`
`
`_ URNAL OF
`
`CLINICAL
`MONITORING
`AND
`COMPUTING
`
`Wan-He 14 Number 6 I‘ll-{gust 1998
`
`I
`
`57977330 BrquRcw'ews
`J" S, (Irmwn‘m'n
`
`L- I I”.
`
`Origina! Articles
`J8 I—jéH INFLUENCE OF THE HEFEHENSE GAS IF PAHAI'NALINETIC
`ONYOEN ANALYZEIIS UN NITROGEN SUNSENTHATIUNS
`OLIHINO ULUSEO—OIHCUITANESTHESIA
`jum F. xl. I-Imdrh'kx', MD, ."IH:."J"('."I._,I. mm me'rrl, MD,
`PhD, :rmffi'lmht’ :‘u'. “'1' I-I’uy‘, FUD
`385—39! AUAILABILITY OI: NEUUOOS IN AN OUTPATIENT
`PHEANESTI'IETII: EUALUATIUN IILINIU
`Com‘m: L. Gibby. AID. mm’ I’I’THN'IH KI Sc'himh. PhD
`39.i—402 "II WVOEIIALUATION OFA ULOSEI LOIP MONITORING
`STRATEGY FOR INOUCEO PAHALYSIS
`Dawn}: mekriflnm, .‘l-IS, KIhmvH' UrMn-Imm', PAD, Mi,
`Km” Kiri”, :‘I'ID,_}(_'[firy Slhu'ehmr, .‘I'IS,
`I’I'lJII-I-I". mm .-HrlI!-:-'rrl'm, PM), PIE. ROIK'II (I. Ebrrhm‘r, PM),
`and .‘vhrIn-rr! DnIl’dr, MS
`
`40374112 HEPLEUTANUE PULSE UNIIYIETHY- PHINUIPLES ANU
`OOSTETRIC APPLICATION IN THE ZURICH SYSTEM
`I'TII'A‘CI‘ Kt'iltflq, Rmrm‘v Hnrh, .md :‘lHIrr: Hm‘h
`4 {3—420 AOSUHAOY OF UOLUME MEASUREMENTS IN MECHANICAL”
`IIEIUTILATEO NEIIIIOOHNS:
`A OOMPAHATIUE STUUY HE UUNIII'IEHOIAL OEUIOES
`Kai Rush, BL'I'N’HHI Fain-"He, Rul'mm’ R. H 1mm] mmi
`(Irm’ SrhumHm‘:
`
`421—424 AN EIIOLUTIONARY SOLUTION TO ANESTHESIA AUTO MATEO
`RECORO IIEEPINU
`rum” :1. BTL'A’FI', I’hiljohn S. (Tam, .UI). and
`HTML} Puppy”, .I-ID
`-
`425—4.?! EUALUATIOIII OF A PITOT TYPE SPIHUI'I'IETEH IN HELIUI’II/
`OXYOEN MIXTURES
`Surf“ Somh‘fiuam‘d. .IID‘ Signrhrwm' KAI-awn. AID.
`Snjlim Lumh'u,
`.‘UD, and ()In Strum-Eu, AID
`4.337459 UETECTION OF LUNG INJ UHY WITH CUNUENTIONAL ANO
`NEURAL NETWORK—BASED ANALYSIS OI: UUNTINUO US OATA
`THAI-2r Rii'sa'rrm. ML), anhun-irr‘nl Lain. .UD
`
`Algorithm
`441F446 A REAL—TIME ALCORITHM TO IMPROIIE THE RESPONSE TIME
`OFA CLINICAL munmns ANALYSER
`1..“me ll-img, .IISI', Rmh Hamilwu. .118. (MB,
`EH68” PaIayIu-n, PHD. and CHM Hm'm, U. PA”
`
`Boole Review
`447 L. C. HENSON AND A. C. LEE [ENS]: SIMULATOHS IN
`ANESTHESIOLORY EOUCATION
`R fi'Imm' A .I'm‘n's
`
`449—451? INFORMATION FOR CONTRIBUTORS
`
`KLUIIIIER ACADEMIC PUBLISHERS
`
`amount JOURNAL or THE EUROPEAN 30mm”
`FOR COMPUTING AND TECHNOLOGY
`IN ANAESTHESIA AND INTENSIVE CARE, AND OF
`
`THE SOCIETY FOR TECHNOLOGY IN ANESTHESIA
`
`JUL 3 U 1399
`r1 :1-.
`
`(cid:36)(cid:83)(cid:83)(cid:79)(cid:72)(cid:3)(cid:44)(cid:81)(cid:70)(cid:17)(cid:3)
`I Applé Inc I H :
`(cid:36)(cid:51)(cid:47)(cid:20)(cid:19)(cid:20)(cid:23)(cid:3)
`APL1014
`(cid:56)(cid:17)(cid:54)(cid:17)(cid:3)(cid:51)(cid:68)(cid:87)(cid:72)(cid:81)(cid:87)(cid:3)(cid:49)(cid:82)(cid:17)(cid:3)(cid:27)(cid:15)(cid:28)(cid:27)(cid:28)(cid:15)(cid:27)(cid:22)(cid:19)
`US. Patent No. 8,989,830
`
`CODEN JCMOEH
`ISSN 138771307
`
`APPLE 1014
`
`I
`(cid:44)
`
`1
`
`1
`
`APPLE 1014
`
`

