'
`
`379-380 B riefReviews
`J. S. Gram*usrrr."H
`
`Lalb
`
`Or1'ginal Articles
`381-384 IHFLIIEHBE [IF THE IIEFEHEHIIE GAS IIF PABAMHBHETIG
`IJHTIIEIII III‘IM.TZEH$ [IN NITHIIHEII IIIIIIIIEIITHHTIIIIIS
`IIIIIIIIIII IIIJISEII-IIIHIIIIIT MIESTHESIII
`
`'
`
`jmu F. ./I. He1:d:'frIe.\', MD, A.I:rJ’.I‘r'/i.]. mmZH.Ink'r.', MUD,
`PHD, rmd Andre M. de fwl/o._‘,C M D
`
`
`
`_
`
`‘
`
`385—391 HIIHILHBIIITT [IF HEIIEIHIIS IN MI IIIITPHTIEIIT
`PIIEIIIIESTHETIII EWIIJIIITIIIII IELIIIIC
`CorJo.u L. Cfllby, IND, a.'ufI'VfHrem K. S(:":Im1'J_ PHD
`
`393402 M’ WHflEIIIII.I.IIITIIIII IIF I HLIIHEII LIJIIP IITIIIIITIIIIIIIII
`STIIHTEIIT EIIII IIIIIIIIIEII PHHHLTSIS
`
`Deepafc Rmm'n'er:'sJ'nm, MS, Khosrow Be!'.=!JeI'.tm.-1', PhD, PE,
`Kevin Kfeirr, M D, _)‘.L_*[',?}'ey Mo:'e;':rr.':', J‘:/IS,
`14/03’ H’. um: Mnhzm":.-r, Ph D, PE. Rn.":.«'r.' C. E1'm'hm'.'. PhD,
`and Mfcfmci Doflnr, M S
`
`4()3—4I2 HEFIEIITINBE I’I.II.5E IIHIIflETIIT— PRINCIPLES MIII
`IIHSTETHIB flI'PI.IIIIITIIIII III THE ZIIIIIIIH 3T3TEHI
`Vofker Kfiwnflq, Renate Hath, r.-ud /lhberr Hm}:
`
`_—T 413-420 HIIIIHHRCT IIF HULIIHE MEISIIHEMENTS III II'|EI:Hfl|'1III:IIlI.T
`Volume 14 Number 6 Asggzrst 1998
`HEtI‘flTH1lI"Il1IIElllI'I‘t'EE“:1'lIl|IJ'I'slIFl:ll||‘|M£Bl:lAl.l]E|llBES
`Km’ Rosier, Berfmm Fofrzfk, Roland R. I»!/am-r, and
`G:-rd Schumh‘.er.-':
`
`421-424 MI EIIIII.IITIIIIIIlHT SIIIIITIIIN TU MIESTHESIH IIIJTEIHIIITEIJ
`IIEITIIHD KEEPING
`
`AIM: A. Bfrl'eeI', P.-':D, join:
`Prmf Poppers, M’ D
`
`'
`
`Cage, MD, and
`
`425—43'.f EW-II.I.|flTIII|'1| IIF A PITIJT TYPE SPIHIIIIIETEH IH HEIIHIIW
`UIWIIEH IHIITIIIIES
`
`Soren .'§oma'e:_'qr:rrr'rf. MD, Sig:r1'b:':g:rI‘ K&ms:m. M D,
`Stqfnti Lm1r."iH, MT), and Ola Sn-nqm'.<.', MD
`
`433-459 IIETEIITIIIH [IF |.IINIi IHJIIHT WITH GIIHIIEHTIIINIIL AND
`IIEI.IIIfll HETWIIHK-HIISEII IIHMJISI5 [IF IIIIIITIHIIIIUS IIIITH
`jrrkkn Ri1's£r'He.u, MD, mm‘ fI.a'mrn’c:'o A. Iron, MD
`
`Algorithm
`44.’-446 II HEM.-TIME IILIIIIHITHTII TII IIIIPHIWE THE HESI'I]H3E TIME
`III: I BLIIIIBHL IIIIILTIIIIIS HHIILTSEH
`Lnwdy Wong, MSc, Run’: Hrum’:'rou. MB, CF18,
`Eifrm Pm'ayIwa, PIID, (Hid C:':'m' Hm‘m, D. Phi!
`
`Boole Review
`
`447 L E. HENSIJH MID H. 3. LEE [EDS]: SIHIIIUITIIHS IN
`IIHESTHESIDLIJIIY EIIIIIIIITIIIN
`Rr'¢'Imrd ."'n'urr.I‘s
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`449-450 INFUHMATIUHEHH IIIIHTBIIIIITIIIIS
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`J U L 3 U 1999
`
`4-
`Apple Inc.
`’“’“°”
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`lSSN1387—1307
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`Ill AIIAESTIIESIA Ann INTENSIVE onus, man or
`41
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`
`Apple Inc.
`APL1014
`U.S. Patent No. 8,989,830
`
`

