(12) United States Patent
`Mendelson
`
`USOO68O1799B2
`(10) Patent No.:
`US 6,801,799 B2
`(45) Date of Patent:
`Oct. 5, 2004
`
`(54) PULSE OXIMETER AND METHOD OF
`OPERATION
`
`(75) Inventor: Yitzhak Mendelson, Worcester, MA
`(US)
`
`rr. A
`(73) Assignee: Cybro Medical, Ltd., Haifa (IL)
`( c: ) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 10/360,666
`(22) Filed:
`Feb. 6, 2003
`(65)
`Prior Publication Data
`US 2003/014.4584A1 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) ................................................ 138884
`(51) Int. Cl." .................................................. A61B 5/00
`(52) U.S. Cl. ........................ 600/330; 600/322; 600/336
`(58) Field of Search ................................. 600/310,322,
`600/323,330,336
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
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`5,349,519 A 9/1994 Kaestle .................. 364/413.09
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`
`WO
`WO
`WO
`
`2/1994
`WO9403102
`8/2001
`WOO154573
`11/2001
`WOO184107
`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, 1996.
`(List continued on next page.)
`Primary Examiner Eric F. Winakur
`(74) Attorney, Agent, or Firm-Howard & Howard
`(57)
`ABSTRACT
`A Sensor for use in an optical measurement device and a
`method for non-invasive measurement of 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 frequency and to generate respective signals.
`The Signals are used to determine the parameter of the blood.
`5 Claims, 6 Drawing Sheets
`
`Apple Inc.
`APL1203
`U.S. Patent No. 9,289,135
`
`001
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`

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`US 6,801,799 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
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`5,355,880 A 10/1994 Thomas et al.
`5,398,680 A 3/1995 Polson et al.
`5,413,100 A 5/1995 Barthelemy et al.
`5,421,329 A 6/1995 Casciani et al.
`5,482,036 A 1/1996 Diab et al.
`5,490,505 A 2/1996 Diab et al.
`5,490,506 A 2/1996 Takatani et al.
`5,494,032 A
`2/1996 Robinson et al. ........... 600/323
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`7/1996 Potratz
`5,632,272 A 5/1997 Diab et al.
`5,645,060 A 7/1997 Yorkey
`5,685,299 A 11/1997 Diab et al.
`5,758,644 A 6/1998 Diab et al.
`5,769,785 A
`6/1998 Diab et al. .................. 600/364
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`5,823.950 A 10/1998 Diab et al. .................. 600/310
`5,842,981 A 12/1998 Larsen et al. ............... 600/323
`5,853,364 A 12/1998 Baker, Jr. et al. ........... 600/500
`5,919,134 A 7/1999 Diab .......................... 600/323
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`6,067.462 A 5/2000 Diab et al. .................. 600/310
`6,081,735 A 6/2000 Diab et al. .................. 600/310
`6,083,172
`7/2000 Baker, Jr. et al. ........... 600/500
`A
`OTHER PUBLICATIONS
`“Reflectance Pulse Oximetry-Principles and Obstetric
`Application in the Zurich System'; Voker 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 Fellow et al. British Journal
`of Obstetrics and Gynecology; Aug. 1997, vol. 104, pp.
`910-916.
`
`
`
`“Design and Evaluation of a New Reflectance Pulse Oxime
`terSensor'; Y. Mendelson, PhD, et al.; 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.; IEEE Transactions 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 of fetal 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 model; Tho
`mas Edrich, Gerhard Rall, Reinhold Knitza; European Jour
`nal if Obstetrics & Gynecology and Reproductive Biology
`72 Suppl. 1 (1997) S29–S34.
`
`* cited by examiner
`
`002
`
`

`

`U.S. Patent
`
`Oct. 5, 2004
`
`Sheet 1 of 6
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`US 6,801,799 B2
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`HELOGLOBINSPECTRAIN
`OX METRY
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`
`
`5000
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`500
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`500
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`600
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`700
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`800
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`900
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`1000
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`WAWELENGTH (nm)
`Figure 1
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`CALIBRATION OF APILSE OXMETER
`
`100
`90
`80
`70
`60
`50
`40
`30
`20
`10
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`O
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`1.0
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`3.0
`2.0
`RMRRATIO
`Figure 2
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`4.0
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`5.0
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`003
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`

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`U.S. Patent
`
`Oct. 5, 2004
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`Sheet 2 of 6
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`US 6,801,799 B2
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`
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`LIGHT
`EMITTING
`DIODES
`
`004
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`

`U.S. Patent
`
`Oct. 5, 2004
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`Sheet 3 of 6
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`US 6,801,799 B2
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`REFLECTIONSENSOR
`
`Figure 5A
`
`REFLECTIONSENSOR
`
`
`
`Figure 6
`
`005
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`

