a2) United States Patent
`US 6,801,799 B2
`qo) Patent No.:
`Mendelson
`Oct. 5, 2004
`(45) Date of Patent:
`
`US006801799B2
`
`(54) PULSE OXIMETER AND METHOD OF
`OPERATION
`
`(75)
`
`Inventor: Yitzhak Mendelson, Worcester, MA
`(US)
`
`(73) Assignee: Cybro Medical, Ltd., Haifa (IL)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`
`(21) Appl. No.: 10/360,666
`
`(22) Filed:
`
`Feb. 6, 2003
`
`(65)
`
`Prior Publication Data
`US 2003/0144584 Al Jul. 31, 2003
`
`Related U.S. Application Data
`
`(62) Division of application No. 09/939,391, filed on Aug. 24,
`2001, now abandoned.
`
`Foreign Application Priority Data
`(30)
`Oct. 5, 2000
`(IL)
`seseessssssssessseesssseesvesssesssseeesesssess 138884
`
`(SL) Tite C0? eee ceceecceeeneceeeeceeneeneanenns A61B 3/00
`(52) U.S. Ch oc 600/330; 600/322; 600/336
`(58) Field of Search 0.0... 600/310, 322,
`600/323, 330, 336
`
`(56)
`
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`
`{1994
`W09403 102
`5/2001
`WO0154573
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`OTHER PUBLICATIONS
`
`“Reflecance Pulse Oximetry at the Forehead of Newborns:
`The Influenece of Varying Pressure on the Probe”; A. Carin
`M. Dassel, MD, et el.; Dept of Obstetrics and Gynecology,
`Univ. Hospital Groningen, Groningen; Journal of Clinical
`Monitoring 12: pp. 421-428, 1990.
`(List continued on next page.)
`Primary Examiner—Eric F. Winakur
`(74) Attorney, Agent, or Firm—Howard & Howard
`ABSTRACT
`(57)
`
`A sensor for use in an optical measurement device and a
`method for non-invasive measurementof a blood parameter.
`The sensor includes sensor housing, a source of radiation
`coupled to the housing, and a detector assembly coupled to
`the housing. The source of radiation is adapted to emit
`radiation at predetermined frequencies. The detector assem-
`bly is adapted to detect reflected radiation at
`least one
`predetermined frequencyand to generate respective signals.
`Thesignals are used to determine the parameter of the blood.
`
`5 Claims, 6 Drawing Shects
`
`17
`
` Oo
`
`18:
`
`0001
`
`Apple Inc.
`APL1053
`U.S. Patent No. 8,923,941
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`Apple Inc.
`APL1053
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`US 6,801,799 B2
`Page 2
`
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`OTHER PUBLICATIONS
`
`“Reflectance Pulse Oximetry—Principles and Obstetric
`Application in the Zurich Systen’’; Vokcr Konig, Renate
`Huch, and Albert Huch; Perinatal Physiology Research
`Dept., Dept. of Obstetrics, Computing 14: pp. 403-412,
`1998,
`“Effect of location of the sensor on reflectance pulse oxim-
`etry”; A.C. M. Dassel, Research Fellowetal. British Journal
`of Obstetrics and Gynecology; Aug. 1997, vol. 104, pp.
`910-916.
`
`“Design and Evaluation of a New Reflectance Pulse Oxime-
`ter Sensor”; Y. Mendelson, PhD,etal.; Worcester Polytech-
`nic Institute, Biomedical Engineering Program, Worcester,
`MA 01609; Association for the Advancement of Medical
`Instrumentation, vol. 22, No. 4, 1988; pp. 167-173.
`
`“Skin Reflectance Pulse Oximetry: In Vivo Measurements
`from the Forearm and Calf’; Y. Mednelson, PhD and M.J.
`McGinn, MSc; Dept. of Biomedical Engineering, Worcester
`Polytechnic Institute, Worcester, MA 01609; Journal of
`Clinical Monitoring, vol. 7, No. 1, 1991; pp. 7-12.