`

`
`
`Inunlinturcumciit mnmrunmu
`um cnmrurms
`_____—___—.___.—-————
`IJFFIBIAI. JIJIIBNAL IIF THE EUROPEAN SOCIETY Fllll BBMPIITINE AND TECHNILIJEY IN ANAESTHESIA flNB INTENSIL‘E CINE [ESllTAlfll
`ANI] llF TIIE SIIGIETT FUN TEIII'INIJLIJIHr ll'il ANESTNESIA [STA]
`
`
`Volume 14 Nmnberé August 1998
`
`
`
`THE PAHENTJflIIflNflLS
`r‘t-I'tlirimri'iiq and Computing,
`lim’riimieimlJournal Qi- Cliniml
`Founded by Omar Prakash in 1984: Reed Gardner. senior
`editor; and the
`il-'lttiiiit)i'iitg. Founded in 1985 by N. Ty
`Journal of. Clim'ml
`Smith, Allen K. Ream. and J. S. Gravenstein.
`
`ENITUNS
`Jeffrey M. Feldman. MD
`Alleghenyr University Hospitals. MCIJ
`Dept. ofAnesthesiology
`3.300 Henry Avenue
`Philadelphia, PA 19129. U,S.A.
`E—nlail: FcldmanJG} aulis.etlti
`
`J. S. Gravenstein, MD. Coordinating Editor
`UFICollege oFMcdicinc
`Dept. offincsthesiology
`13.0. Box 100254-
`Gainesville, FL 32610, USA.
`E-mail: Jgravensfiianest2.anest.ufi.edu
`llkka Kalli, MD, PhD
`City Maternity Hospital
`Dept. of Anaesthesiology
`P.O. Box 61
`FlNallOWi Helsinki. Finland
`E—mail:
`ilkkakallifii iki.Fi
`Publisher
`Kluwer Academic Publishers
`HO. Box 990
`330i] AZ Dordrecht,The Netherlands
`
`E-iuail: Loes.Wils@ \\'kap.nl
`
`ENITNNIM. ENTIRE
`Paul G. Bardsll. MD
`' Gerard Bashcin, MD. Phi)
`Charlotte Bell, MD
`Jan E. W, Beneken, PhD
`J. A. Blom, PhD
`J. G. Bovill, Ml), PhD
`Peter Carstcnsen
`William P. Coleman, PhD
`Jeffrey E. Cooper, PhD
`John Doyle. MD
`Ronald Dileck, MD
`Peter (3. Duke, MD
`
`ISSN [387-1307
`All rights resel'ved
`
`f.) I998 [(qu-er Academic Publishers
`No part of the material protected by this copyright notice may be
`reprodnred or utilised in any form or by any means. electronic or
`mechanical. Including photocopying. recording or by any inl‘ornia—
`rim: storage and retrieval system. without written permission from
`the copyright owner.
`Printed in the Netherlands
`
`Primer! on llflfljl-l'fl' paper
`
`W q ,
`E" r
`
`..,
`
`,
`
`I
`
`_
`
`,
`
`_
`
`- J
`
`.
`
`JUL 3 0 1999
`
`Mgr”
`W'Lm'” "
`
`.
`*‘I‘I-iu-omc lit.
`
`.
`
`Harveyr L.‘ Editionds, MD
`John H. Eiehhorn.M|)
`TammyY. Euliauo, MD
`M. Fisher. MD
`A. Dean Forbes
`Wolfgang Fricsdorf, MD
`Yasuhiro Fukui. PhD
`Leslie Geddes, PhD
`Nikolaus Gravenstein, MD
`A. Gel-son Greeuhurg. MD. PhD
`0. Hedenstierna, MD
`Wolfgang Heinrichs. MD
`11. C. G. Gallandat Huet, MD
`Kazuyuki lkeda, MD
`VilleJantti, MI)
`Gavin N. C. Kenny, MD
`Samson Lampotang, l’hD
`J. W. R. McIntyre, Ml)
`James H. Philip, Mr)
`Michael L. Quinn, Plil)
`Pradeep Rainaaya. MI)
`Ira]. Rampil, MD
`Gunter Rnu, MD
`Michael F. lloizen, MD
`J.“A. Rolnautl, MD
`William B. Runcimnn, MD
`Keith Ruskin, MD
`David A. Sainsbury, MD
`H. Seliillings, MD
`John W. Severinghaus, MD
`M. Michael Shabot, MD
`Dean Forrest Sittig, I’l'il)
`Bradley E. Smith. Ml)
`Richard Tepliek. MD
`Kevin K. Tremper, MD
`Jorge Urzua. MD. PhD
`Karel H. Wesseling, PhD
`D Wayne Westcnskow, PhD
`J. C. Wyatt. PhD
`
`llflllllflE l4. 8 ISSUES. 1993
`Issues include:
`Brief Reviews. Editorials, Clinical Articles, Research Ar-
`ticles, Educational Articles, Book Reviews, Correspondence,
`Selections From the Anesthesia Patient Safety Foundation
`Newsletter.
`
`Killil’t'l‘flt‘m‘lt'lillt' l’iiblisheir. Dorelrecht, Boston, London
`
`Kluwcr Academic Publishers Online
`Visit our website at http:/ll \\-‘\vw.wkap.nl
`
`.Uouiwriug am." Computing is indexed in J'mlm'
`Thejnm'iml (If Chilled-l
`Mrdl'ms, Cum-m Commis/(L‘h'uiml :luh'i‘llll'llll'.
`[External
`.l-lrd'ioi. Clo-mu
`a“lll'sil'{’”li'.\.\' in Bililqeiml Stir-"res, and the Bil-inrifiml Engineering Ciriiiimi
`index. and is indexedlabslractcd in [Engineering Index,
`.Si'iwm- Gram”.-
`index, lisp-1min." Ri‘s‘mn'h Alert, .-\ {will}! Dm'riim'imm'arr St'rm'rt'.
`
`II
`(cid:44)(cid:44)
`
`2
`
`

