 38 is received by a preamplifier 46. In a preferred embodiment, preamplifier 46 includes a differ-30 ential current-to-voltage ampllfier 48 and a single-ended output amplifier 50.
, To understand the advantages of using the differential amplifier 48, it may first be helpful to consider the operatlon of a conventional current-to-voltage amplifier as shown in FIGURE 12. As shown, a current-to-voltage amplifier 52 is substantially comprised of an operational amplifier 54 and g~in determination 35 resistor RF. With a detector 38 connected to the inputs of the amplifier as shown, a current ID is input to the amplifier upon the detection of suitable ~h wavelength light. The output of amplifier 52 is designated V0 and, as win be '~"!; appreciated, is equal to the product of the detector current ID and the gain o lc - m~

: `
..

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determination resistor RF. The primary problem with suoh a ¢onstruotlon 18 that It also amplifies the extern~l interterenoe noise produoed, m~king the slgnal extraoted less acourate.
Adoption of the differentisl current-to-voltage amplItier 48, when combined with the slngle-ended output amplifier 50 8S shown in FIGVRE 13, however, eliminates this problem. As shown, the differential amplifier 48 produces positive and negative versions o~ the output, the absolute value of each version being equal to the product of the gain determination resistance RF and the detector current ID. These outputs are then supplied to the single-ended 1 0 output amp 50, which provides unity gain, thus producing an output signal having a magnitude that is twice that of the inputs. An advantage of this arrangement is that external interference noise is cancelled at the single-ended output ampli-fier 50 by the opposing signs of the two transimpedance amplifier outputs. In addition, twice the signal is produced with the current noise or~y increasing by a magnitude of 1.414. Therefore, an improved signal-to-noise ratio results.
At this point, the mixed signal indicative of the red and infrared wavelength responses of detector 38 hss been amplified and is input to a demoduiator 56 to extract the red pulsatile and infrared pulsatile waveforms shown in FIGURES 9 and 10. In a preferred arrangement, the demoduiator 56 includes a sample-and-hold (S/H) circuit 60 that responds to the detector signalproduced in response to red wavelength light and a sample-and-hold (S/H) circuit 58 that responds to the infrared wavelength response of detector 38. Thetlming of circuits 58 and 60 is controlled so that each circuit samples the signal input to demodulator 56 during the portion of the signal corresponding to the wavelength to which it respond~. In this manner, two signals are reconstructed ' from the single input to demoduiator 56. As noted above, these signals correspond to the red pulsatile signal and infrared pulsatile signals shown in FIGURES 9 and 10.
To remove high-freguency noise from the outputs of circuits 58 and 60, they are input to lowpass filters 62 and 64. In a preferred embodiment, the "red" lowpass filter 62 and "infrared" 10WPQSS filter 64 each include two stages.
The flrst stage of each filter utilizes a fifth-order, monolithic integrated circuit switched capacitor filter because of its low cost, relatively small physical size ~ and accuracy. Since both the "red" and "infrared" signals pass through nearly ¦ ~ 35 identical first-stage filters due to monolithic IC matching, their gain and phase frequency responses are matched. The second stage of each filter is a second-order Bessel filter having a slightly higher roll-off freguency than the first stage.
This ensures that the first-stage filter is the dominant filter of the two-stage ' ~ 3~'4503 combination, producing the desired fUterlng accuracy. The second stage then filters the switching noise from the flrst-stQge output.
The filtered red and infrared pulsQtIle signals are next prepared tor conversion and transmission to the mlcrocomputer 1~. As will be discussed in 5 greater detail below, this process involves the use ot a programmable DC
subtractor or offset 66 followed by a programmable gain amplifier 68 having a gain range from approximately one to 256. The appropriately processed signals are combined at multiplexer 70, sampled and held at n, and converted to digital form by A/D converter 72 for transmission to microcomputer 16.
Before a more complete discussion of the operation of pro-grammable subtractor 66, programmable gain amplifier 68, multiplexer 70, S/H 71, and A/D converter 72 is provided, several details regarding the signals to be transferred to microcomputer 16 should be noted. For example, as shown in FIGURES 9 and 10, the signal produced by detector 30 in response to light at 15 each wavelength includes components that, for convenience, are termed baseline and pulsatile. The baseline component approximates the intensity of light received at detector 38 when onty the "fixed" nonpulsatile absorpUve component i8 present in the finger. This component of the signal is relatively constant over short intervals but does vary with nonpulsatile physiological changes or system ~,, 20 changes, such as movement of sensor 12 on the finger. Over a relatively long intervd this baseline component may vary significantly. As wilt be appreciated, the magnitude Or the baseline component at a given point in time is substantiatly equQI to the level identified in FIGURE 9 as RH. For convenience, however, the ~, baseline component may be thought of as the level indicQted by RL, with the 25 pulsatile component varying between the values for RH and RL over a given pulse. That pulsatile component is attributable to light transmission ohanges through the finger resulUng from blood volume ¢hanges in the finger during a ¦ cardiac putse. Typicatly, the pulsatile component mQy be retatively smatl in compQrison to the baseline component and is shown out of proportion in J 30 FIGURES 9 and 10.
The determination of RoS in accordance with equation (22) re-quires accuratety measured vatues for both the baseline and pulsatile signat ~mponents. Because the putsatile components are sma~ter, however, greater ¢are must be exercised with respect to the measurement of these components.
35 As wilt be readily appreciated, if the entire signat shown in FIGURES 9 and 10, ~; inctuding the baseline and pulsatite components, was amplirled and converted to a digitat format for use by microcomputer 16, a great deat of the accuracy of the conversion woutd be wasted because a substantiat portion of the resolution `: :
,`
.