`

`U.S. Patent
`U.S. Patent
`
`Oct. 5, 2004
`Oct. 5, 2004
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`Sheet 4 of 6
`Sheet 4 of6
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`US 6,801,799 B2
`US 6,801,799 B2
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`17
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`MICROPROCESSOR
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`
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`DISPLAY
`
`006
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`006
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`

`U.S. Patent
`
`Oct. 5, 2004
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`Sheet 5 of 6
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`US 6,801,799 B2
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`COMPUTEDATA
`FORPD
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`S
`DATA
`ACCEPTABLE
`p
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`REJECTDATA POINT
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`
`CALCULATEW/W
`(NEARAND FAR)
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`NO
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`ACCEPT POINT
`GO TO STEP3
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`RELECT POINT:
`
`TIRN ON ALARM
`(ADLISTSENSOR POSITION)
`
`Figure 10A
`
`007
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`

`U.S. Patent
`
`Oct. 5, 2004
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`Sheet 6 of 6
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`US 6,801,799 B2
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`
`
`IS
`THE QUALITY OF EACH
`PHOTOPLETHYSMOGRAM
`ACCEPTABLE
`?
`
`
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`REJECT POINT:
`TRN ON ALARM
`(MOVEMENT/BREATHING ARTIFACTS)
`
`
`
`YES
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`
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`ACCEPT POINT
`GO TO STEP3
`
`Figure 10B
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`CALCULATEW/W,
`AND WAW,
`(NEARAND FAR)
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`ABS(W/W - W/W3) < TH2
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`ACCEPT POINT
`COMPLITE AND PDATE
`DISPLAY WITH NEWSpo, VALLIE
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`RELECT POINT:
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`TAKEANOTHER
`MEASUREMENT
`
`
`
`Figure 10C
`
`008
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`