`

`
`
`I This material may be protected by Copyright law [Title 17 U.S. Code]
`
`403
`403
`
`
`
`BEFLEBTflNGE PULSE UXIMETHY- PRINCIPLES AND
`flBSTETHIIi APPLIBATIUN 1" THE ZUHIBH SYSTEM
`Vcillccr Kin-rig, Renate Htttlr, and Albert Hurlt
`
`From the l’e’rinatn] Physiology Research Department, Department
`of Obstetrics, Zurich University Hospital. CH—SU‘JI Zurich. Switzer—
`land.
`
`Received Nov I3, 1997, and in revised form Jun 23, 1998, ACccptcd
`i‘or pubiication Aug 4, l998.
`
`Address correspondence to Voikt‘l' Kfinig, l’erinntal Physiology Re—
`search Department, Department of Obstetrics, Zurich University
`Hospital. iii-178091 Zurich. Switzerland.
`E-utail: vlsgttt'fltkalszch
`
`Julfflifllihrc1llfllt‘dli‘u'illlflli'lJl'i-JgfllfllCllfll'pil'lltl'lt: 14: 403—412. 1998.
`{C} 1998 Kliimet'Armlemir Publishers, Printed in the mammals,
`
`Kiinig V. liuch R. Hucli A. Reflectance pulse oxiinetry — principles
`and obstetric application in the Zurich system.
`J Clin Monit 1998: 14:403—412
`
`MISTHM". Transmission and reflectance are the two main
`
`Inodcs ofpulse oximetry. In obstetrics, due to the absence ofa
`transilltnniuable feta] part For transmission oximetry, the only
`Feasible option is the reflectance mode, in which sensor and
`detector are located on the same surface of the body part.
`[-lowever, none of the reflectance pulse oximeters developed
`For intrapartum use are fully satisfactory, as indicatL’d by the
`Fact that none have entered routine use. We have designed,
`developed, constructed and tested a reflectance pulse oximeter
`with the possibility to adjust the electronic circuits and signal
`processing in order to determine the effects of various para m-
`eters on signal amplitude and wave—form and to optimize the
`sensitivity and spatial arrangement ofthe optical elements.
`Following an explanation of the principles of reflectance
`pulse oximetry, we report our experience with the design,
`development, construction and field-testing of an in~housc
`reflectance pulse oximetry system for obstetric application.
`
`KEY WflflflS. Oxygen saturation, reflectance pulse uximetry.
`intrapartuni fetal monitoring.
`
`INTRIIDIIBTHIN
`
`Pulse oximetry is the combination of spectrophotome—
`try and plethysmography. It permits rapid noninvasive
`measurement of arterial oxygen saturation with the
`added advantages oFsimple sensor application and direct
`measurement, requiring neither calibration not pre—
`adjustment. Pulse oxitnctcrs are thus in widespread 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 a light emitter on one side of the body
`part to a 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 “transiNominated.”
`In obstetrics, fetal oxygen status during labor is a
`crucial parameter. However, no transilluminablc Fetal
`part is available. The only option in this case is reflec—
`tance oximetry [2], using a sensor with its light emis~
`sion and detection elements on the same surface of
`
`the body part. Various types of such a reflectance pulse
`oxilnctcr have been developed For intrapartum use at
`various locations. However, for a wide variety of reasons,
`all are still experimental and not in full routine Lise [3—7].
`Basically a reflectance measurement can be achieved
`using planar sensors — which can be produced,
`for
`example. by modifying conventional transmission sen-
`sors — and a sensitive modern pulse oximcter. However,
`
`3
`
`

`