-18-' ~ 3~4503 would be used to measure the baseline component. For example, with an A/D
converter employed having an input range of between +10 and -10 volts, a signal having a baseline component referenced to -10 volts that is four times that of the pulsatile component can be amplified until the baseline component is 5 represented by a 16-volt difference and the pulsatile signal represented by a 4-volt difference. With a 12-blt A/D converter 72, the totsl signal can be resolved into 4096 components. Therefore, the number of incremental levels representing the pulsatile signal would be approximately 819. If, on the other hand, the baseline component is removed prior to the conversion, the gained 10 pulsatile signal could be resolved into 4096 intervals, substantially improving accuracy.
The disclosed invention empoys this technigue, as the first half of a constructio~reconstruction process, in the manner schematically outlined in FIGURE 14. As shown, an input signal Vl (corresponding to the signals shown in 1 5 FIGURES 9 and 10) is received from each filter 62 and 64. V1 includes both the baseline and pulsaffle components discussed above. As will be described, subseguent operations upon Vl subtract off a substantial "offset voltage" portion of the baseline component, then gain up the remaining substanffally pulsatile signal for conversion by A/D converter 72. A digital reconstrucffon of the 20 origimll signal is then produced by reversing the process, wherein digitally provided information allows the gain to be removed and the offset voltage added back. This step is necessary because the entire signal, including both the baseline and pulsatile components, is used in the oxygen saturaffon measurement , ~ process.
Feedback from microcomputer 16 to I/O circuit 14 is also reguired } to maintain the values for the offset voltage, gain, and driver currents at levels appropriate to produce optimal A/D converter 72 resolution. Proper control reguires that the microcomputer continuaUy analyze, and respond to, the offset voltage, gain, driver currents and the output of A/D converter 72 in a manner to be described next.
Briefly, with reference to FIGURE 15, thresholds L1 and L2 slightly below and above the maximum positive and negative excursions L3 and L4 allowable for the A/D converter 72 input are established and monitored by microcomputer 16 at the A/D converter 72 output. When the magnitude of the signal input to, and output from, A/D converter 72 excee<~; either of the thresholds L1 or L2, the driver currents ID are read~usted to decrease the intensity of light impinging upon the detector 38. In this manner, the A/D
converter 72 is not overdriven and the margin between L1 and L3 and between ~ , ~
, ~ ~