`

`
`
`JOURNAL [IF GLINIIIAL NIIJNITIIBING
`AND BIIMPIITINII
`
`E|FFIllllll. Jfllllllllll. [IF THE ElIlllJl’Ell|"l SIJSIETY Fllll lllJ|lIl'Il'l'lll[l Mill] TElIHl'lllI.IIll‘l' Ill llll|llESTIlE8lA Mill! IHTEHSIVE BIIHE IESIITAIIII
`Mlll [IF THE SIIIIIETT Fllll TEl3HllIJl.lIlliY Ill MIESTIIESIA [STll]
`
`Volume 14 Number6 August 1998
`
`WEST?” Hon -
`
`.5 Y
`
`Harvey L: Edrnouds, MD
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`
`
`
`TIIE PIIIIENT Jllll lllilfils
`flalui-riIai’ing rmrl Cmripnffiig,
`lHrei'mm'o.Iml _]orrrn.1l qf Clinirnl
`founded by Omar Prakash in 1984; Reed Gardner, senior-
`editor; and the
`jarrriral qf Clirricnl Mrmfrerfng, Founded in 1935 by N.Ty
`Smith, Allen K. Ream, and J. S. Gravenstein.
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`Allegheny University Hospitals, MCI’
`Dept. oFAnesthesiology
`3300 Henry Avenue
`Philadelphia, PA19129, U.S.A.
`E—niail: Fcld1na1i]@aL1hs.cd1t
`
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`UF,lCollege of Medicine
`Dept. offlnesthesiology
`P.O. Box 100254
`Gainesville, FL 32610. U.S.A.
`E—n1ail: jgravens@anest2.anest.ufi.edu
`
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`City Maternity Hospital
`Dept. ofAnae-sthesiology
`P.O. Box 61
`
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`
`403
`
`BEFLEBTANBE PULSE llX|METflY— PRINCIPLES Alllll
`OBSTETBIS APPLIIIATIUN IN THE Zlllllflll SYSTEM
`
`Kiinig V. lluirh R, Huch A. Reflectiince pulse o_\'i1nel'ry — principles
`and obstetric application in the 7.uriCl1 system.
`J Clin Monit 1998: I4: -103--H2
`
`I/ol!'cei' Kilt-rig, Rcna.te Hitch, and /‘lll){.’l"l Hut.-'i
`
`HIISTINWT. Transmissioii and reflectance are the two main
`
`modes ofpulse oximetry. In obstetrics, due to the absence ofn
`trzlnsilliniiinable feta] part For transinission oximetry. the only
`Feasible option is the 1'eflcct;1ncC mode, in which sensor and
`detector are located on the same surface of the body part.
`However, none of the teflectance pulse oxiineters developed
`For intraptirturn use are fully szltisftictiny, :15 lllCllC:1|I€Cl by the
`Fact that none have entered routine use. We have designed,
`developed, constructed and tested a reflectaiicc pulse oximeter
`with the possibility to adjust the electronic circuits and signal
`processing in order to determine the effects ofvarious paramu
`eters on signal amplitude and wave—for1r1 and to optimize the
`sensitivity and spatial arrangement ofthe optical elements.
`Following an explamtioii of the principles of reflectance