`4m jam-naillif‘Clinimlil-lanimrhiq.imlComparing l'iill-l Nan August'1998
`
`such instruments come with a “black—box" microproces-
`sor—controlled mode ofoperation making constructional
`adjustments to the electronic circuits and signal process—
`ing virtually impossible. As a result. it becomes diflicult
`to determine the effect of various parameters on signal
`amplitude and wave-Form, optimize sensor sensitivity In
`light intensity and the arrangement of the optical ele-
`ments, and hence assess the dependence ofarterial oxygen
`saturation measurement 011 key physical, technical and
`above all physiological variables. This was the aim
`driving our decision to design, develop and construct
`in-housc a system dedicated to obstetric applications.
`Following a brief review of the principles of pulse
`oxinietry, we report our experience with the develop—
`ment of the new device,
`together with some field-
`testing results.
`
`
`
`
`PBINBIPLES IJF PULSE DXIMETRY
`
`Signal recording
`
`Light is absorbed on passing through matter. The de-
`gree oFabsorption depends on the nature of the trans-
`illmninated material and the wavelength of the light
`employed. All optical techniques For determining arte-
`rial oxygen saturation use the marked dill—erence 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 can be measured, for example, as the
`photocurrent l(t) ofa photodiode, and is obtained From
`the Lambert—Beer absorption law as:
`
`Em.
`
`d(t)
`
`absorption coeflicient of reduced hemoglobin
`(Function of wavelength)
`time Function ofinean pulsatile change in artery
`thickness, with amplitude d = d(diastole) —
`d(systole)
`
`transmitted
`Measurement may be impaired by light
`directly from the light source to the receiver or light
`which does not pass through arterially perfused tissue.
`If this “direct light" 1,“, is taken into account, Equation
`(I) changes to:
`
`[(0 = ltmuu ’K'NP(’(S()2'51-Iisti+
`(1 7 $03) - EH“ 1“”) + [dir
`
`(la)
`
`where
`
`lrissnc : [ll
`
`’ CXP(_5Ei~s\{s‘ ' 5)
`
`Since the pulsatile component of the absorption is at
`most a few percent,
`i.e. the exponent of the second e
`function in l:quation (I) is very small, we can Lise the
`approximation:
`
`cxp(x} : 1+ x
`
`For N ((1
`
`to obtain the very close approximation:
`
`lit) :
`
`.
`.
`h..<i.."(1’i502 ' 5HM} + (I _ 502) ‘ SIH‘ ‘ ‘10)) + l.IIr
`
`(lb)
`
`This light intensity is measured in the photodiodes and
`can be broken down electronically into two compo—
`nents. a time—independent signal
`
`]-)C : lllnue + Idir
`
`with amplitude equal to the value oFthis signal
`
`(2)
`
`(3)
`
`1m = L. . u— - s) - opt—(so: - also +
`(I i 502) '5I-Ibl ‘Llitll
`
`r)
`I
`
`at- = or; = 1,.,,,,,. + 1,",
`
`where
`
`intensity ofincident light
`h.
`5mm in ‘an absorption coefficient oftissue (function of
`wavelength)
`mean thickness oftransilluminated tissue
`
`s
`
`303 oxygen saturation to be determined (2 HbOf
`(HbO + Hb), Le. ratio of oxygenated hemoglo—
`bin concentration to sum of oxygenated and
`reduced hemoglobin concentrations)
`Ema) absorption coel‘licicut til-oxygenated hemoglobin
`(function ofn-'a\-'elengtli)
`
`and a signal which varies in time with the pulsatile
`change in artery thickness
`
`A(: : Itluue ' (St-J2 ' Elli“) + (1 ‘_‘ 5(32) '
`
`|Ellb) ' (Kt)
`
`(4)
`
`with amplitude
`
`ac = 1(tliastolc} — [{systole) =
`
`hit-up ' ($02 ' mm + i1 — 503) '
`
`€th "1-
`
`The ratio between the ac and dc amplitudes is then
`
`4
`
`