`
`“Experimental and Clinical Evaluation of a Noninvasive
`Reflectance Pulse Oximeter Sensor”; Setsuo Takatani, PhD,
`et al; Dept. of Surgery, Baylor College of Medicine, One
`Baylor Plaza, Houston, TX 77030; Journal of Clinical
`Monitoring, vol. 8, No. 4, Oct. 1992; pp. 257-266.
`
`“Wavelength Selection for Low-Saturation Pulse Oxim-
`etry”; Paul D. Mannheimer, et al; JEEE ‘lransactions on
`Biomedical Engineering, vol. 44, No. 3, Mar. 1997; pp.
`148-158.
`
`“Noninvasive Pulse Oximetry Utilizing Skin Reflectance
`Photoplethysmography”; Yitzhak Mendelson and Burt D.
`Ochs; IEEE Transactions on Biomedical Engineering, vol.
`35, No. 10, Oct. 1988; pp. 798-805.
`
`“Physio—-optical considerations in the design offetal pulse
`oximetry sensors’; P.D. Mannheimer, M.E. Fein and J.R.
`Casciani; European Journal of Obstetrics & Gynecology and
`Reproductive Biology 72 Suppl. 1 (1997) S9-S19.
`“Fetal pulse oximetry: influence of tissue blood content and
`hemoglobin concentration in a new in-vitro mode?”; ‘Vho-
`mas Edrich, Gerhard Rall, Reinhold Knitza; European Jour-
`nal if Obstetrics & Gynecology and Reproductive Biology
`72 Suppl. 1 (1997) $29-S34.
`
`* cited by examiner
`
`0002
`
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`0002
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`

`

`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 1 of 6
`
`US 6,801,799 B2
`
`HELOGLOBIN SPECTRA IN
`OXIMETRY
`
`It{iIIItI ttt
`
`EXTINCTION
`
`COEFICIENT
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`
`
`OXYGENSATURATION
`
`10
`500
`
`600
`
`700
`
`800
`
`900
`
`1000
`
`WAVELENGTH (nm)
`
`Figure 1
`
`CALIBRATION OF A PULSE OXIMETER
`
`0
`
`1.0
`
`2.6
`
`3.0
`
`4.0
`
`5.0
`
`R/IR RATIO
`Figure 2
`
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`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 2 of 6
`
`US 6,801,799 B2
`
`
`
`EMITTING
`DIODES
`
` LIGHT
`
`VEIN
`
`ARTERY
`
`Figure 3
`
`
`
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`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 3 of 6
`
`US 6,801,799 B2
`
`REFLECTION SENSOR
`
`1F gu
`
`re5A
`
`REFLECTION SENSOR
`
`SuFFi
`
`re 5B
`
`guFi
`
`re6
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`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 4 of 6
`
`US 6,801,799 B2
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`MICROPROCESSOR
`
`
`
`DISPLAY
`
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`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 5 of 6
`
`US 6,801,799 B2
`
`REJECT DATA POINT
`
`
`
`
`
`
`GOTO STEP 3 Figure 10A
`
`CALCULATE W/W;
`(NEAR AND FAR)
`
`NO
`
`ACCEPT POINT:
`
`REJECT POINT:
`
`
`TURN ON ALARM
`(ADJUST SENSOR POSITION)
`
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`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 6 of 6
`
`US 6,801,799 B2
`
`
`
` REJECT POINT:
`Is
`
`
`THE QUALITY OF EACH
`
`
`TURN ON ALARM
`PHOTOPLETHYSMOGRAM
`
`(MOVEMENT/BREATHING ARTIFACTS)
`
`ACCEPTABLE
`
`?
`
`
`
`ACCEPT POINT:ON
`Figure 10B
`
`
`
`
`CALCULATE W, /W>
`AND W, /W,
`(NEAR AND FAR)
`
` REJECT POINT:
`
`?
