(12) United States Patent
`US 6,801,799 B2
`(10) Patent No.:
`Mendelson
`(45) Date of Patent:
`Oct. 5, 2004
`
`U8006801799B2
`
`4,740,080
`4,773,422
`4,796,636
`4,802,486
`4,819,649
`4,819,752
`4,854,699
`4,859,057
`4,867,557
`4,892,101
`4,928,692
`4,934,372
`4,960,126
`5,190,038
`5,224,478
`5,348,004
`5,349,519
`
`>>>>>>>>>>>>>>>>>
`
`4/1988
`9/1988
`1/1989
`2/1989
`4/1989
`4/1989
`8/1989
`8/1989
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`5/1990
`6/1990
`10/1990
`3/1993
`7/1993
`9/1994
`9/1994
`
`356/326
`
`Donohue et al.
`Isaacson et al.
`Branstetter et al.
`Goodman et al.
`Rogers et al.
`Zelin
`.................. .. 356/41
`Edgar, Jr.
`Taylor et al.
`..
`.. 356/41
`Takatani et al.
`............ .. 356/41
`Cheung et al.
`Goodman et al.
`Corenman et al.
`Conlon
`Polson et al.
`Sakai et al.
`Hollub
`Kaestle ................ .. 364/413.09
`
`W0
`W0
`W0
`
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`W'O9403 102
`/1994
`[/2001
`W'00154573
`11/2001
`W'00184107
`OTHER PUBLICATIONS
`
`PULSE OXIMETER AND METHOD OF
`OPERATION
`
`Inventor:
`
`Yitzhak Mendelson, Worcester, MA
`(US)
`
`Assignee: Cybro Medical, Ltd., Haifa (IL)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`Appl. No.: 10/360,666
`
`Filed:
`
`Feb. 6, 2003
`Prior Publication Data
`
`US 2003/0144584 A1 Jul. 31, 2003
`
`Related US. Application Data
`
`(62) Division of application No. 09/939,391, filed 011 Aug, 24,
`2001, [low abandoned.
`
`Foreign Application Priority Data
`(30)
`Oct. 5, 2000
`(IL)
`.............................................. .. 138884
`
`Int. Cl.7 ................................................ .. A61B 5/00
`(51)
`(52) us. Cl.
`...................... .. 600/330; 600/322; 600/336
`(58) Field of Search ............................... .. 600/310, 322,
`600/323, 330, 336
`
`(56)
`
`References Cited
`
`US. PATENT DOCUMENTS
`
`US. Patent No. 8,923,941
`
`“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 Exarniner%ric F. Winakur
`(74) Attorney, Agent, or Firm—Howard & Howard
`(57)
`ABSTRACT
`
`3,638,640
`3,799,672
`3,847,483
`3,998,550
`4,086,915
`4,167,331
`4,266,554
`4,357,105
`4,407,290
`4,446,871
`4,714,341
`
`>>>>>>>>>>>
`
`2/1972
`3/1974
`11/1974
`12/1976
`5/1978
`9/1979
`5/1981
`11/1982
`10/1983
`5/1984
`12/1987
`
`Shaw
`Vurek ....................... .. 356/41
`Shaw ..... ..
`356/41
`Konishi et al.
`............. .. 356/39
`Kol‘sky et al.
`Nielsen ..................... .. 356/39
`Hamagnri
`Loretz ....................... .. 356/40
`Wilber
`Imura
`Hamaguri et al.
`
`.......... .. 356/41
`
`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
`
`10\,\
`
`Apple Inc.
`APLl 053
`
`Apple Inc.
`APL1053
`U.S. Patent No. 8,923,941
`
`0001
`
`FITBIT, Ex. 1053
`
`