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.3~ S03 L2 and L4 help~ assure thIs even for rapidly varylng signals. An operable voltage margln for A/D converter 7a exlsts outslde of the thresholds, allowing A/D
converter 7a to continue operating while the appropriate feedback ad~ustments to A and Vos are made.
When the signal from A/D oonverter 72 exceeds positlve and negative thresholds L5 or L6, microcomputer 16 responds by signaling the programmaMe subtractor 66 to increase the offset voltage being subtracted.
This is done through the formation and transmission of an oftset code whose magnitude is dependent upon the level of the slgnal reoeived from converter 72.
Three different gain adjustments are employed in the arrangement graphically depicted in FIGURE 15. For example, if microcomputer 16 determines that the A/D converter 72 signal has not exceeded positive and negative thresholds L7 and L8, the current value of a gain code is increased.
This revised gain code is then transmitted to the programmable amplifier 68, 5 which makes the appropriate adjustment to the gain A. If the A/D converter signal exceeds positive and negaffve thresholds L9 and L10, the gain code is adjusted downward as a function of the signal magnitude. Simil~rly, if separate lower positive and negative thresholds Lll and L12 are exceeded, the gain oode i8 also adjusted downward as a separate function of the sign~l magnitude.
j 20 The manner in which the various thresholds are established and the relationship of the gain and offset codes to the signal received can be altered to produce substanffally any form of control desired. Thus, the arrangement shown in PIGURE 15 is illustrative or~y and represents the currently preferred embodi-ment.
As will be appreciated from FIGURE 16, the instrucffons for the microcomputer program that controls the signal construction-reconstrucffon .l discussed above are stored in erasable, programmable, read-o~y memory(EPROM) 74 of microcomputer 16. Similarly, values for RH, IRH, RL, IRL, and signal period ~pulse durstion) are determined pursuant to peak-detection soft-30 ware contained in EPROM 74. These values are stored in random-access memory (RAM) 76 for operation upon by a central processing unit (CPU) 78 in accordance with further computational instrucffons stored in EPROM 74. Interfaces 80 act as input and output buffers for microcomputer 16.
The computational software in EPROM 74 a~ows CPU 78 to de-- 35 termine both oxygen saturation and pulse rate. Initially the program directs CPU 78 to determine the present value for Ros by substituting the measured values for RH, IRH, RL, and IRL into eguation (22):