`
`TAKE ANOTHER
`MEASUREMENT
`
`
`
`
`
`
`
` ACCEPTPOINT:
`
`COMPUTE AND UPDATE
`DISPLAY WITH NEW SpO» VALUE
`
`
`
`
`
`Figure10C_
`
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`US 6,801,799 B2
`
`1
`PULSE OXIMETER AND METHOD OF
`OPERATION
`
`This application is a divisional application of U.S. patent
`application Ser. No. 09/939,391 filed Aug. 24, 2001, now
`abandoned.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`10
`
`This invention is generally in the field of pulse oximetry,
`and relates to a sensor for use in a pulse oximeter, and a
`method for the pulse oximeter operation.
`2. Background of the Invention
`Oximetry is based on specirophotometric measurements
`of changes in the color of blood, enabling the non-invasive
`determination of oxygen saturation in the paticnt’s blood.
`Generally, oximetry is based on the fact that the optical
`property of blood in the visible (between 500 and 700 nm)
`and near-infrared (between 700 and 1000 nom) spectra
`depends strongly on the amount of oxygen in blood.
`Referring to FIG. 1,
`there is illustrated a hemoglobin
`spectra measured by oximetry based techniques. Graphs G1
`and G2 correspond,respectively, to reduced hemoglobin, or
`deoxyhemoglobin (Hb), and oxygenated hemoglobin, or
`oxyhemoglobin (HbO,), spectra. As shown, deoxyhemoglo-
`bin (Hb) has a higher optical extinction (i.e., absorbs more
`light) in the red region of spectrum around 660 om, as
`compared to that of oxyhemoglobin (HbO,). On the other
`hand, in the near-infrared region of the spectrum around 940
`nm,
`the optical absorption by deoxyhemoglobin (Hb) is
`lower than the optical absorption of oxyhemoglobin (HbO.).
`Prior art non-invasive optical scnsors for measuring artc-
`rial oxyhemoglobin saturation (SaO,) by a pulse oximeter
`(termed SpO.,) are typically comprised of a pair of small and
`inexpensive light emitting diodes (LEDs), and a single
`highly sensitive silicon photodetector. A red (R) LED cen-
`tered on a peak emission wavelength around 660 nm and an
`infrared (IR) LED centered on a peak emission wavelength
`around 940 nm are used as light sources.
`Pulse oximetry relics on the detection of a photoplethys-
`mographic signal caused by variations in the quantity of
`arterial blood associated with periodic contraction and relax-
`ation of a patient’s heart. The magnitude of this signal
`depends on the amount of blood ejected from the heart into
`the peripheral vascular bed with each systolic cycle,
`the
`optical absorption of the blood, absorption by skin and tissue
`components, and the specific wavelengths that are used to
`wn5
`illuminate the tissue. SaO, is determined by computing the 5
`relative magnitudes of the R and IR photoplcthysmograms.
`Electronic circuits inside the pulse oximeter separate the R
`and IR photoplethysmogramsinto their respective pulsatile
`(AC) and non-pulsatile (DC) signal components. An algo-
`rithm inside the pulse oximeter performs a mathematical
`normalization by which the time-varying AC signal at each
`wavelength is divided by the corresponding time-invariant
`DC component which results mainly from the light absorbed
`and scattered by the bloodlesstissue, residual arterial blood
`when the heart is in diastole, venous blood and skin pig-
`mentation.
`
`bomn
`*
`
`£a
`
`QDa
`
`Since it is assumed that the AC portion results only from
`the arterial blood component, this scaling process provides
`a normalized RAR ratio (ie., the ratio of AC/DC values
`corresponding to R- and IR-spectrum wavelengths,
`respectively), which is highly dependent on SaO., but is
`largely independentof the volumeofarterial blood entering
`
`2
`the tissue during systole, skin pigmentation, skin thickness
`and vascular structure. Hence, the instrument does not need
`to be re-calibrated for measurements on different patients.
