00906689/88/16’]~»’I73/$02.00/O
`MEDICAL INSTRUMENTATION
`Copyright © ‘IQBB by the Association for the Advancement of Medical Instrumentation
`
`Vol. 23?, No.
`
`It
`
`Design and Eyaiuation cf a New
`Pulse @xirneter Sensor
`
`
`
`eflectance
`
`Y. MENDELSON, PHD, J. C. KENT, MS, B. L. YOCUM, BS, AND WI. d. BIBLE, BS
`
`The design and construction of a new optical reflectance sensor suitable
`‘for noninvasive monitoring of arterial hemoglobin oxygen saturation
`with a pulse oximeter is described. The reflectance sensor was inter-
`faced to a Datascope ACCUSAT pulse oximeter that was 'specially
`adapted for this study to perform as a reflectance oximeter. We eval-
`uated the reflectance sensor in a group of 10 healthy adult volunteers.
`SpO2 obtained from the forehead with the reflectance pulse oximeter
`and SpO2 obtained from a finger sensor that was connected to a standard
`ACCUSAT transmittance pulse oximeter were compared simultane~
`ously to arterial blood samples analyzed by an IL 282 CO—Oximeter.
`The equation for the best fitted linear regression line between the
`reflectance SpO2 and HbO2 values obtained from the reference IL 282
`CO-Oximeter in the range between 62 and 100% was: SpO2 (%) =
`4.78 + 0.96 (IL); 11 = 110. The regression analysis revealed a high
`degree of correlation (r = 0.98) and a relatively small standard error
`of the estimate (SEE = 1.82%). The mean and standard deviations for
`the difference between the reflectance SpO2 and IL 282 measurements
`was 1.88 and 1.85%, respectively. This study demonstrates the ability
`to acquire accurate SpO2 from the forehead using a reflectance sensor
`and a pulse oximeter.
`
`The recent development of transmittance pulse ox-
`imeters by combining optical plethysrnography with the
`Spectrophotometric determination of hemoglobin oxygen
`saturation in arterial blood (SpOZ) has provided a widely
`used technique for monitoring hypoxemia.
`With transmittance pulse oximeters, sensor applica-
`tion is limited to several peripheral locations Where light
`can be readily transmitted and detected, such as the
`finger tips, ear lobes, and toes on adults, and the foot or
`palms on infants. Alternatively, skin reflectance oximetry
`could enable SpO2 measurement from more centrally
`located parts of the body such as the forearms, chest,
`and forehead, which cannot be monitored using conven-
`tional transillumination techniques.
`It appears that reflectance pulse oximetry may be par-
`ticularly suitable for direct assessment of fetal distress
`resulting from hypoxia during delivery, if used in addi—
`‘ tion to monitoring fetal heart rate by a scalp ECG elec-
`trode. Another suggested application of noninvasive re-
`flectance pulse oximetry is for monitoring SpO2 in the
`external carotid artery through a sensor applied to the
`skin near the superficial temporal artery.1
`In this article we describe the design and construction
`
`From the Worcester Polytechnic Institute, Biomedical Engineering
`Program, Worcester, MA 01609.
`Address correspondence and requests for reprints to Yitzhak Men-
`delson, PhD, Biomedical Engineering Program, Worcester Pontech-
`nic Institute, 100 Institute Ftd., Worcester, MA 01609.
`
`of an optical reflectance sensor suitable for noninvasive
`monitoring of SpO2 with a pulse oximeter. The experi-
`mental evaluation of the new sensor and verification that
`
`SpO2 obtained with the reflectance sensor compare fa»
`vorably with:
`(a) SpO2 measured simultaneously by a
`finger sensor connected to a standard transmittance pulse
`oximeter, and (b) HbO2 measured by the IL 282 CO—
`Oximeter from samples of arterial blood in a group of 10
`healthy adult volunteers is presented.
`
`PULSE QXIMETRY
`
`The principle ofpulse oximetry was proposed by Aoy~
`agi et al2 and further developed by Yoshiya at (11.3 This
`unique approach is based on the change in light absorp—
`tion by tissue. The change is caused primarily by arterial
`blood pulsation. The pulsating arterioles in a vascular
`bed, by expanding and relaxing, modulate the amount
`of light absorbed by the tissue and thus produce a char~
`acteristic photoplethysmographic waveform. The changes
`in light absorption are used to measure SpO2 noninva-
`sively.