---` 13~!4S03 i RoS = ln (RL/RH) (22) ln (IRL/IRH) Then, the computational software instructs CPU 78 to determine the oxygen saturation from RoS by use Or a caiibration curve such as the one depicted in FIGURE 7. The calibration curve is a plot of the reiationship between independently determined oxygen saturations corresponding to values Or RoS
produced by oximeter 10 in accordance with the technique described above.
With sufficiently large space in EPROM 74, enough points along the calibration curve can be stored in a look-up table to allow CPU 78 to extract an1 accurate indication of oxygen saturation from the value of RoS input to EPROM 74. The storage of a sufficient number of calibration curve data points may, however, necessitate the use of an undesirably large capacity EPROM 74.
For that reason, a second method of storing the calibration curve information ispreferred.
1 5 Pursuant to that method, once independently derived data associ-ating RoS with the oxygen saturation is obtained, a mathematical expression between the two can be derived from a plot of the curve. The basic formula and the coefficients of the formula's variables are then stored in EPROM 74. When a value for RoS is measured, CPU 78 extracts the coefficients from EPROM 74 20 and computes a value for the oxygen saturation. This technique allows information completely identifying the entire calibration curve, or a family of such curves, to be stored witNn a relatively small amount of EPROM 74 space.
Before discussing the manner in wNch informaffon extracted by oximeter 10, as outlind above, is made availaMe to an operator, a brief 25 description of power source 18 is provided. As shown in FIGURE 16, grounded ~¦ AC power is input to power source 18 from, for example, a waU ou'det. An isolation transformer 82 i8 employed between the AC supply and a battery charger 84. The battery charger 84 allows battery 86 to charge whenever the oximeter 10 i9 plugged into a waU ou'det. Battery 94 is constructed to operate 30 oximeter 10 for awroximately one hour in the absence of AC power input. A
converter 88 conditions the output of battery 86 to provide the various DC
suwly vaatages required by the I/O circuit 14 and microcomputer 16 to produce the various information to be displayed.
The output of information produced by oximeter 10 is now dis-35 cussed in greater detail with reference to FIGURE 17, which iUustrates the front i panel of an oximeter constructed in accordance with this invenffon. As shown, panel 90 supports user input keyboard 22, as weU as the various displays 20.
Keybo~rd 22 includes a POWER switch 92, which, for example, maybe a momentary, tactile membrane switch. When switch 92 is depressed, ~ .
; ', , -al- 13Q4503 oximeter 10 is nctivated. The oximeter 10 will operate off AC power when it is avsilable; otherwise, DC power from battery 86 is used. The battery ch~rger 84 will charge battery 86 ss long ss oximeter 10 is plugged in, snd is not dependent on the operstion Or POWER switch 92.
A SELECT switch 100 is used to sequence through the various operstional limits of oximeter 10, altowing them to be sd~usted, as described ingreater detail below. The partic~ar limit selected can be identified by, for example, the flashing of a predetermined portion or the enffrety of a correspon-ding displsy on panel 90. Momentary depression oi switch 100 csuses sn incremental progression through a predetermined order of the limits, while continued depression provides automsffc stepping through the various limits.
When switch 100 has not been depressed for a predetermined smount of time, microcomputer 16 indicates that none of the limits are to be adjusted and ~ returns oximeter 10 to normal operaffon. The SELECT switch 100 progresses J 15 through the various operating limits in a predetermined sequence, prefersbly beginning with an iniffal, normat opersffng mode. In this mode, none of the operating limits of oximeter 10 can be sdiusted.
RAISE snd LOWER switches 102 snd 104 (indicated on keyboard 22 by upwardty and downwardly directed triangles) are used in conjunction with LIMIT SELECT switch 100 in the following msnner. With switch 100 used to setect the partlcutar limit to be sdjusted, the levet of the timit can be raised by depressing switch 102 snd lowered by depressing switch 104. Once sdjusted, a parffcutsr limit is set by another depression of switch 100. Simitar to the operstion of LIMIT SELECT switch 100, momentary depression of the RAISE snd LOWER switches 102 snd 104 produces an incrementat sdjustment in the operaffng limit while continuous depression produces continuous chsnge. The ~1 operating limit may inctude an "O~E" position at the upper limit of adjustment reached by depressing the RAISE switch 102 as wetl as an "OF~" position at the lower limit of adjustment reached by depressing LOWER switch 104.
The visual display of the basic oxygen saturation and pulse rate produced by oximeter 10, are achieved by displays 20. As shown in FIGURE 17, these displays include a vetffcal column of light-emitting diodes (LEDs) snd a plurality of digital LED displays.
t~ Addressing now the digital display elements of displsy psnel 20, ss ;~ ' 35 shown in FIGURE 17, the oxygen saturation display 106 includes a three-digit display 120 that indicates the oxygen saturation currently determined by s ~ microcomputer 16. As wi~ be apprecisted, three-digit displsy 120 aUows oxygen sstursffon to be expressed in one-percent increments over the entire range of 1 .. ~, .. . .
~ ' ' -.

, ~
`` 1.3(~4503, oxygen saturations to be encountered. The pulse display 108 also ~ncludes a three-digit display 122 of the pulse rate currently measured by oximeter 10.
This display allows the pulse to be expressed in, for example, one-beat-per-minute increments over the entire range of pulse rates to be encountered.
f 5 Displays 120 and 122 are substanffally the same slze.
The displays included on panel 90 that are of particular Interest are the oxygen saturation trend displays 132 and 134 and the pulse perfwion bar display 136. In a preferred embodiment, trend display 132 is an upwardly directed, triangular LED display, which indicates that the oxygen saturation 1 0 monitored by oximeter 10 has increased at a rate in excess of some predeter-mined minimum. Similarly, trend display 134 is a downwardly directed, triangular LED display, which indicates that the oxygen saturation has decreasedat a rate in excess of some predetermined minimum. As will be appreciated, displays 132 and 134 provide the operator of oximeter 10 with gualitative 1 5 information about oxygen saturation activity in the patient that may be inconvenient to obtain from display 120. For example, unless a physical or mental record of the oxygen saturaffon indicated at display 120 is kept, a physician may be unable to guickly determine whether the oxygen saturstion has been progressively drifffng in a particular direcffon. In some instances, this change in oxygen saturation may presage the occurrence of potenffally harmful physiological changes, even before the upper and lower oxygen saturation alarm limits have been reached. Displays 132 and 134 provide the operator with this informaffon in a convenient, easily read manner.
~; The manner in which such qualitative displays are controlled by ¦ 25 microcomputer 16 is as follows CPU 78 stores the value of the oxygen saturation output by display 120 in RAM 76 at a first ffme and at a second time,some predetermined ffme later. The interval between these nrst and second measurement ffmes may be Or fixe~d duration, for example, two minutes, or a function of the pldse, for example, 150 beats. CPU 78 then compares the oxygen saturation stored at the two ffmes and, if the change in oxygen saturation is greater than, for example, three percent, and the sign of the current slope corresponds to the sign of the slope over the specified interval, the appropriate trend indication display is lit. Once lit, displays 132 and 134 remain on until the rate at which the oxygen saturaffon changes drops below a second predetermined level, for example, a 2.5 percent full-scale change over a two-minute interval or j the sign of the current slope of the oxygen saturation no longer corresponds to j ~ the direcffon of the lit trend indicator display. CPU 78 repeats this test at intervals that may be of fixed duration or Q function of pulse. Thus, the change ~ ....... ,. - -- ' .