`pulse oximetry, we report our experience with the design,
`development, construction and field-testing of an in~hoi.isc
`reflectance pulse oximetry system for obstetric application.
`
`KEY WIJHBS. O.\'\_,-‘gen saturation, refiectance pulse oximetry,
`intrapartum fetal inonitoring.
`
`lN'|'|iI]lJ|lBTl{I|l|
`
`Pulse oximetry is the combination of spectrophotome-
`try and plethysmography. It permits rapid noninvasive
`1'nez1surement of arterial oxygen saturation with the
`added advantages of simple sensor application and direct
`nieasurenient, requiring neither calibration nor pre-
`adjustinent. Pulse oxinietcrs are thus in vvidesprezid and
`Fast—increasing use, e.g. in intensive care, anesthetics and
`neonatology [1]. All these applications employ “trans-
`mission” pulse oximetry, so called because the light
`used to determine blood oxygen saturation is “trans-
`mitted" from :1 light emitter on one side of the body
`part to :1 light receiver on the other side; suitable sites
`are the fingers in adults or hands and feet in neonates or
`children, which are said to be “transilluminated.”
`
`In obstetrics, fetal oxygen status during labor is a
`crucial pzitanictcr. However, no transilluminable Fetal
`part is available. The only option in this case is reflec-
`tance oximetry [2], using :1 sensor with its light emis~
`sion and detection elements on the same surface of
`
`the body part. Various types of such a reflcctance pulse
`oxinietcr have been developed for intrapalrtuni use at
`various locations. I-Iowever, for :1 wide variety OF‘I'CZlS0l"1S,
`
`all are still experimental and not in full routine use [3—7].
`Basically a refiectancc measurement can be achieved
`using planar sensors — which can be produced,
`for
`example. by modifying conventional traiistnission sen-
`sors — and :1 sensitive modern pulse oxinicter. Hoivever,
`
`From the l’erin:It:il Pliysiology Rcsentcli Departnient. Departinent
`of Obstetrics, Zurich University Hospital. CH—8{I‘)I Zurich. Switzer-
`land.
`
`Received Nov I3, 1997, and in revised Formjun 23,
`for publication Aug 4, I998.
`
`l‘J‘)8, Accepted
`
`l-’l.'l'll'I4'lE£ll l’hysiology Re-
`Address c01'i'esp0l1Llei1cC to V0llCL‘I' Kfiliig,
`search Departnient. Department of 0bstet1'i:.‘s. Zurich University
`Hospital. Cl-l—R0‘Jl Zurich. Sivitzerlantl.
`E-mail:
`\'l£gQlf_._fl1l(.llSZ.Cl]
`
`_,I'unriml qfCli:n'mu' Mmiimr:'iIg rmrl Cniiipnrirng 14: -"H13--I I2. 1998.
`CC‘ I998 Klnnrer/lrmlrmft Pnlilr'sl'n'i's. Print:-n‘ in the !\7erl'ir'n'niiri's_
`
`403
`
`