`‘Typical calibration of a pulse oximeteris illustrated in FIG.
`2 by presenting the empirical relationship between SaO, and
`the normalized R/IR ratio, which is programmed by the
`pulse oximeters’ manufacturers.
`Pulse oximeters are of two kinds operating, respectively,
`in transmission andreflection modes. In transmission-mode
`pulse oximetry, an optical sensor for measuring SaO, is
`usually attached across a fingertip, foot or earlobe, suchthat
`the tissue is sandwiched between the light source and the
`photodetector.
`In reflection-mode or backscatler type pulse oximetry, as
`shown in FIG. 3,
`the LEDs and photodetector are both
`mounted side-by-side next to cach other on the same planar
`substrate. This arrangement allows for measuring SaO, from
`multiple convenient locations on the body (e.g. the head,
`torso, or upper limbs), where conventional transmission-
`mode measurements are not feasible. or this reason, non-
`invasive reflectance pulse oximetry has recently become an
`important newclinical technique with potential benefits in
`fetal and neonatal monitoring. Using reflectance oximetryto
`monitor SaO,
`in the fetus during labor, where the only
`accessible location is the fetal scalp or checks, or on the
`chest
`in infants with low peripheral perfusion, provides
`several more convenient locations for sensor attachment.
`
`Reflection pulse oximetry, while being based on similar
`spectrophotometric principles as the transmission one, is
`more challenging to perform and has unique problems that
`can not always be solved by solutions suitable for solving
`the problems associated with the transmission-mode pulsc
`oximetry. Generally, comparing transmission and reflection
`pulse oximetry,
`the problems associated with reflection
`pulse oximetry consist of the following:
`In reflection pulse oximetry, the pulsatile AC signals are
`generally very small and, depending on sensor configuration
`and placement, have larger DC components as compared to
`those of transmission pulse oximetry. As illustrated in TIG.
`4, in addition to the optical absorption and reflection due to
`blood, the DC signal of the R and IR photoplethysmograms
`in reflection pulse oximetry can be adversely affected by
`strong reflections from a bone. ‘This problem becomes more
`apparcnt when applying mcasurcments at such body loca-
`tions as the forehead and the scalp, or when the sensor is
`mounted on the chest over the ribcage. Similarly, variations
`in contact pressure between the sensor and the skin can
`cause larger errors in reflection pulse oximetry (as compared
`to transmission pulse oximetry) since someof the blood near
`the superficial layers of the skin may be normally displaced
`awayfrom the sensor housing towards deeper subcutaneous
`structures. Consequently,
`the highly reflective bloodless
`tissue compartment near the surface of the skin can cause
`large errors even at body locations where the bone is located
`too far awayto influence the incident light generated by the
`sensor.
`
`Another problem with currently available reflectance sen-
`sors is the potential for specular reflection caused by the
`superficial layers of the skin, when an air gap exists between
`the scnsor and the skin, or by direct shunting of light
`between the LEDsand the photodetector through a thin layer
`of fluid which may be due to excessive sweating or from
`amniotic fluid present during delivery.
`It is important to keep in mind the two fundamental
`assumptions underlying the conventional dual-wavelength
`pulse oximetry, which are as follows:
`
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`3
`illuminating
`(1) the path of light rays with different
`wavelengths in tissue are substantially equal and, therefore,
`cancel each other; and (2) each light source illuminates the
`same pulsatile change in arterial blood volume.
`Furthermore,
`the correlation between optical measure-
`ments and tissue absorptions in pulse oximetry are based on
`the fundamental assumption that light propagation is detcr-
`mined primarily by absorbable due to Lambent-Beer’s law
`neglecting multiple scattering effects in biologicaltissues. In
`practice, however, the optical paths of different wavelengths
`in biological tissues is known to vary more in reflectance
`oximetry compared to transmission oximetry, since it
`strongly depends on the light scattering properties of the
`illuminated tissue and sensor mounting.