`" 1.3~ 51)3 `
in oxygen saturation may be redetermlned, for example, every five seconds or every five pulses.
As wlll be appreciated, the oxygen saturatlon trend Ind~cation displays outlined above can be alternatlvely implemented and supplemented in a variety of fashions. For example, a9 shown in FIGURE 18, a display 20 or inclusion on panel 90 also includes a digital oxygen saturation trend display 138 and a trend-rate ad~ustment display 140. In this arrangement, CPU 78 continually updates oxygen saturation information In RAM 76 and then processes it to determine whether it has been increasing or decreasing at a rate in excessof the rates stored in RAM 76 in the following manner.
LIMIT SELECT switch 100 is used to access second and third operating limit ad3ustment modes. These modes allow adjustment of the rates that must be exceeded to light triangular trend displays 132 and 134, re-spectively. For example, with LIMIT SELECT switch 100 depressed until the 1 5 second mode is reached, triangular LED 132 and adjustment display 140 are both flashed continuously. Then, with the aid of RAISE and LOWER switches 102 and 104, the rate above which the oxygen saturation must increase to light display 132 can be increased or decreased. When the proper rate h~s been set, LIMIT
SELECT switch 100 is again depressed and the new rate stored in RAM 76.
In a similar manner, the rate above which oxygen saturation must decrease to light display 134 can be adiusted when the third mode has been selected by LIMIT SELECT switch 100. In this case, trend display 134 and trend adjustment display 140 flash and the threshold rate is adjusted by RAISE and LOWER switches 102 and 104. Although not depicted in I~IGURE 18, it will be appreciated that a second trend-rate adjustment display can be employed in cor~unction with display 140 to allow the rates of oxygen saturation change ~< necessary to extinguish LEDs 132 and 134 to be adjusted. These rates, however, may be a fixed percentage of the rates set above.
An alarm produced by a speaker can be employed to simultaneously sound with the lighting of displays 132 and 134. The volume Or the audible alarmis adjusted by repeatedly operating LIMIT SELECT switch 100 unffl a "set alarm volume" mode is reached. Then RAISE and LOWER switches 102 and 104 are ~, used to raise and lower the alarm volume. The alarm volume, as set, determines the volum e of the audible tone produced by the speaker when the oxygen saturation has changed sufficiently to light LEDs 132 and 134. An ALARM
VOLUME indicator 118 indicates that the alarm volume adjustment mode in the ~i alarm limit sequence 118 activated by LIMIT SELECT switch 100 has been , .