`

`404 jom':MiofC.I'niimI'M'airf.'ui'irn3aiid Cumpnfing I/m')'4 N06 /lugusrI998
`
`em,
`
`d(t)
`
`absorption coefficient of reduced hemoglobin
`(function of wavelength)
`time function of mean pulsatile change in artery
`thickness, with amplitude d : d(diastole) —
`d(systole)
`
`Measurement may be impaired by light transmitted
`directly from the light source to the receiver or light
`which does not pass through arterially perfused tissue.
`Ifthis “direct light“ 1,1,, is taken into account, Equation
`(1) changes to:
`
`I“) = [tissue ' exP(_(SO2 ' EHb0+
`(1 ‘ 502) '€Hbl
`‘ d(t)) + Idir
`
`(M)
`
`where
`
`Ilissut : Iii
`
`'9xP(“5tissi1i.- ' 5)
`
`Since the pulsatile component of the absorption is at
`most a few percent, i.e. the exponent of the second e
`function in Equation ('1) is very small, we can use the
`approximation:
`
`exp(x) = l + x
`
`for
`
`<<l
`
`to obtain the very close approximation:
`
`I(t) =
`
`[tissue '
`
`_
`
`' EHIIO +
`
`_
`
`' EHI) '
`
`+ Idir
`
`(lb)
`
`This light intensity is measured in the photodiodes and
`can be broken down electronically into two compo-
`nents, a time-independent signal
`
`DC = Itissuc + Idir
`
`with amplitude equal to the value ofthis signal
`
`such instruments come with a “black—box" 1nicroproces—
`sor—controlled mode ofoperation making constructional
`adjustments to the electronic circuits and signal process-
`ing virtually impossible. As a result, it becomes difficult
`to determine the effect of various parameters on signal
`amplitude and wave-form, optimize sensor sensitivity to
`light intensity and the arrangement of the optical ele-
`ments, and hence assess the dependence ofarterial oxygen
`saturation measurement on key physical, technical and
`above all physiological variables. This was the aim
`driving our decision to design, develop and construct
`in—house a system dedicated to obstetric applications.
`Following a brief review of the principles of pulse
`oximetry, we report our experience with the develop-
`ment of the new device,
`together with some field-
`testing results.
`
`
`
`PBINBIPLES [IF PIlI.SEOXIl'i1ET|IY
`
`Sig:-rel recording
`
`Light is absorbed on passing through matter. The de-
`gree of absorption depends on the nature of the trans-
`illuminated material and the wavelength of the light
`employed. All optical techniques for determining arte-
`rial oxygen saturation use the marked difference in the
`absorption of red light between oxygenated and re-
`duced hemoglobin.
`The absorption of light passing through bone or
`nonpulsatile tissue is constant over time. Oxygenated
`and reduced hemoglobin in the arterial vascular bed, on
`the other hand, cause changes in absorption timed by
`the heart rate due to the pulsatile variation in artery
`thickness. The total intensity of the light after passing
`through tissue ca11 be measured, for example, as the
`photocurrent I(t) ofa photodiode, and is obtained from
`the Lambert—Beer absorption law as:
`
`I(t) = In ‘ exp(—E,;,,m. - s) - exp{—(SO3 - EH50 +
`(1 — so?) - Enbl « dun
`
`7|
`"’
`
`dc E DC = Itissuc + Idir
`
`where
`
`intensity ofincidcnt light
`In
`I-:,i_,_,,,._. mean absorption coefficient of tissue (function of
`wavelength)
`mean thickness oftransilluminated tissue
`
`5
`
`SO;
`
`oxygen saturation to be determined (3 I-IbO]
`(HbO + Hb), i.e. ratio of oxygenated hemoglo-
`bin concentration to sum of oxygenated and
`reduced hemoglobin concentrations)
`t-:H;,.;) absorption coefficient of oxygenated hemoglobin
`(function of wavelength)
`
`and a signal which varies in time with the pulsatile
`change in artery thickness
`AiC = ltimu.
`- (S03 - E|.u,o + (l — S03) '5H[,) ' d(t)
`
`(4)
`
`with amplitude
`
`ac = I(diastole] — I(systolc) =
`
`Itissue '
`
`'5Hl:0 + (I — S02) ' "5-Hls)
`
`‘ d-
`
`Thc ratio between the ac and dc amplitudes is then
`
`