`

`Ktiiiitlrtrri: '1"ananirhOlrsrcn-ir itefiertantt'Putin-(lrimrn'r 4%
`
`1‘: SIC/dc :T' “lustre/(Insane + [thru-
`(502 ‘El-llit)+(l —502l'511hl‘d-
`
`6
`
`l
`
`(
`
`'10],
`However, despite various theoretical models |‘),
`the scatter coefiicients of the various tissue types are
`not known with sufficient accuracy to permit exact
`
`this ratio r is
`in the case that "direct light" 1,“, : 0,
`independent of the incident light intensity I“ and ofthe
`absorption in the nonpulsating tissue value 1“,“:
`
`calculation. Experimental calibration thus has to be
`performed by directly comparing the pulse oximeter
`readings with arterial blood sample values.
`
`1- : ac/dc =
`(SOE'EI-Iho—l'll "SOEl'Sl-Ilwl‘d
`
`{01' [dir = U.
`
`(a
`lb)
`
`Ti'flllSHlfSSftHl' pulse (trimt’fry
`
`is then dependent only on the oxygen
`This ratio r
`saturation 803 to be determined, the known absorption
`coefiicients 614130 and 5H1“ and the mean pulsatilc change
`d in the thickness of the arterial vessels in the trans-
`
`illuminated region.
`To eliminate this dependence on d, the measurement
`is performed at two wavelengths with maximally dif-
`fering absorption coefficients. On the assumption that
`the d values are the same for both wavelengths, we
`obtain a variable
`
`R = r,,.,1/r,-r = {ac/dclmi/[ae/dchr
`ll
`
`(502 ’ 5“so ‘i' (I — 502) ' El-llilrt-d/
`
`(SO: ’ Ellbt‘) + (l — 502) ' Ens)”
`
`(F)
`
`(73)
`
`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 attachment 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
`of the 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 803 is readily calculated
`without knowing the incident light intensity or tissue
`thicknesses.
`
`Calculation assumes the Following physical prerequi—
`sites:
`
`I No light must be measured that has not passed
`through the pulsatile vascular bed e.g. light passing
`directly from light source to receiver (I‘m).
`0 The pulsatile changes in artery thickness must be the
`same for both wavelengths,
`i.e. both wavelengths
`must transilluminate the same tissue region
`I 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 ofpulsatile changes in optical
`thickness, c.g. hemoglobin derivatives, circulating
`pigments, pulsatile changes in thickness produced
`mechanically in nonartcrially perfused tissue by car—
`diac action, and, above all, venous pulsation.
`0 To simplify description ofthc principle behind meas-
`urement and its limitations,
`the Lambert—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 oflimited applicability
`[8L The exact absorption coefficients must be cor—
`rected by taking the scattering effect into account.
`
`Reflectance pulse oximetry
`
`In this method the light backseattered 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 indiccs. This means
`
`that on physiological grounds, strong reflections can be
`expected on the entry of light into bone. The trans-
`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 acfdc ratio
`off).[]0lill.0()5 can be achieved on the forehead. Perfuu
`sion is also good over the sternum. One method of
`signal enhancement is to h ‘at the measurement site to
`induce hyperpcrfusion, which can safely be performed
`up to 42°C. A rubcfacient, e.g. nicotinic acid (Rubri-
`inent), can also be applied to the measurement site.
`The principal physical limitations are the following:
`
`o The sensor design must eliminate “direct light," i.e.
`light passing directly from the light sources to the
`photodiodes or that is only scattered in the outer part
`ofthc skin.
`
`I The measured AC signals are some 10 times weaker
`
`5
`
`