`

`5,355,880
`5,398,680
`5,413,100
`5,421 ,329
`5,482,036
`5,490,505
`5,490,506
`5,494,032
`5,517,988
`5,533,507
`5,632,272
`5,645,060
`5,685,299
`5,758,644
`5,769,785
`5,782,237
`5,823,950
`5,842,981
`5,853,364
`5,919,134
`5,995,856
`6,011,986
`6,031,603
`6,036,642
`6,067,462
`6,081,735
`6,083,172
`
`>>>>>>>>>>>>>>>>>>>>>>>>>>>
`
`......... .. 600/323
`
`.
`
`.. 600310
`.. 600/323
`.. 600/500
`.. 600323
`.. 600322
`.. 600323
`356/41
`600364
`.. 600310
`. . . . . . . . . .. 600310
`......... .. 600/500
`
`“Reflectance Pulse Oximetry—Princip/es and Obstetric
`Application in the Zurich System”; Vokcr Konig, Renate
`Huch, and Albert Huch; Perinatal Physiology Research
`Dept., Dept. of Obstetrics, Computing 14: pp. 403—412,
`1998.
`“Eflect 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-
`ter Sensor”; 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, W'orcester, MA 01609; Journal of
`Clinical Monitoring, v01. 7, N0. 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, N0. 4, Oct. 1992; pp. 2577266.
`
`“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.
`1487158.
`
`“Noninvasive Pulse Oximetry Utilizing Skin Reflectance
`P/Iotoplet/1ysmography”; Yitzhak Mendelson and Burt D.
`Ochs; IEEE Transactions on Biomedical Engineering, vol.
`35, N0. 10, Oct. 1988; pp. 7984305.
`
`“Physio—optical considerations in the design offetal pulse
`oximetry sensors”; RD. Mannheimer, M.Jz'. hem and JR.
`Casciani; European Journal of Obstetrics & Gynecology and
`Reproductive Biology 72 Suppl. 1 (1997) 597819.
`“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) 829—834.
`
`US 6,801,799 132
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`10/1994 Thomas et a1.
`3/1995 Polson et al.
`5/1995 Barthelemy et a].
`6/1995 Casciani et a].
`1/1996 Diab et a1.
`2/1996 Diab ct a1.
`2/1996 Takatani et al.
`2/1996 Robinson et a1.
`5/1996 Gerhard
`7/1996 Potratz
`5/1997 Diab ct a1.
`7/1997 Yorkey
`11/1997 Diab et a1.
`6/1998 Diab et a].
`6/1998 Diab et a1. ................ .. 600364
`7/1998 Casciani ct a1.
`10/1998 Diab et a1.
`.
`.
`12/1998 Larsen et a1.
`12/1998 Baker, Jr. et a].
`7/1999 Diab ..
`11/1999 Mannheimcr ct a .
`1/2000 Diab et a1.
`2/2000 Fine et al.
`3/2000 Diab et a].
`.
`5/2000 Diab et a1.
`. . . .
`6/2000 Diab ct a1.
`7/2000 Baker, Jr. et a1.
`OTHER PUBLICAI‘IONS
`
`* cited by examiner
`
`0002
`
`FITBIT, Ex. 1053
`
`

`

`US. Patent
`
`Oct. 5, 2004
`
`Sheet 1 0f 6
`
`US 6,801,799 B2
`
`WAVELENGTH (nm)
`
`figure 1
`
`CALIBRATION OF A PULSE OHMETER
`
`HELOGLOBIN SPECIRA IN
`OXIMETRY
`
`SATURATION
`
`E"
`
`EXTINCTION
`
`COEFICENT
`
`OXYGEN
`
`0003
`
`FITBIT, Ex. 1053
`
`

`

`US. Patent
`
`Oct. 5, 2004
`
`Sheet 2 0f 6
`
`US 6,801,799 B2
`
`LKHH‘
`EMH‘HNG
`Dflflflfi
`
`REHECTANCE SENSOR
`‘IIIIIIIII
`
`VHN
`
`59,5!!!"
`mgggccccss 0
`‘;
`ar-
`C
`v
`'
`
`AKHmY
`
`0004
`
`FITBIT, Ex. 1053
`
`

`

`60103teehS
`
`US 6,801,799 B2
`
`U S. Patent
`
`Oct. 5, 2004
`
`REFLECTION SENSOR
`
`REFLECTION SENSOR
`
`0005
`
`FITBIT, Ex. 1053
`
`

`

`Oct. 5, 2004
`
`Sheet 4 0f 6
`
`US. Patent
`
`US 6,801,799 B2
`
`0006
`
`FITBIT, Ex. 1053
`
`

`

`US. Patent
`
`Oct. 5, 2004
`
`Sheet 5 0f 6
`
`US 6,801,799 B2
`
`IS
`
`DATA —ACCEPTABLE REIECT DATA POINT
`
`
`
`GO TO STEP 3
`
`?
`
`REIECT POINT:
`
`TURN ON ALARM
`
`(ADIUST SENSOR POSITION)
`
`ACCEPT POINT:
`
`0007
`
`FITBIT, Ex. 1053
`
`

`

`US. Patent
`
`Oct. 5, 2004
`
`Sheet 6 0f 6
`
`US 6,801,799 B2
`
`IS
`THEQIIALITYOFEACH
`PHOTOPLE'IHYSMOGRAM
`ACCEPTABLE
`?
`
`ACCEPTPOINT:
`comm-2193
`
`Figure IOB
`—
`
`CALCULATE W1 /w2
`AND W1 lw2
`(NEARANDFAR)
`
`.
`REJECFPOM‘
`TURN ON ALARM
`(MOVEMENT/BREATHING ARWACTS)
`
`DISPLAY mmmawsw2 VALuE
`
`AB“ W1/ Wz'WI/ “’3’ <1?”
`
`REJECI‘POINT:
`
`TAKEANO‘IHER
`MEASUREMENT
`
`ACCEPT POINT:
`COMPUTE AND UPDATE
`
`figure 10C
`
`0008
`
`FITBIT, Ex. 1053
`
`