`

`Kiiiiig or al: The ZIrn'r."r Obsnerrfr Rtjfiermiire Pulse ().\'lHl£'l(’l'
`
`405
`
`r : acid‘: = (llissiicxulissiic + I(|ir))'
`(502 ' €Hbo + ('1 ' 302) ‘Final ‘ d-
`
`6
`
`}
`
`(
`
`this ratio r is
`In the case that “direct light” 1.1,, = 0,
`independent of the incident light intensity In and ofthe
`absorption in the nonpiilsating tissue value ltism:
`
`However, despite various theoretical models [9, l0],
`the scatter coefficients of the various tissue types are
`not known with sufficient accuracy to permit exact
`calculation. Expcrinieiital calibration thus Iias to be
`performed by directly comparing the pulse oxinietcr
`readings with arterial blood sample values.
`
`r = ac/dc =
`(503 - EH50 + (l - S02) - E31,) - d
`
`for Edi, = 0.
`
`(6a)
`
`'Ii'ansuiissi'on pi-Ilse om‘:-iieti'y
`
`then dependent only on the oxygen
`This ratio r is
`saturation S0; to be determined, the known absorption
`coeflicients EH50 and EH5, and the mean pulsatile change
`d in the thickness of the arterial vessels in the trans-
`
`illuniinated region.
`To eliminate this dependence on d, the measurement
`is performed at two wavelengths with maximally dif-
`fering absorption coeflicients. On the assumption that
`the d values are the same for both wavelengths, we
`obtain a variable
`
`R = rm;/ri, = (ac/dclmi/(ac/dc)ir
`
`= (502 ' 5HbO + (1 F 5O2l '5Hbln-.i/
`(502 ' 5HbO + (1 — Sozl '5Hblir
`
`(7)
`
`7
`( 3)
`
`The optical elements are located on opposite sides ofa
`body part. The sensors are applied mainly to the fingers
`and toes. Ears and nose are used only rarely due to poor
`perfusion. In neonates the sensor is applied around the
`hand or foot. This arrangement largely ensures that the
`optical paths are the same for both wavelengths. Never-
`theless, incorrect sensor attachnient can give spurious
`results, e.g. if some of the transmitted light reaches the
`receiving diodes around the outside ofa finger as “direct
`light.”
`Signal magnitudes are an important determinant of
`measurement accuracy: in normal fingertips, the ratio
`ofthe signal due to absorption in pulsating blood (ac)
`to the signal due to absorption in total tissue (dc), r =
`acfdc, is 0.02—0.05.
`
`from which the unknown S02 is readily calculated
`without knowing the incident light intensity or tissue
`thicknesses.
`
`Calculation assumes the following physical prerequi-
`sites:
`
`0 No light must be measured that has not passed
`through the pulsatile vascular bed e.g. light passing
`directly from light source to receiver (ldir).
`0 The pulsatile changes in artery thickness must be the
`same for both wavelengths,
`i.e. both wavelengths
`must transilluminate the same tissue region.
`0 Valid measurement assumes that the pulsatile signal
`originates only from varying absorption by arterial
`oxygenated and reduced hemoglobin. The results are
`falsified by other causes of pulsatile changes in optical
`thickness, e.g. hemoglobin derivatives, circulating
`pigments, pulsatile changes in thickness produced
`mechanically in nonarterially perfused tissue by car~
`diac action. and, above all, venous pulsation.
`0 To simplify description ofthe principle behind meas-
`urement and its limitations,
`the Lanibert—Beer law
`was assumed valid for the passage of light through
`tissue. However, as light is not only absorbed in tissue
`but also scattered, the law is of limited applicability
`[8]. The exact absorption coefficients must be cor-
`rected by taking the scattering effect into account.
`
`Reflerrm-ice pulse oxii-iierry
`
`In this method the light backscattered in the body is
`used to determine oxygen saturation. The optical ele-
`ments are thus located on the same plane on the same
`body surface. Reflection originates from nonhomo—
`geneity in the optical path, i.e. at the interfaces between
`materials with different refractive iiidices. This means
`that on physiological grounds, strong reflections can be
`expected on the entry of light into bone. The traiis—
`illuminated tissue must also be well perfused to obtain
`as strong a signal as possible. Not all body parts are as
`well perfused as the fingers or hands, but an ac/dc ratio
`of 0.00l—0.005 can be achieved on the forehead. Perfu-
`sion is also good over the sternum. One method of
`signal enhancement is to heat the measurement site to
`induce hyperperfusion, which can safely be performed
`up to 42 °C. A rubefacient, e.g. nicotinic acid (Rubri-
`ment), can also be applied to the measurement site.
`The principal physical limitations are the following:
`
`o The sensor design rriust eliminate “direct light,” i.e.
`light passing directly from the light sources to the
`photodiodes or that is only scattered in the outer part
`ofthe skin.
`
`0 The measured AC signals are some 10 times weaker
`
`