`

`406 journaligf'CliniralMonitoringand Computing Valli-“F Nné August 1998
`
`than in the transmission method. The conditions
`
`governing the heating of the light—emitting diodes
`(LED) limit
`the potential
`for producing stronger
`signals by increasing the incident light intensity: not
`only can high uncontrolled 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 he
`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—
`her. 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 of sensor.
`0 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 iu~
`corp orated in the sensor unit.
`
`Numerous types of sensor meeting the above require-
`ments were built. A sensor used for intrapartuni 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 ofapproxin‘iately 100 mbar.
`
`
`THE leIIIIIH HEFLEBTAI‘IBE PULSE flXlMETEB
`
`Electronics
`
`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 l).
`
`o For maximal independence from local tissue differ—
`ences. a radially—symmetric pattern was selected for
`the photoclcmcnts. The light source — a chip with
`two LEDs for the wavelengths red = 660 11111 and
`infrared Z 920 nm — was placed in the center of the
`sensor and surrounded by a radial photodiodc 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 3X33 photodiodcs
`(Siemens) were used with a mean radius ofl7 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,
`I Unlike with transmission sensors,
`
`fixation to the
`
`measurement site can pose problems. Aluminum 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 analogfdigital converter (ADC),
`including signal amplification, filtering, and separation
`into DC and AC signals. all handled electronically ; thC
`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:
`
`I Time control ofmeasurement
`
`From a rectified mains power supply signal (100 Hz),
`a phase-locked loop (I’LL) — 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 I’LL clock drives a
`‘ 7-bit counter that addresses an EPROM giving a data
`cycle ofl kHz, 64 pulses long. The output pulses of
`the El-‘ROM control
`the entire sequence of light
`emission. signal acquisition and signal processing.
`a LED drive
`
`The oppositely poled red and infrared LEDs are
`located in the output circuit of a current—stabilizer‘l
`push-pull output stage. They are triggered by digital
`signals from the control unit via analog switches at
`the l kHz sampling rate in the equally spaced sc—
`
`6
`
`

`

`Kr'im'g (‘r a}:
`
`'I'Frr' Km mu {)hsrm'r‘r Rqflr’r'hrm‘c' Pm'xt' U.\'rmr’u'r
`
`4| 1?
`
`
`
`
`
`@ Vacuum-tube
`(D Photodiodes
`® Signal—cable
`(9 Red and Infrared LED's
`® Light barrier
`
`Fig. '1" (Truss-senior: Hu'orrgh a silirunr-rnl’flm‘ rcjfim'rfon rm_~'or_,"orfurrapn-rmm UH'l’UHJ'L'lHl'HF u ‘oxygr-n nmnmr'mr.
`
`7
`
`

`

`408 jmu'ual’afCiim‘miMonitoringand Computing Vol“ M16 Aii‘girsr1998
`
`
`
` Pre—
`
`Amplifier
`
`S & H
`
`
`12 bit
`
`- Dark-Curr.
`ADC
`
`
`Subtract.
`Computer
`
`
`
`
`
`
`
`Ext. Device
`
`
`Oscillator/Control Unit
`
`(HP. Nelieor)
`
`
`
`Fig. 2. Sriienmrir representation afmmsm'emrm elerfrmtirs.
`
`qucnce: 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 photocorrent sup-
`plied bv the photodiodes is converted to a voltage
`and then amplified. Six switch positions permit am—
`plifications of 50 menA to 5 anA. Afterwards,
`three sample—and—hold (SM-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 subs
`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—time signals at the
`SStl-I output are reconvened 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 of0.1 Hz. The AC components are passed to :1
`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 DC... DCM, AG, and
`ACrcd are thus available at
`the output. As oxygen
`saturation is calculated from the ratios (acidchT and
`(acfdc)rcd, 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.
`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 Still—410°C to an accuracy of0.1 ”C.
`Vacuum pump
`The sensor is fixed to the skin using a small pump
`producing a maximal -30{l inbar vacuum. The pump
`
`8
`
`