`Initial attempts to develop a noninvasive oximeter that
`can measure oxygen saturation by analyzing the absolute
`light intensity that is diffusely reflected from the skin
`were only partially successful, mainly because of limited
`accuracy associated with variations in tissue attenuation
`and differences in skin pigmentation. Recently, we
`showed that accurate SpO2 measurements can be made
`utilizing a reflectance sensor and the concept of pulse
`oximetry." 5 We found that SpO2 can be calculated from
`the ratio of the reflected red and infrared photople—
`thysmograms based on a normalization in which the pul-
`satile (ac) component of the red and infrared photople—
`thysmograms is divided by the respective nonpulsatile
`(dc) component. The conversion of the red/infrared ratios
`to SpOg is performed by an empirical calibration of the
`oximeter, This process is performed by comparing the
`red/infrared ratios measured by the pulse oximeter with
`blood HbO2 values obtained from an in vitro oximeter.
`The excursions of photoplethysmographic signals de—
`tected by reflectance and transmittance sensors when
`placed on the forehead and finger, respectively, are in-
`versely related to changes in arterial blood pulsations.
`Although the amplitude of the pulsatile component of
`the two waveforms is different,- the shapes of the pho-
`
`167
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`APPLE 1015
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`168
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`MEMO/XL INS’I HUMEN l‘A'l‘lON
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`Volume 22, No. 4, August Hit-it;
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`toplethysmograms are virtually identical, as illustrated
`in Figure 1.
`
`$ENSQR EglGN
`
`The basic optical sensor of a pulse oximeter consists
`of a red and an infrared light emitting diode (LED) and
`a suitable photodetector. In a transmittance pulse ox~
`imeter sensor,
`the LEDs and the photodetector are
`mounted in opposition, whereas in a reflectance sensor,
`the LEDS and the photodetector are mounted side by
`side. The wavelength of the red LED is typically chosen
`from regions of the spectra where the absorption coef-
`ficient of Hb and HbO2 are markedly different (6.5;, 660
`nm). The infrared wavelength, 011 the other hand,
`is
`typically chosen from the spectral region where the dif—
`ference in absorption coefficients of Hb and HbOZ is
`relatively small (e.g., 930 nm). The spectral response of
`the photodetector must overlap the emission spectra of
`the red and infrared LEDs.
`
`Practically, the major limitation in reflection pulse ox—
`imetry is the comparatively low-level photoplethysmo-
`grams typically recorded from low—density, vascular areas
`of the skin. The feasibility of reflection pulse oximetry,
`therefore, is essentially dependent on the ability to de—
`sign a sensor that can detect sufficiently strong reflection
`photoplethysmographic signals from various locations on
`the body.
`The light from the LEDs in the reflectance sensor is
`diffused by the skin in all directions. This suggests that
`to detect most of the backscattered radiation from the
`
`skin, the photodetector must be able to detect light from
`an area concentric with the LE Ds. The intensity of the
`backscattered light decreases in direct proportion to the
`square of the distance between the photodetector and
`the LEDs; thus the photodetector should be mounted
`close to the LEDs. We found experimentally that a sep—
`aration of 4—5 mm between the LEDs and photodetector
`provides the best sensitivity in terms of detecting ade-
`
`quately large pulsatile components. We also found that
`when multiple photodetectors are arranged at equal dis,-
`tances around the LE Us,
`the total amount of backseat
`tered light that can be detected by the reflectance sensor
`is directly proportional to the number of photodetectors.