`

`406 jmrnml ofCh'niml Moiiftnrirtg and Coiiipiiriiig Val 14 Na 6 .r‘ll'l'gI'I$I I998
`
`than in the transmission method. The conditions
`
`governing the heating of the light—emitring diodes
`(LED) limit
`the potential for producing stronger
`signals by increasing the incident light intensity: not
`only can high uncont1'olled temperatures damage
`tissue at the measurement site, but the wavelength
`of the emitted light changes as the LEDs become
`warmer. For this reason the photodiode area must be
`as large as possible.
`0 As in the transmission mode, the principle of meas-
`urement is the determination of absorption, except
`that this now refers to incoming reflected light. The
`light path is less well defined than in transmission
`mode, and thus may differ between the two wave-
`lengths. The effective absorption coefficients of the
`calibration inserted in the Lambert-Beer law must be
`
`checked and if necessary corrected by comparison
`with photometrically measured arterial blood values.
`
`
`
`neonatal head. Experimentation led to the choice ofa
`vacuum system using sensors cast from silicone rub-
`ber, with a suction groove for fixation, a guard ring
`against direct light, and a connector for a suction
`pump. The photoelectric components are identical in
`both types ofsensor.
`I Some metal sensors were fitted with a resistance-wire
`
`heating coil of maximal output 200 mW' to induce
`local hyperemia. The temperature was monitored by
`a negative temperature coefficient (NTC) resistor in-
`corporated in the sensor unit.
`
`Numerous types of sensor meeting the above require-
`ments were built. A sensor used for intrapartum meas-
`urements — cast from silicone rubber with suction
`
`channel and pump connection — is shown in cross-
`section in Figure 1. It is attached to the fetal head with a '
`vacuum of approximately 100 mbar.
`
`TIIE ZIIIIIBH IIEFLEIITANIIE PIILSE IIKIIIIETEB
`
`Elecrroiiics
`
`Sensors
`
`In constructing planar reflection sensors, i.e. sensors in
`which the photoelectric emitting and receiving elements
`lie next to each other in virtually the same plane, special
`attention was paid to the following points (Figure 1).
`
`0 For maximal independence from local tissue differ-
`ences, a radially—symrnetric pattern was selected for
`the photoelements. The light source ~ a chip with
`two LED5 for the wavelengths red = 660 inn and
`infrared = 920 nm — was placed in the center of the
`sensor and surrounded by a radial photodiode array
`for detecting the reflected light. To obtain a good
`signal at minimal
`light intensity,
`the area of the
`photodiodes had to be as large as possible. After some
`preliminary experiments,
`six BX33 pliotodiodes
`(Siemens) were used with a mean radius of 17 mm.
`Their connections are led outwards individually, so
`that the sensor as a whole remains operational if a
`wire breaks or an individual diode is lost. This arrange-
`ment gives an external sensor diameter of22 mm.
`o A guard ring around the LEDS acts as a barrier to
`"direct light." The sensor must also fit snugly to the
`skin to minimize the risk of ambient light reaching
`the photodiodes.
`9 Unlike with transmission sensors, fixation to the
`
`measurement site can pose problems. Aluniinuni sen-
`sor units are readily fixed with double—sided adhesive
`ECG rings. However,
`these rigid sensor heads are
`unsuited to the small radii of curvature of the fetal]
`
`Initially, we decided to separate signal processing in the
`analog part before the analogldigital converter (ADC),
`including signal amplification, filtering, and separation
`into DC and AC signals, all handled electronically ; the
`post—ADC digital part was handled by software which
`input the data, averaged and evaluated amplitudes, cal-—
`culated saturation and heart rate, and produced a some-
`what complicated screen display. Now, using modern
`techniques, we have a system in preparation in which a
`fast separate microprocessor unit handles most signal
`processing and digital filtering tasks.
`Figure 2 shows the block diagram of the current
`apparatus with the following individual units:
`
`0 Time control ofmeasurernent
`
`From a rectified mains power supply signal (100 Hz),
`a phase—locked loop (PLL) — connected as a frequency
`multiplier — generates a square—wave signal of 64
`kHz. Coupling to the line frequency eliminates data
`acquisition faults due to interferences with the line
`frequency. The 64 kHz from the PLL clock drives a
`' 7-bit counter that addresses an EPROIVI giving a data
`cycle ofl kHz, 64 pulses long. The output pulses of
`the EPROM control
`the entire sequence of light
`emission, signal acquisition and signal processing.
`0 LED drive
`
`Theoppositely poled red and infrared LEDs are
`located in the output circuit of a current—stabilized
`push—pull output stage. They are triggered by digital
`signals from the control unit via analog switches at
`the 1 kHz sampling rate in the equally spaced se-
`
`

`

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`
`
`
`
`
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`
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`
`
`
`@ Vacuum-tube
`G Photodiodes
`@ SignaI—cab|e
`@ Red and Infrared LED's
`
`6) Light barrier
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`
`