`

`US 6,801,799 B2
`
`1
`PULSE OXMETER 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
`
`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 oximeter is 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 and reflection modes. In transmission-mode
`pulse Oximetry, an optical Sensor for measuring SaO is
`usually attached acroSS a fingertip, foot or earlobe, Such that
`the tissue is Sandwiched between the light Source and the
`photodetector.
`In reflection-mode or backScatter type pulse Oximetry, as
`shown in FIG. 3, the LEDs and photodetector are both
`mounted Side-by-Side next to each other on the same planar
`Substrate. This arrangement allows for measuring SaO2 from
`multiple convenient locations on the body (e.g. the head,
`torso, or upper limbs), where conventional transmission
`mode measurements are not feasible. For this reason, non
`invasive reflectance pulse Oximetry has recently become an
`important new clinical technique with potential benefits in
`fetal and neonatal monitoring. Using reflectance oXimetry to
`monitor SaC) in the fetus during labor, where the only
`accessible location is the fetal Scalp or cheeks, 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 pulse
`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 FIG.
`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
`apparent when applying measurements 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 Some of the blood near
`the Superficial layers of the skin may be normally displaced
`away from 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 away to influence the incident light generated by the
`SCSO.
`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 Sensor and the skin, or by direct shunting of light
`between the LEDs and 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:
`
`1. Field of the Invention
`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 Spectrophotometric measurements
`of changes in the color of blood, enabling the non-invasive
`determination of oxygen Saturation in the patient'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 nm) 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 (HbO2), spectra. As shown, deoxyhemoglo
`bin (Hb) has a higher optical extinction (i.e., absorbs more
`light) in the red region of spectrum around 660 nm, as
`compared to that of oxyhemoglobin (HbO2). 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 Sensors for measuring arte
`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 relies 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
`illuminate the tissue. SaO is determined by computing the
`relative magnitudes of the R and IR photoplethysmograms.
`Electronic circuits inside the pulse Oximeter Separate the R
`and IR photoplethysmograms into 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 bloodless tissue, residual arterial blood
`when the heart is in diastole, Venous blood and skin pig
`mentation.
`Since it is assumed that the AC portion results only from
`the arterial blood component, this Scaling proceSS provides
`a normalized R/IR ratio (i.e., the ratio of AC/DC values
`corresponding to R- and IR-spectrum wavelengths,
`respectively), which is highly dependent on SaO, but is
`largely independent of the Volume of arterial blood entering
`
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`(1) the path of light rays with different illuminating
`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 deter
`mined primarily by absorbable due to Lambent-Beer's law
`neglecting multiple Scattering effects in biological tissues. 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 values 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 changes in the
`ratio of blood to bloodless tissue volumes may occur
`through venous congestion, vasoconstriction/vasodilatation,
`or through mechanical pressure exerted by the Sensor on the
`skin.
`Additionally, the empirically derived calibration curve of
`a pulse Oximeter can be altered by the effects of contact
`preSSure exerted by the probe on the skin. This is associated
`with the following. The light paths in reflectance oximetry
`are not well defined (as compared to transmission oximetry),
`and thus may differ 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 tissues without 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 Studies
`are disclosed in the following publications:
`1. Dassel, et al., “Effect of location of the sensor on
`reflectance pulse oximetry”, British Journal of Obstetrics
`and Gynecology, vol. 104, pp. 910–916, (1997);
`2. Dassel, et al., “Reflectance pulse oximetry at the
`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. Edrich et al., “Fetal pulse oximetry: influence of 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.
`S29–S34, (1997).
`
`45
`
`50
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`55
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`60
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`65
`
`4
`Improved Sensors for application in dual-wavelength
`reflectance pulse Oximetry have been developed. AS dis
`closed in the following publication: Mendelson, et al.,
`"Noninvasive pulse OXimetry utilizing skin reflectance
`photoplethysmography”, IEEE Transactions on Biomedical
`Engineering, vol. 35, no. 10, pp. 798-805 (1988), the total
`amount of backScattered light that can be detected by a
`reflectance Sensor is 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, et al., “Design and evaluation of a new
`reflectance pulse oXimeter Sensor, Medical
`Instrumentation, vol. 22, no. 4, pp. 167-173 (1988); and
`5. Mendelson, et al., “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 Summed electronically
`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 showed that this
`Sensor configuration could be used Successfully to accu
`rately measure SaC) from the forehead, forearm and the calf
`on humans. However, this Sensor requires a means for
`heating the skin in order to 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 LEDs are 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 LEDs is
`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 in all of the above
`publications, only LEDs of two wavelengths, R and IR, are
`used as light Sources, and the computation of SaO is based
`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 tissue inhomogeneity, human and animal Studies
`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
`
`010
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`US 6,801,799 B2
`
`S
`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 fetal pulse OXimetry Sensors”, European Jour
`nal of Obstetrics and Gynecology and Reproductive
`Biology, vol. 72, Suppl. 1, pp. S9-S19, (1997); and
`7. Mannheimer at 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
`HbO2 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 dyes
`injected into the blood Stream, have been developed and are
`disclosed, for example, in WO 00/32099 and U.S. Pat. No.
`5,842,981. The techniques disclosed in these publications
`are aimed at providing an improved method for direct digital
`Signal formation from input signals produced by the Sensor
`and for filtering noise.
`None of the above prior art techniques provides a Solution
`to overcome the most essential 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 heteroge
`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 lengths 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
`SaO.
`
`SUMMARY OF 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 inaccuracies 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 neonataland
`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
`CCS.
`Another object of the invention is to eliminate or reduce
`the effect of variations in the calibration of a reflectance
`
`6
`pulse Oximeter between Subjects, Since perturbations caused
`by contact preSSure remain one of the major Sources of 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
`ment of oxygen Saturation by a dual-wavelength reflectance
`pulse Oximeter. Another object of the invention is to provide
`accurate measurement of oxygen Saturation in the fetus
`during delivery.
`The basis for the errors in the oxygen Saturation 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 components.
`Therefore, the R and IRDC signals practically measured by
`photodetectors contain a relatively larger proportion of light
`absorbed by and 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 R/IR 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 Sp02, 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 nmi-20 nm (IR spectrum), one additional
`measurement Session 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, i.e., in the NIR-IR spectrum (having the
`peak emission value above 700 nm). In a preferred embodi
`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 least two
`IR-light Sources.
`Preferably, the selection of the IR wavelengths is based on
`certain criteria. The IR wavelengths are Selected to coincide
`with the region of the optical absorption curve where HbO
`absorbs slightly more light than Hb. The IR wavelengths are
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`US 6,801,799 B2
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`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 changes in 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 least three unknowns (i.e., the relative
`concentrations of HbO and Hb, which are used to 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
`either of the IR-wavelengths. In a preferred embodiment,
`changes in these ratios are tracked and compared in real
`time to determine which ratio produces a more Stable or leSS
`noisy Signal. That ratio is used predominantly for calculating
`SaO.
`The present invention utilizes collection of light reflected
`from the measurement location at different detection loca
`tions arranged along a closed path around light emitting
`elements, which can be LEDs or laser 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” information (i.e., AC and DC values which would
`result in accurate calculations of Sp0) and “bad” informa
`tion (i.e., AC and DC values which would result in inaccu
`rate calculations of Sp0).
`There is thus provided according to one aspect of the
`present invention, a Sensor for use in an optical measurement
`device for non-invasive me