`

`Kantian! al': The Zurirh Obstetric Rtjllrrmm'r l’rrlsr Uxma'trr
`
`wllJ‘}
`
`
`
`60 Min
`0 sec File: LRUBL922.PX2
`Start: 23. 1.9? 8:19:21 Tot
`RPDX
`302 Z
`Lxxxxxxx Rxxx 92b. 20.03.63 US Datun 23.
`
`
`
`HPDX HFI bDH
`
`
`
`
`
`
`
`
`
`llt’fl” rare, hear! "‘1l"Jl'U"‘ HP CTG '“l’l’lmll
`Fig. 3. Screen display erI 10-minute measurement. From top to lmlfmt'.‘ tlA'jflt’t'Ii WINNING”.
`incrim- murmrtionjiwu HP CTC mauitm: L-'Iidrrireatl'1 the DC and .‘IC signals-jar infrared and red lighrjiir the Mar 5 strands (”are the
`d[fli’i‘trrr ordinate stalesftu' DCand AC).
`
`is maintained electronically via a manometer at a
`preset negative pressure. In normal medical use, ad—
`equate fixation is achieved with a vacuum of about
`—l(](l Inbar.
`
`Software
`
`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 (Metrabyte). A counter on this card
`triggers analogfdigital conversion at 400 Hz, which
`starts an interrupt program in the computer For read-
`ing the 4 measured values DC", DC,“ and AC”,
`AC,“ into a cyclic buffer of 5-sccond length, From
`the signal (ACi,,+ACn.d) the maxima and minima are
`determined for each cardiac cycle, and hence the
`instantaneous heart rate. Oxygen saturation; are calw
`culated from the amplitudes ac and dc oFthe AC and
`DC signals.
`Saturation is calculated from the Lambert—Beer
`
`the
`law using the absorption eoefiicients [11] For
`as
`nominal wavelengths of our LEDS. However,
`the actual wavelengths may deviate from these nom—
`inal values and the applicability ofthe Lambert-Beer
`law is limited by scattering in tissue, experimental
`
`9
`
`

`

`410 journal nfCiiuimi r'l/[mrimriug and Computing Vol 14 No 6 August 1998
`
`calibration ofthe measured saturation values is essen—
`tial.
`
`Calibration
`
`Saturation and heart rate can be averaged over 1—9
`cardiac cycles and are displayed every second. Analog
`heart rate and uterine contraction signals from any
`CTG monitor with analog output (eg. 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 DCir. DCmi and AG“
`ACmd, 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.
`I 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.
`
`
`MEASIIREMENTS fiflll BISEIISSIIIN
`
`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~
`ranieters not previously measured in both physiolog—
`ical and pathological situations. but also to evaluate
`the diagnostic significance ofsuch 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), (iraz (A), Otilu (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 =1',.\:I/r;I : (ac/dcjmd/[ac/dc)ir
`
`cf {7)
`
`on the basis of an experimental or theoretical relation-
`ship. Initially we used the absorption coefficients of
`Zijlstra ct all [11]. The general problems of calibrating a
`pulse oximeter have been discussed elsewhere [12, l3|.
`For fine calibration we performed the Following inves-
`tigations:
`
`o Tests in the arterial oxygen saturation range 88—
`10(an 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 catheterization, refer—
`ence values were provided by a MINOLTA PUL—
`SOX 8 transmission pulse oximcter attached to the
`index finger. Data analysis [13] showed a 4.5”u differ-
`ence in oxygen saturation between the MINOLTA
`and the preliminary results of our reflectance system
`based on the absorption coefficents onisztra et a].
`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].
`O Low saturations in viva can also be measured in the
`
`fetal Iainb [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 l[}~1()()% saturation range by a factor of
`1.045.
`
`Fixation
`
`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 ofthe 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 ma mhar is the most suitable pressure
`
`10
`
`10
`
`

`

`at which to maintain the sensor as it ensures good
`sensor-skin contact and reliable continuous Fetal oxygen
`saturation values. Mea