`

`408 jairrriai t5fCl'ii:ftau' Moiiitoriitg mm‘ Coiirptirii-ig V0114 N06 August 1998
`
`
`
`Computer
`
`
`
`
`
`Pre-
`
`Amplifier
`
`Vacuum
`
`Pump
`and
`
`
`
`Control
`
`
`
`
`
`
`Ext. Device
`
`
` Oseillatorfcontrol Unit
`
`(HP, Nellcor)
`
`
`Fig. 2. Stireiimtic represeiitnfioii ofriieasureincm eietrrairirs.
`
`quence: infrared—dark-red—-dark (overall length: 1 ms).
`Light intensity is determined by the amplitude of the
`signal driving this output stage. lnput signal intensity
`can be selected in two ways:
`* Manually: The red and infrared LED intensities
`can be manually adjusted independently using two
`potentiometers (Helipot). This permits the use of
`any desired light intensity within the limits stipu-
`lated for test and research purposes.
`— Automatically: DC voltage as the input signal
`controls the LEDS so that the DC voltages at the
`computer input for both wavelengths are 2.0 :i: 0.5
`V. Outside this range the control circuit changes
`the LED currents to reset the DC voltages to 2.0 V.
`This setting is used for normal clinical applications.
`To prevent skin damage from overheating even in
`the event of electronic component failure, maximal
`LED intensity is limited by an electronic circuit.
`Input amplifier and sample—and—hold stages
`Using operational amplifiers the photocurrent sup-
`plied by the photodiodes is converted to.a voltage
`and then amplified. Six switch positions permit am-
`plifications of 50 mV/nA to 5 V]nA. Afterwards,
`three sample—and—hold (S35!-I)
`stages — switched
`by the corresponding signals from the control stage -
`resolve the signals into the three components infra-
`red, red and dark. The dark currents are then sub-
`
`tracted from the red and infrared signals in a subtrac-
`tion stage which also eliminates small ambient light
`components that may have reached the photodiodes.
`
`o Filters
`
`Using low—pass filters the discrete—tirne signals at the
`S&I-I output are reconverted to continuous—time sig-
`nals and trimmed of high—frequency components
`using eighth-order Bessel
`filters with a cut—off
`frequency of 7 Hz. The DC components are then
`separated using a low—pass filter with a cut—off fre—
`quency ofO.1 1-12. The AC components are passed to a
`further Bessel high-pass 0.7 Hz filter for separation of
`slow motion artifacts, and then to a 40-fold amplifi-
`cation stage. The four signals DCi,, DC,,d, ACi, and
`AC,,d are thus available at the output. As oxygen
`saturation is calculated from the ratios (ac/dc);, and
`(ac/dc),cd, it was essential to ensure by careful com-
`ponent selection that the amplifications for the am-
`plification and separation stages were as near as possi-
`ble identical for both wavelengths.
`0 Patient insulation
`
`the LED controller output
`For patient protection,
`stage and photodiode input stage were electrically
`‘isolated from the mains using Burr & Brown isola-
`tion amplifiers. These units were powered by an
`insulated power pack.
`Heating
`Sensors incorporating a resistance coil were fitted with
`a precision controller stabilizing temperature within
`the range 38.0—4l.O °C to an accuracy of0.] °C.
`Vacuum pump
`The sensor is fixed to the skin using a small pump
`producing a maximal -300 mbar vacuum. The pump
`
`

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`
`Fig. 3. Srm-n display qf :1 I0-minute h‘l£’d'$HI"t’li'h‘.'l"l'f. From top to banom: oxygen saturation, heart rare, hear: mrefram HP CTG mauirnr,
`uterine roirrracriorifiuiii HP CTC mcmflor. Undermmrh the DC and AC sigrrals for infrared am} red ligirrfur the his! 5 secomis (mm the
`dnflereiwt o.rriinare scaicsfor DC and AC).
`
`is maintained electronically via a manometer at a
`preset negative pressure. ln normal medical use, ad-
`equate fixation is achieved with a vacuum of about
`—l00 mbar.
`
`Sofiware
`
`Apart from various test programs, a program for data
`acquisition, display, calculation and storage and a pro—
`gram for subsequent data postprocessing were written
`in PASCAL.
`
`0 Signal acquisition, calculation and display
`For data acquisition a 486 DOS computer was
`equipped with a 12-bit analogfdigital conversion
`
`(ADC) card (Mctrabyte). A counter on this ca1'd
`triggers analogjdigital conversion at 400 Hz, which
`.starts an interrupt program in the computer for read-
`ing the 4 measured values DC,“ DC,,_.,; and AC,“
`AC,,.,g into a cyclic buffer of 5-second length. From
`the signal (AC,,.+AC,cd} the rnaxirna and minima are
`determined for each cardiac cycle, and hence the
`instantaneous heart rate. Oxygen saturations are cal-
`culated from the amplitudes ac and dc of the AC and
`DC signals.
`Saturation is calculated from the Lambert-Beer
`law using the absorption coefficients [11]
`for
`the
`nominal wavelengths of our LEDs. However, as
`the actual wavelengths may deviate from these nom-
`inal values and the applicability of the Lambert—Beer
`law is limited by scattering in tissue, experimental
`
`