`The optical reflectance sensor used in this study con“
`sists of two red (peak emission wavelength: 660 nm) and
`two infrared (peak emission wavelength: 930 nm) LED
`chips (dimensions: 0.3 X 0.3 mm), and six silicon pho~
`todiodes (active area: 2.74 X 2.74 mm) arranged sym»
`metrically in a hexagonal configuration as shown in Fig
`ure 2. To maximize the fraction of backseattered light
`collected by the sensor, the currents from all six pho»
`todiodes were summed. The LEDs and photodiode chips
`were mounted with conductive epoxy (Epo-tek H31,
`Epoxy Technology, Inc. Billerica, Massachusetts) on a
`ceramic substrate (dimensions: 13.2 X 13.2 X 0.25 mm)
`that was housed in a standard 24—pin (dimensions: 19 X
`19 mm) microelectronic package (AIRPAX, Cambridge,
`Maryland), which is commonly used for packaging elec-
`tronic circuits. The optical components were intercon=
`nected and wired to the package pins with 1—mil (0.025le
`mm diameter) aluminum wires, by a conventional ultra:
`sonic bonding technique. To minimize the amount of
`light transmission and reflection between the LEDs and
`the photodiodes within the sensor, a ring—shaped, opti=
`cally opaque shield of black Delrin (Dupont, Wilming=
`ton, Delaware) was placed between the LEDs and the
`photodiode chips. The optical components were encapw
`sulated inside the package using optically clear adhesive
`(NOA—63, Norland Products, Inc., New Brunswick, New
`Jersey). The microelectronic package was mounted in
`side a black Delrin housing (dimensions: 3.2—cm diamn
`eter >< 1.5—cm high). The sensor can be attached to the
`skin by means of double-sided adhesive tape. The weight
`of the entire sensor assembly is approximately 11 g.
`
`SIGNAL PROCESSING
`
`The optical reflectance sensor was interfaced to a com~
`
`
`
`
`INFRARED REFLECTANCE PLETHYSMOGRAMS
`
`2 sec
`
`emu-mi
`
`INFRARED TRANSMITTANCE PLETHYSMOGRAMS
`
`
`
`Figure 1. Relative infrared reflectance and transmittance photoplethysmograms recorded from the forehead and finger, respectively.
`
`2
`
`2
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`

`

`A NEW FlEFLE’CT/WCF PLJI,,SE OXIMEI'F
`
`H Simeon (I‘Viendelaon at al.)
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`RED & INFRARED LEDS
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`PHOTODIODE
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`SUBSTRATE
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`CONDUCTIVE 7,Lm
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`Figure 2. Diagram (A and B) and photograph (C) of the reflectance pulse
`
`oximeter sensor.
`
`(B)
`
`mercially available ACCUSAT (Datascope, Paramus, New
`Jersey) pulse oximeter.6 The oximeter circuitry generates
`separate digital pulses to energize alternately the red
`and infrared LEDs in the sensor. The time—multiplexed
`current pulses from the photodiodes, which correspond
`to the red and infrared light intensities reflected from
`the skin, are first converted by the oximeter to propor—
`tional voltage pulses. The pulses are subsequently de-
`multiplexed into two separate channels. The red and
`infrared photoplethysmographic signals are then ampli—
`fied and high-pass filtered to separate the ac and dc
`components of each photoplethysmographic waveform.
`Before the study began, an ACCUSAT pulse oximeter
`was modified by adjusting the intensities of the red and
`infrared LEDs so that the dc component of each pho-
`toplethysmogram was approximately equal to the cor—
`responding dc level obtained from transmittance pho-
`toplethysmograms as measured by a standard ACCUSAT
`sensor designed for a finger. The adjustment was per—
`formed while the reflectance and transmittance sensors
`
`were applied to the forehead and right index finger of a
`white subject breathing ambient air. N 0 further adjust—
`ments were made throughout the study. The reflectance
`oximeter was adapted to provide a continuous readout
`of the ac and dc components of the red and infrared
`photople thysmograms .
`
`In addition to the modified ACCUSAT pulse oximeter,
`a second standard ACCUSAT transmittance pulse 0x"
`imeter was used to measure Sp02 with a finger sensor.
`SpO2 from each of the two pulse oximeter-s was acquired
`every 2 s (0.5 Hz) using an AT&T 6300 personal com-
`puter. The conversion of the transmitted and reflected
`red/infrared ratios measured by each ACCUSAT pulse
`oximeter to SpO2 was performed using the same internal
`calibration algorithms. The exact algorithm for calculat-
`ing SpO2 was unavailable.
`
`IN VIVO EVALUATION
`
`The purpose of this study was to evaluate the per-
`formance of the reflectance pulse oximeter sensor during
`progressive steady—state hypoxia in humans and to com-
`pare the values obtained with the reflectance sensor to
`those from an ACCUSAT transmittance pulse oximeter
`and from the IL CO-Oximeter detecting the Hb02 of
`simultaneously drawn arterial blood samples.
`Tests were performed on 10 healthy, nonsmoking, adult
`volunteers of different ages and skin pigmentations in
`compliance with the University of Massachusetts Med—
`ical Center review guidelines for humans experiments.