`Several human validation studies, backed by animal
`investigations, have suggested that uncontrollable physi-
`ological and physical parameters can cause large variations
`in the calibration curve of reflectance pulse oximeters pri-
`marily at low oxygen saturation valucs below 70%. It was
`observed that the accuracy of pulse oximeters in clinical use
`might be adversely affected by a number of physiological
`parameters when measurements are made from sensors
`attached to the forehead, chest, or the buttock area. While the
`exact sources of these variations are not fully understood, it
`is generally believed that there are a few physiological and
`anatomical factors that may be the major source of these
`errors. It is also well known for example that changesin the
`ratio of blood to bloodless tissuc volumes may occur
`through venous congestion, vasoconstriction/vasodilatation,
`or through mechanical pressure exerted by the sensor on the
`skin.
`
`10
`
`15
`
`30
`
`Additionally, the empirically derived calibration curve of
`a pulse oximeter can be altered by the effects of contact
`pressure exerted bythe probe on the skin. ‘This 1s associated
`with the following. The light paths in reficctance oximetry
`are not well defined (as compared to transmission oximetry),
`and thus maydiffer between the red and infrared wave-
`lengths. Furthermore, the forehead and scalp areas consist of
`a relatively thin subcutaneous layer with the cranium bone
`underneath, while the tissue of other anatomical structures,
`such as the buttock and limbs, consists of a much thicker
`layer of skin and subcutaneous lssues withoul a nearby
`bony support that acts as a strong light reflector.
`Several in vivo and in vitro studies have confirmed that
`
`
`uncontrollable physiological and physical parameters (e.g.,
`
`
`different amounts of contact pressure applied by the sensor
`on the skin, variation in the ratio of bloodless tissue-to-blood
`content, or site-to-site variations) can often cause large
`errors in the oxygen saturation readings of a pulse oximeter,
`which are normally derived based on a single internally-
`programmed calibration curve. The relevant in vivo studics
`are disclosed in the following publications:
`1. Dasscl, ct al., “Effcet of location of the scnsor on
`reflectance pulse oximetry”, British Journal of Obstetrics
`and Gynecology, vol. 104, pp. 910-916, (1997);
`the
`2. Dassel, et al., “Reflectance pulse oximetry at
`forehead of newborns: The influence of varying pressure on
`the probe”, Journal of Clinical Monitoring, vol. 12, pp.
`421-428, (1996).
`‘The relevant in vitro studies are disclosed, for example in
`the following publication:
`3. Edrichet al., “etal pulse oximetry: influenceof tissue
`blood content and hemoglobin concentration in a new
`in-vitro model”, European Journal of Obstetrics and Gyne-
`cology and Reproductive Biology, vol. 72, suppl. 1, pp.
`$29-S34, (1997).
`
`40
`
`wn5
`
`55
`
`60
`
`65
`
`4
`Improved sensors for application in dual-wavelength
`reflectance pulse oximetry have been developed. As dis-
`closed in the following publication: Mendelson, ct al.,
`“Noninvasive pulse oximetry utilizing skin reflectance
`photoplethysmography”, IEEE ‘Iransactions on Biomedical
`Engineering, vol. 35, no. 10, pp. 798-805 (1988), the total
`amount of backscattered light that can be detected by a
`reflectance sensoris directly proportional to the number of
`photodetectors placed around the LEDs. Additional
`improvements in signal-to-noise ratio were achieved by
`increasing the active area of the photodetector and optimiz-
`ing the separation distance between the light sources and
`photodetectors.
`Another approach is based on the use of a sensor having
`six photodiodes arranged symmetrically around the LEDs
`that is disclosed in the following publications:
`4. Mendelson, ct al., “Design and cvaluation of a ncw
`reflectance pulse oximeter sensor”, Medical
`Instrumentation, vol. 22, no. 4, pp. 167-173 (1988); and
`5. Mendelson,etal., “Skin reflectance pulse oximetry: in
`vivo measurements from the forearm and calf’, Journal of
`Clinical Monitoring, vol. 7, pp. 7-12, (1991).