`

`US 6,801,799 B2
`
`1
`PULSE OXIMETER AND METHOD OF
`OPERATION
`
`This application is a divisional application of US. patent
`application Ser. No. 09/939,391 filed Aug. 24, 2001, now
`abandoned.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`0009
`
`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 11m) 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 (I-Ib), and oxygenated hemoglobin, or
`oxyhemoglobin (HbOZ), 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 (HbOz), 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 (HbOZ).
`Prior art non-invasive optical sensors for measuring arte-
`rial oxyhemoglobin saturation (SaOz) by a pulse oximeter
`(termed SpOz) 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. SaO2 is determined by computing the g
`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.
`
`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 SaO2 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 SaO2 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 SaO2 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 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. 'lhis 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
`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 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:
`
`'
`
`V
`
`1
`
`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 SaOZ, but is
`largely independent of the volume of arterial blood entering
`
`0009
`
`FITBIT, Ex. 1053
`
`

`

`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. 7987805 (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 SaO2 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. 2577266 (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.
`
`0010
`
`3
`illuminating
`(1) the path of light rays with different
`wavelengths in tissue are substantially equal and, therefore,
`cancel each other; and
`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 dilfer 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.,
`di ‘erent 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);
`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. 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.
`329—334, (1997).
`
`
`
`
`US 6,801,799 B2
`
`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 oximetryiprinciples and obstetric appli—
`cation in the Zurich system”, Journal of Clinical Monitoring,
`1 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 SaO2 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 di erences 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
`
`0010
`
`FITBIT, Ex. 1053
`
`

`

`0011
`
`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 backseattered from a homogenous
`mixture of blood and bloodless tissue components.
`Therefore, the R and IR DC 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-\ 'avelength 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 SpOZ, 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 nm120 nm (IR spectrum), one additional
`measurement session is carried outwith 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 NlR-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 HbO2
`absorbs slightly more light than Hb. The IR wavelengths are
`
`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 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-
`rac1es.
`
`Another object of the invention is to eliminate or reduce
`the effect of variations in the calibration of a reflectance
`
`US 6,801,799 B2
`
`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 e ects 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. 59—319, (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 US. 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
`re Jroducible 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
`re y on closely spaced LED wavelengths in order to mini-
`mize the differences in the optical path lengths of the
`di erent 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
`SaOz.
`
`
`
`
`
`
`
`
`0011
`
`FITBIT, Ex. 1053
`
`

`

`US 6,801,799 B2
`
`8
`illuminating a measurement location with at least three
`different wavelengths M, A2 and M, the first wave-
`length 7t1 lying in a red (R) spectrum, and the at least
`second and at least third wavelengths 12 and M lying
`substantially in the infrared (IR) spectrum;
`detecting light returned from the measurement location at
`different detection locations and generating data indica-
`tive of the detected light, wherein said different detec-
`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 blood
`parameter.
`
`BRIEF DESCRIPTION OF TIIE 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
`' wherein:
`
`FIG. 1 illustrates hemoglobin spectra as measured by
`oximetry based techniques;
`FIG. 2 illustrates a calibration curve used in pulse oxim-
`etry as typically programmed by the pulse oximeters manu-
`facturers;
`FIG. 3 illustrates the relative disposition of light source
`and detector in reflection-mode or backseatter type pulse
`oximetry;
`FIG. 4 illustrates light propagation in reflection pulse
`oximetry;
`FIGS. 5A and 5B illustrate a pulse oximeter reflectance
`sensor operating under ideal and practical conditions,
`respectively;
`FIG. 6 illustrates variations of the slopes of calibration
`curves in reflectance pulse oximetry measurements;
`FIG. 7 illustrates an optical sensor according to the
`invention;
`FIG. 8 is a block diagram of the main components of a
`pulse oximeter utilizing the sensor of FIG. 7;
`FIG. 9 is a flow chart of a selection process used in the
`signal processing technique according to the invention; and
`FIGS. 10A to 10C are flow charts of three main steps,
`respectively, of the signal processing method according to
`the invention.
`
`0012
`
`7
`in the spectral regions where the extinction coefficients of
`both Hb and HbO2 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 HbO2 and Hb, which are used to calculate
`SaOz, and the unknown variable fraction of blood-to-tissue
`volumes that effects the accurate determination of SaOZ),
`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
`SaO2 based on the ratio formed from the R-wavelength and
`either of the Ill-wavelengths. In a preferred embodiment,
`changes in these ratios are tracked