`The subject distribution was: one deeply pigmented black,
`two subjects of lightly pigmented Oriental descent, and
`
`3
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`

`

`170
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`Memo/a, lNSTHUMkN i’ATiON
`
`Volume 22, No. ti, August i988
`
`two darkly tanned and five lightly tanned Whites. Subject
`age ranged from 22—39 years (mean fl: SD: 29.2 "J; 5.6
`years). Measured hematocrit was in the range of 35414%
`(mean i SD: 40 :i: 2.65%). Each volunteer was informed
`of the complete procedure and possible risks associated
`with arterial cannulation and hypoxic gas breathing. Each
`volunteer received monetary compensation for partici-
`pation in the study.
`A modified Allen’s test for assessing the radial and
`ulnar arterial blood circulation to the hand was per“
`formed on each subject prior to arterial cannulation. A
`, Teflon cannula (ZZ-gauge, 3.2-cm long) was inserted into
`the radial or ulnar artery of each subject after the sub-
`cutaneous tissue around the puncture site was anesthe—
`tized locally with a l—ml injection of 1% lidocaine hy-
`drochloride (Xylocaine).
`All instruments warmed up for at least 30 min before
`the study. The transmittance sensor of the ACCUSAT
`pulse oximeter was attached to the index finger on the
`hand opposite that of the arm with the arterial cannula.
`The sensor of the reflectance pulse oximeter was attached
`to the middle of the forehead. Samples of arterial blood
`(approximately 1 nil/sample) were drawn into 3—ml hep-
`arinized syringes and analyzed immediately by the
`
`2
`
`Instrumentation Laboratories iL Z82 CO~Oximeter
`
`(Instrumentatimi Laboratories, Lexington, Massachw
`setts). Simultaneous measurements of total hemoglobin
`(Hb), oxyhemoglobin (HbOg), carboxyhemoglobin
`(HbCO), and methemoglobin (Hi) were obtained from
`each blood sample. The arterial cannula was flushed with
`0.9% normal heparinized saline solution (1000 units/250
`ml) between blood samplings. Care was taken to ensure
`that the arterial line and the bloodwsampling syringes
`were free of air bubbles.
`
`A standard lead I ECG and the end-tidal CO2 were
`continuously monitored by a Hewlett—Packard 78345A
`patient monitor (Hewlett-Packard, Andover, Massachu~
`setts). Each subject was placed in the supine position.
`A face mask was tightly fitted over the subject’s mouth
`and nose, and the subject was asked to breathe spon—
`taneously different 02 and N2 gas mixtures. The inspired
`02/N2 gas was supplied by a modified Heidbrink anes=
`thesia machine (Ohio Medical Products, Madison, Wis-
`consin). The breathing circuit was equipped with a C02
`scrubber (soda lime). Inspired 02 concentration was ad»
`justed between 10 and 100% and was monitored continu
`uously with an IL 408 oxygen monitor that was inserted
`in the inspiratory part of the breathing circuit.
`
`
`
`R/IRREFLECTANCERATIO
`
`
`
`
`
`
`
`50
`
`80
`
`70
`
`80
`
`90
`
`10!
`
`Figure 3. Comparison of the lL 282 CO-Oximeier (x-axis) and the red/infrared ratios measured by the reflectance pulse oximeter (y-axis)
`during progressive steady~state hypoxia in 10 subjects. y = 3.51 — 0.030x; r = l— 0.98; SEE = 0.060; n = 110; p < 0.001. The solid line
`represents the best fitted linear regression line.
`
`IL 282 CU-DXIMETER (K)
`
`4
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`

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`A NEW HEPLECTANCE PULSE ()XIMETEH SENSOR (Mendelson at 61/.)
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`IL 282 CD-OXIMETER (Z)
`Figure 4. Comparison of SpO2 measurements obtained from the reflectance pulse oximeter (y-axis) and the IL 282 CO-Oximeter (x-axis)
`during progressive steady-state hypoxia in 10 subjects. y = 4.78 + 0.96x; r = 0.98; SEE = 1.82; n = 110; p < 0.001 . The solid line represents
`the best titted linear regression line. The dashed line represents identity.