' `~ 13~4503 reached. Thus, light 118 notlfies the operator that the alarm volume can now be raised or lowered by use of RAISE and LOWER switches 102 and 104.
The other display shown in FIGURE 17 to be particularly noted is the pulse/perfusion bar display 136. In the arrangement shown, bar display 136 5 includes a plurslity of verticaUy aligned individual LEDs 142. For example, a display 136 comprised of 20 LEDs 142 has been found to provide acceptable resolution. As will be described in greater detail below, bar display 136 provides a visual indication of pulse signal quality, pulse level, and perfusion over a broad range of perfusions. Signal quality basicaUy is an indication of the influence of 10 noise on the signat received. The signal level is a measure of the signal level relative to that required by the oximeter to operate properly. Perfusion indicates the relative volume of the arterial blood flowing in the tissue. Thesevarious parameters are combined ;n a useful manner by bar display 136.
As will be appreciated from the basic discussion of oximetry theory 1 5 and oximeter 10 provided above, the pulse rate can be extracted from the signals input to microcomputer 16 by use of a peak detection program stored in EPROM
74. In addition, because the intensity of light impinging upon detector 38 is a funcffon of the blood volume in the tissue analyzed, the signal received by microcomputer 16 from I/O circuit 14 can be used to indicate the perfusion.
20 This informaffon is provided on display 136 as a "filled" bar with the origin at the bottom of the display. Thus, an increasing number of LEDs 142 wiU be lit as arterial blood volume in the ffssue approaches a maximum and they will be progressively extinguished as the blood volume returns to a minimum. Taken as a whole, display 136 will appear to rise and fall once for the occurrence of each 25 pulse.
ln operation, any signal greater than a predetermined minimum threshold is automaffcally scaled to the level necessary to light all of the individual LEDs 142 when maximum blood volume is present. As noted previously, the peak detection software in EPROM 74 allows the peaks of the 30 signal input to microcomputer 16 to be determined and it is these peaks that are autoscaled by CPU 78 in accordance with further EPROM 74 instructions to produce full-scale deflection on display 136. While a linear display could be ; employed in con~unction with the automaffc scaling described above, certain advantages may be produced by representing the varying indication of perfusion 35 derived from display 136 between the full-scale occurrences of each pulse as a logarithmic funcffon of the perfuslon. Thus, for example, a tenfold drop in baood volume between maximum and minimum blood valumes wo~d or~y produce a half-scale drop in display 136. As will be appreciated, use of such logarithmic ., ..~
, , ~ , .

13C! 4S03 -2~

scaling provides greater resolution for sm~ll changes near full-scale denection but renders display 136 substantially unresponsive to similar changes wetl belowfull-scale. In this manner, the attenuation in the signal produced by the pulsatile component, which, after all, is the part that indicates blood volume, i9 S emphasized in displ~y 136 and the portion of the attenuation attributable to the fixed components is substantially ignored.
When the minimum threshold required for automatic scaling of the signal to produce peak dellection of display 136 has not been reached, the incoming signal amplitude may be either l}nearly or logarithmicatly scaled on 1O display 136. Use of logarithmic scaling retains the advantages of improved resolution discussed above even when the perfusion of the patient being monitored is relatively low. If the peak amplitude produced by the signal on display 136 falls below a predetermined threshold (eg., l/aO full-scale deflec-tion), one LED is lit and is stepped up the pulse bar until an acceptable pulse is 15 locked onto again.
While the invention has been described with ref erence to a preferred embodiment, it is to be clearly understood by those skilled in the artthat the invention is not limited thereto, and that the scope of the invention is to be interpreted or~y in conjunction with the fonowing claims.

I

' .

Claims (4)

1. An apparatus, for processing signals containing information about the oxygen saturation of arterial blood flowing in tissue, comprising, processing means for determining the oxygen saturation of said arterial blood flowing in said tissue from said signals; and display means for producing a numeric representation of the change in said oxygen saturation during a specified interval.

2. The apparatus of claim 1, further comprising first and second trend indication displays, said first trend indication display producing an output when said oxygen saturation has increased by a first predetermined amount during said specified interval, said second trend indication display producing an output when said oxygen saturation has decreased by a second predetermined amount during said specified interval.

3. A method of processing signals containing information about the oxygen saturation of arterial blood flowing in tissue comprising the steps of, producing an indication of the oxygen saturation of said arterial blood flowing in said tissue from said signals; and producing a numeric representation of the change in said oxygen saturation during a specified interval.

4. The method of claim 3, wherein said step of displaying said output further comprises the steps of producing a first display when said oxygen saturation has increased by a first predetermined amount during said specified interval and producing a second display when said oxygen saturation is decreased by a second predetermined amount during said specified interval.