`According to this approach,
`in order to maximize the
`fraction of backscattered light collected by the sensor, the
`currents from all six photodiodes are summedelectronically
`by internal circuitry in the pulse oximeter. This configuration
`essentially creates a large area photodetector made of six
`discrete photodiodes connected in parallel
`to produce a
`single current that is proportional to the amount of light
`backscattered from the skin. Several studies showedthatthis
`sensor configuration could be used successfully to accu-
`rately measure SaO,, from the forehead, forearm and the calf
`on humans. However,
`this sensor requires a means for
`heating the skin in ordcrto increase local blood flow, which
`has practical limitations since it could cause skin burns.
`Yet another prototype reflectance sensor is based on eight
`dual-wavelength LEDs and a single photodiode, and is
`disclosed in the following publication: Takatani et al.,
`“Experimental and clinical evaluation of a noninvasive
`reflectance pulse oximeter sensor”, Journal of Clinical
`Monitoring, vol. 8, pp. 257-266 (1992). Here, four R and
`four IR LEDsare spaced at 90-degree intervals around the
`substrate and at an equal radial distance from the photo-
`diode.
`
`A similar sensor configuration based on six photodetec-
`tors mounted in the center of the sensor around the LEDsis
`disclosed in the following publication: Konig, et al.,
`“Reflectance pulse oximetry—principles and obstetric appli-
`cation in the Zurich system”, Journal of Clinical Monitoring,
`vol. 14, pp. 403-412 (1998).
`According to the techniques disclosed inall of the above
`publications, only LEDs of two wavelengths, R and IR, are
`used as light sourecs, and the computation of SaO, is bascd
`on reflection photoplethysmograms measured by a single
`photodetector, regardless of whether one or multiple photo-
`diodes chips are used to construct the sensor. This is because
`of the fact that the individual signals from the photodetector
`elements are all summed together electronically inside the
`pulse oximeter. Furthermore, while a radially-symmetric
`photodetector array can help to maximize the detection of
`backscattered light from the skin and minimize differences
`from local tissuc inhomogencity, human and animal studics
`
`confirmed that this configuration can not completely elimi-
`
`
`
`nate errors caused by pressure differences and site-to-site
`variations.
`
`The use of a nominal dual-wavelength pair of 735/890 nm
`was suggested as providing the best choice for optimizing
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`5
`accuracy, as well as sensitivity in dual-wavelength reflec-
`
`tance pulse oximetry,
`in U.S. Pat. Nos. 5,782,237 and
`
`
`
`5,421,329. This approach minimizes the effects of tissue
`heterogeneity and enables to obtain a balance in path length
`changes arising from perturbations in tissue absorbance.
`This is disclosed in the following publications:
`6. Mannheimer at al., “Physio-optical considerations in
`the design of fctal pulse oximctry sensors”, European Jour-
`nal of Obstetrics and Gynecology and Reproductive
`Biology, vol. 72, suppl. 1, pp. S9-S19, (1997); and
`7. Mannheimerat al., “Wavelength selection for low-
`saturation pulse oximetry”, IEEE Transactions on Biomedi-
`cal Engineering, vol. 44, no. 3, pp. 48-158 (1997)].
`However, replacing the conventional R wavelength at 660
`nm, which coincides with the region of the spectrum where
`the difference between the extinction coefficient of Hb and
`HbO,is maximal, with a wavelength emitting at 735 nm, not
`only lowers considerably the overall sensitivity of a pulse
`oximeter, but does not completely eliminate errors due to
`sensor placement and varying contact pressures.
`Pulse oximeter probes of a type comprising three or more
`LEDs for filtering noise and monitoring other functions,
`such as carboxyhemoglobin or various indicator dycs
`injected into the blood stream, have been developed and are 3.
`disclosed, for example, in WO 00/32099 and U.S. Pat. No.