`
`Progressive hypoxemia was gradually induced by
`changing the inspired fractions of O2 and N2. To provide
`a relatively uniform distribution of SpO2 data points,
`samples were recorded during both desaturation and
`reoxygenation. Initially, the inspired 02 concentration
`was changed in step decrements, each producing ap-
`proximately a 5% decrease in SpO2 as determined from
`the ACCUSAT transmittance pulse oximeter display. The
`inspired 02 was maintained at each level until the pulse
`oximeter readings were stable. When the inspired O2
`reached 10%, corresponding to a saturation of approxi-
`mately 65%, the process was reversed, and the inspired
`02 was increased in a similar stepwise manner to 100%.
`SpO2 from the ACCUSAT and the reflectance pulse ox—
`imeters during blood sampling was acquired every 2 s
`(0.5Hz), using an AT&T 6300 personal computer.
`None of the subjects showed ECG abnormalities be
`fore or during the study. All subjects tolerated the pro—
`cedure well, without adverse reactions.
`
`DATA ANALYSIS
`
`For each step change in inspired 02, readings from
`
`the ACCUSAT transmittance and reflectance pulse ox-
`imeters were averaged for 10 s before and after blood
`sampling and compared with the corresponding HbO2
`values measured by the IL 282 CO-Oximeter. To avoid
`operator biases, the data from each pulse oximeter were
`acquired automatically by the computer and later sub
`jected to the same statistical tests. Averaged readings for
`the 10 subjects were pooled and a least—squares linear
`regression analysis was performed. Student’s t test de"
`termined the significance of each correlation; p < 0.001
`was considered significant.
`The SpO2 displayed by two—wavelength pulse oxime—
`ters account only for the presence of HbOZ and Hb in
`the blood. The presence of HbCO, Hi, or any other
`interfering substance in the blood is not accounted for.
`Therefore, the term often used to represent SpOZ meas-
`ured by pulse oximeters is functional saturation,
`i.e.,
`HbOz/(Hb + HbOZ). The IL 282 CO-Oximeter, on the
`other hand, displays the percentage of oxygenated hemo-
`globin expressed as a fraction of the total hemoglobin
`present in the blood, 1.6., HbOQ/(Hb + HbOz + HbCO
`+ Hi). To compare SpO2 measured by the pulse ox—
`imeters with corresponding readings from the IL 282
`
`5
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`

`

`“i723
`
`MEDICAL lNSTHUI‘I/lizhl m HON
`
`Volume 22., No. 4, August ”9823
`
`CO=Oximeter, both HbCO and Hi values from each blood
`sample were used to convert
`the IL 282 readings to
`functional SpOQ, according to the following relationship7:
`
`p
`2
`%S 02 (functional) 3 HbO X 100/
`ad)— HbCO —‘Ha
`
`RESULTS
`
`A total of 110 pairs of data points were used in the
`regression analysis, which gave the estimated slopes and
`intercepts 0f the regression lines. An average of 11 blood
`samples was collected from each subject. Each pair of
`data points represents a different hypoxic level. Regresw
`sion analysis of the HbO2 values obtained from the IL
`282 CO-Oximeter (x—axis) vs the normalized red/infrared
`ratios (y—axis) as measured by the reflectance pulse ox-
`imeter is shown in Figure 3. The equation for the best
`fitted linear regression line was: y = 3.51 — 0.030x;
`r = —0.98; SEE 2 0.060; p < 0.001. A comparison of
`SpO2 readings from the reflectance pulse oximeter
`(y-axis) and the IL 282 CO-Oximeter (x-axis) is shown
`in Figure 4. The equation for the best fitted linear regres—
`sion line was: y = 4.78 + 0.962;; r = 0.98; SEE = 1.82;
`
`p < 0.001. Figure 5 shows the comparison ofSpIO2 values
`measured by the ACCUSA’I‘ reflectance pulse oximeter
`(y=axis) and the ACCUSAT transmittance pulse oximeter
`(x-axis). The linear regression equation for this compar~
`0 98; SEE: 2. 23; p <
`*ison is: y r: 5.85 + 0.95x;1 4:
`0.001. The standard deviations of the mean differences
`
`between the reflectance oximeter SpO2 and IL 282 HbO2
`values for four different saturation ranges are summa-
`rized in Figure 6.
`
`DISCU$$IQN
`
`Pulse oximetry has become a widely utilized medical
`technology, particularly in anesthesia and intensive care.
`Pulse oximeters offer significant monitoring advantages
`because of their reliability, simple operation, and the
`benefit of providing continuous SpO2 monitoring.