`

`410 _[orrr.Iim"qfClr'nicaI A/Ioirfroring rmri CompHH'n_g V0314 No 6 x1HgI:sr1998
`
`calibration of the measured saturation values is essen-
`tial.
`
`CalflJr'ation
`
`Saturation and heart rate can be averaged over 1-9
`cardiac cycles and are displayed every second. Analog
`hea1't rate and uterine contraction signals from any
`CTG monitor with analog output
`(e.g. Hewlett
`Packard model 8040) can be input to the ADC every
`second and likewise displayed.
`The measurement display (Figure 3) shows, along the
`bottom, the 4 measured values DC;,, DCM and AC;,.,
`AC,cd, over the last 5 seconds of measurement. This
`serves to monitor signal quality during acquisition.
`Poor—signal periods can be marked and excluded from
`data processing. Along the top, measured arterial
`oxygen saturation and heart rate values per second are
`displayed cyclically over a 10-minute interval, with
`the CTG heart rate and contraction input under-
`neath. Any time point can be marked for subsequent
`identification and all values and comment stored in a
`
`file at any time.
`0 Data analysis
`The values from a stored file can be redisplayed in
`measurement mode using an evaluation program.
`Time intervals can be marked with the arrow keys or
`mouse. Means and standard deviations — including
`the CTG data — are then calculated and displayed.
`
`MEASUREMENTS Mil IIISIIIJSSIIIH
`
`Clinical application ofany new instrument or measure-
`ment system presumes:
`
`— mechanical reliability, accuracy and calibration,
`— feasibility in the clinical situation, including accept-
`ance by both medical personnel and patients,
`— the ability not only to determine physiological pa-
`rameters not previously measured in both physiolog-
`ical and pathological situations, but also to evaluate
`the diagnostic significance of such parameters, in this
`case oxygen saturation.
`For the first two more technical points is to say, that
`our instrument required calibration before clinical
`use, together with field tests ofthe suction device and
`long—term oximeter performance during birth.
`For
`these points controls and clinical
`trials were
`performed in our own unit and with colleagues in
`Copenhagen (DK), Graz (A), Oulu (SF) and Berlin
`(D). The major investigations comprised:
`
`To calibrate a pulse oximetcr, an oxygen saturation
`value must be assigned to the measured variables
`
`R = rm]/rir = [ac[dc)m.]/(ac/dc)“,
`
`cf (7)
`
`on the basis of an experimental or theoretical relation-
`ship. Initially we used the absorption coefficients of
`Zijlstra er al [11]. The general problems of calibrating a
`pulse oximeter have been discussed elsewhere [12, 13].
`For fine calibration we performed the following inves-
`tigations:
`
`0 Tests in the arterial oxygen saturation range 88-
`100% were conducted in 14 healthy adult volunteers
`breathing normal air and then air with approxi—'
`mately 80% normal oxygen content for 10 minutes
`in each case. The reflection sensor was fixed to the
`
`forehead or sternum with an adhesive ring. Owing to
`the invasive nature of arterial cathcterization, refer-
`
`ence values were provided by a MINOLTA PUL-
`SOX 8 transmission pulse oximeter attached to the
`index finger. Data analysis [13] showed a 4.5% differ-
`ence in oxygen saturation between the MINOLTA
`and the preliminary results of our reflectance system
`based on the absorption coefiicents ofZijlstra et al.
`0 At lower saturation levels, measurements were per-
`formed in cyanotic children before surgery. The chil-
`dren had arterial lines, permitting direct comparison
`with arterial blood readings [14].
`0 Low saturations in viva can also be measured in the
`
`fetal lamb [15]. We used this method to compare our
`pulse oximeter readings directly with arterial values
`in the oxygen saturation range 10-80% [16].
`
`Preliminary evaluation of these data shows that our
`previous calculations of oxygen saturation have to be
`corrected in the 1[}—10()% saturation range by a factor of
`1.045.
`
`Fixa than
`
`The first experiments in sensor fixation to the head and
`other parts of the human body were performed in
`adults [17] and neonates [18].
`Attachment is simple in practice, even during birth.
`After rupture of the membranes, the sensor can be fixed
`to the fetal head once the cervix has dilated to at least 2
`cm. Initial fixation takes 30-60 seconds and allows full
`
`freedom ofmovement [19].
`Approximately 100 mbar is the most suitable pressure
`
`

`

`at which to