`5,842,981. The techniques disclosed in these publications
`are aimedat providing an improved methadfor direct digital
`signal formation from input signals produced by the sensor
`and forfiltering noise.
`Noneof the above prior art techniques provides a solution
`to overcome the most cssential
`limitation in reflectance
`pulse oximetry, which requires the automatic correction of
`the internal calibration curve from which accurate and
`reproducible oxygen saturation values are derived, despite
`variations in contact pressure or site-to-site tissue heterage-
`neity.
`In practice, most sensors used in reflection pulse oximetry
`rely on closely spaced LED wavelengths in order to mini-
`
`mize the differences in the optical path Icngths of the
`
`different wavelengths. Nevertheless, within the wavelength
`range required for oximetry, even closely spaced LEDs with
`closely spaced wavelengths mounted on the same substrate
`can lead to large random error in the final determination of
`SaQ,.
`
`10
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`toCc
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`30
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`
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`£a
`
`SUMMARYOF THE INVENTION AND
`ADVANTAGES
`
`The object of the invention is to provide a novel sensor
`design and method that functions to correct the calibration
`relationship of a reflectance pulse oximeter, and reduce
`measurement inaccuracics in general. Another object of the
`invention is to provide a novel sensor and method that
`functions to correct the calibration relationship of a reflec-
`tance pulse oximeter, and reduce measurement inaccuracies
`in the lower range of oxygen saturation values (typically
`below 70%), which is the predominant range in neonatal and
`fetal applications.
`Yet another object of the present invention is to provide
`automatic correction of the internal calibration curve from
`which oxygen saturation is derived inside the oximeter in
`situations where variations in contact pressure or site-to-site
`tissue heterogeneity may cause large measurement inaccu-
`racies.
`
`Another object of the invention is to eliminate or reduce
`the effect of variations in the calibration of a reflectance
`
`wn5
`~
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`QDa
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`65
`
`6
`pulse oximeter between subjects, since perturbations caused
`by contact pressure remain one of the major sourcesof errors
`in reflectance pulse oximetry. In fetal pulse oximetry, there
`are additional factors, which must be properly compensated
`for in order to produce an accurate and reliable measurement
`of oxygen saturation. For example, the fetal head is usually
`the presenting part, and is a rather easily accessible location
`for application of reflectance pulse oximetry. However,
`uterine contractions can cause large and unpredictable varia-
`tions in the pressure exerted on the head and by the sensor
`on the skin, which can lead to large errors in the measure-
`mentof oxygen saturation by a dual-wavelength reflectance
`pulse oximeter. Another object of the inventionis to provide
`accurale measurement of oxygen saturation in the fetus
`during delivery.
`The basis for the errors in the oxygensaturation readings
`of a dual-wavelength pulse oximeter is the fact
`that,
`in
`practical situations, the reflectance sensor applications affect
`the distribution of blood in the superficial layers of the skin.
`This is different from an ideal situation, when a reflectance
`sensor measures light backscattered from a homogenous
`mixture of blood and bloodless tissue componcnts.
`Therefore, the R and IR DCsignals practically measured by
`photodetectors contain a relatively larger proportion of light
`absorbed byand reflected from the bloodless tissue com-
`partments. In these uncontrollable practical situations, the
`changes caused are normally not compensated for automati-
`cally by calculating the normalized RAR ratio since the AC
`portions of each photoplethysmogram, and the correspond-
`ing DC components,are affected differently by pressure or
`site-to-site variations. furthermore, these changes depend
`not only on wavelength, but depend also on the sensor
`geometry, and thus cannot be eliminated completely by
`computing, the normalized R/IR ratio, as is typically the case
`in dual-wavelength pulse oximeters.
`The inventor has found that the net result of this nonlinear
`effect is to cause large variations in the slope of the cali-
`bration curves. Consequently, if these variations are not
`compensated automatically, they will cause large errors in
`the final computation of SpO.,, particularly at low oxygen
`saturation levels normally found in fetal applications.