`Noninvasive monitoring of oxygen saturation based
`upon skin-reflectance spectrophotometry was first de—
`scribed by Brinkman and Zijlstra.8 They showed that
`changes in Oxygen saturation can be recorded noninva-
`sively from an optical sensor attached to the forehead.
`The use of light reflection instead of tissue transillumi-
`
`as
`
`so 1
`
`
`
`REFLECIANCESpflz
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`70
`
`80
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`
`TRANSMITTANCE Spflg
`Figure 5. Comparison of SpO2 measured by the reflectance pulse oximeter (y-axis) and the finger transmittance pulse oximeter (x- axis)
`during progressive steady--siaie hypoxiaIn 10 subjects. y = 5 85 + O. 95x; r = 0. 98; SEE = 2. 23; n = 110; p < 0 001. The solid line represents
`the best fitted linear regression line. The dashed line represents identity
`
`6
`
`

`

`A Niiw FtCFLECTANCE Put cl? OXIMETH‘t ESL-uses (Mentielson at al.)
`
`1753
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`STANDARDDEVIAFZDNBFMEANDIFFERENCES
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`w
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`r BELOW 70
`70-80
`80-90
`
`ABOVE 907
`
`IL 282 CO-DXIMETER (K)
`
`Figure 6. Standard deviations of the mean differences between the
`reflectance pulse oximeter and the lL 282 CO-Oximeter. N is the
`number of paired data points included in the statistical analysis.
`
`nation was suggested to enable noninvasive monitoring
`from virtually any skin surface. More recently, Cohen
`and Wadsworth9 and Takatani10 attempted to develop a
`skin reflectance oximeter utilizing a similar spectropho—
`tometric approach. In those three reflectance oximeters,
`oxygen saturation was calculated from the absolute light
`intensity diffusely reflected (backseattered) from the skin.
`Although these developments represent significant ad—
`vancements in noninvasive oximetry, the major problems
`were limited accuracy, poor reproducibility, and diffi-
`culties in absolute calibration.
`
`Available transmittance pulse oximeters can be used
`only on a few specific peripheral locations. The approach
`presented in this article demonstrates that SpO2 can be
`measured from an alternate site, specifically the fore-
`head. This technique provides a clinically acceptable al-
`ternative to presently available transmittance pulse
`oximeters. Although we found that reflectance photo—
`plethysmograms can be detected from several locations
`on the body (e.g., forearm, chest, and back), the rela—
`tively small, photoplethysmographic signals lead to prac-
`tical problems when processed by the pulse oximeter.
`Therefore, the choice of the forehead as a site for our
`study was based on the fact that at this location, relatively
`large reflectance photoplethysmographic signals can be
`detected.
`
`The relationship between the red/infrared ratios meas-
`ured by the reflectance pulse oximeter and HbO2 meas-
`ured by the IL 282 CO-Oximeter produced a regression
`relationship similar in slope and intercept to that ob-
`served from transmittance pulse oximeters.ll This sug—
`gests that SpOz monitoring from the forehead can be
`
`performed successfully using a reflectance sensor con—
`nected to a standard transmittance pulse oximeter wiihw
`out significant modifications of hardware and software.
`
`SUMMARY
`
`We compared simultaneous $1302 from a reflectance
`pulse oximeter sensor attached to the forehead and from
`a transmittance pulse oximeter with a sensor attached to
`a finger with HbO2 from arterial blood samples in a group
`of 10 healthy adult volunteers. A high degree of cone
`lation was found for SpO2 between 6.2 and 100%. Relative
`to arterial blood samples, the SEE for the reflectance
`pulse oximeter was 1.82%. We conclude that in situations
`in which a transmittance pulse oximeter cannot be used
`reliably, the forehead may be cOnsidered as a suitable
`alternative site for monitoring SpO2 with a reflectance
`pulse oximeter sensor.
`
`We gratefully acknowledge the clinical assistance of Albert
`Shahnarian, PhD, Gary W. Welch, MD, PhD, and Robert M.
`Giasi, MD, Department ofAnesthesiology, University of Mas—
`sachusetts Medical Center, Worcester, Massachusetts. We are
`indebted to Paul A. Nigrbni, Datascope Corporation, Paramus,
`New Jersey, and Kevin Hines, Semiconductor Division, Analog
`Devices, Wilmington, Massachusetts, for their technical as-
`sistance. Financial support for this study was provided by the
`Datascope Corporation.
`
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