`Another object of the present invention is to compensate
`for these variations and to provide accurate measurement of
`oxygen saturation. The invention consists of, in addition to
`two measurement sessions typically carried out in pulse
`oximetry based on measurements with two wavelengths
`centered around the peak emission values of 660 nm (red
`spectrum) and 940 nm+20 nm (IR spectrum), one additional
`measurementsession is carried out with an additional wave-
`length. At
`least one additional wavelength is preferably
`chosen to be substantially in the IR region of the electro-
`magnetic spectrum,ie., in the NIR-IR spectrum (having the
`peak emission valuc above 700 nm). In a preferred embadi-
`ment
`the use of at
`least
`three wavelengths enables the
`calculation of an at least one additional ratio formed by the
`combination of the two IR wavelengths, which is mostly
`dependent on changes in contact pressure or site-to-site
`variations. In a preferred embodiment, slight dependence of
`the ratio on variations in arterial oxygen saturation that may
`occur, is easily minimized or eliminated completely, by the
`proper selection and matching of the peak emission wave-
`lengths and spectral characteristics of the at
`lcast
`two
`IR-light sources.
`Preferably, the selection of the IR wavelengthsis based on
`certain criteria. The IR wavelengths are selected to coincide
`with the region of the optical absorption curve where HbO.,
`absorbsslightly more light than Hb. The IR wavelengths are
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`US 6,801,799 B2
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`8
`illuminating a measurement location with at least three
`different wavelengths 41, A2 and A3, the first wave-
`length A1 lying in a red (R) spectrum,andtheat least
`second and at least third wavelengths 42 and A3 lying
`substantially in the infrared (IR) spectrum;
`detecting light returned from the measurementlocation at
`different detection locations and generating data indica-
`tive of the detected light, wherein said different detcc-
`tion locations are arranged so as to define at least one
`closed path around the measurement location; and
`analyzing the generated data and determining the bload
`parameter.
`
`BRILI DESCRIPTION OF THE DRAWINGS
`
`Other advantages of the present invention will be readily
`appreciated as the same becomes better understood by
`reference to the following detailed description when con-
`sidered in connection with the accompanying drawings
`toCc
`) wherein:
`
`7
`in the spectral regions where the extinction coefficients of
`both Hb and HbO, are nearly equal and remain relatively
`constant as a function of wavelength, respectively.
`In a preferred embodiment, tracking changesin the ratio
`formed by the two IR wavelengths, in real-time, permits
`automatic correction of errors in the normalized ratio
`obtained from the R-wavelength and each of the
`IR-wavelengths. The term “ratio” signifies the ratio of two
`values of AC/DC corresponding to two different wave-
`lengths. ‘This is similar to adding another equation to solve
`a problem with at Icast three unknowns(i.c., the relative
`concentrations of HbO, and Hb, whichare usedto calculate
`SaO,, and the unknown variable fraction of blood-to-tissue
`volumes that effects the accurate determination of SaO,),
`which otherwise must rely on only two equations in the case
`of only two wavelengths used in conventional dual-
`wavelength pulse oximetry. In a preferred embodiment, a
`third wavelength provides the added ability to compute
`SaO, based on the ratio formed from the R-wavelength and
`cither of the IR-wavelengths. In a preferred embodiment,
`changes in these ratios are tracked and compared in real-
`time to determine whichratio produces a more stable or less
`noisy signal. Thatratio is used predominantly for calculating
`SaQ,.
`bomn
`The present inventionutilizes collectionof light reflected +
`from the measurement location at different detection loca-
`tions arranged along a closed path around light emitting
`elements, which can be LEDsorlaser sources. Preferably,
`these detection locations are arranged in two concentric
`rings, the so-called “near” and “far” rings, around the light
`emitting elements. This arrangement enables optimal posi-
`tioning of the detectors for high quality measurements, and
`enables discrimination between photodetectors receiving
`“good”