Bergmans et al, Measurement and processing of fetal Pco2 levels 369

Original articles

J. Perinat. Med.15 (1987) 369

Measurement and processing of fetal transcutaneous Pco2 levels

Martin G. M. Bergmans, Herman P. van Geijn, Herman van Kessel, Jerome I.Puyenbroek, and Nico F. Th. Arts

Department of Obstetrics and Gynecology, Free University of Amsterdam, TheNetherlands

1 Introduction

The purpose of each obstetrician is to have allchildren delivered in an optimal condition. Sur-veillance of the fetal heart rate should be a guar-antee in this regard. Indeed, an optimal cardio-tocogram is in 95% of the cases associated withan optimal fetal condition. Although electronicfetal monitoring has contributed to a considerabledecrease in perinatal morbidity [25] it must bereminded that asphyxia is not the only factordetermining the fetal heart rate. Other influencesare e. g. fetal age, behavioral state, congenitalanomalies, maternal infections and drug usage [20,21]. Therefore, a non-optimal cardiotachogram(fetal heart rate pattern) can be a sign of fetaldistress, but this is not necessarily always true.In case of a suspicious cardiotachogram furtherevaluation of the fetal condition is advisable, es-pecially determination of the acid-base equili-brium, which has been shown superior in predict-ing neonatal condition [13, 26, 27].However, the method of fetal blood sampling hasseveral disadvantages. The measured acid-baseequilibrium is only representative for a very shortperiod of time, since fetal pH may decrease rapidlyduring labor [8]. Therefore, repeated micro bloodsampling is often necessary. Since each samplingoften requests a long lasting vaginal examination,the procedure interferes with the normal laborprocess and is cumbersome to the laboringwoman. With regard to the fetus: each samplingis invasive and fetal bleedings and infections havebeen described, possibly contributing to an in-crease of fetal morbidity and mortality [11].

Continuous non-invasive surveillance of fetal oxy-gen supply or its reflection on the acid-base equili-brium, such as fetal transcutaneous Pco2 (tcPco2)does not have these drawbacks and appears to bea more acceptable and preferable technique, ifreliable and adequate assessment of the fetal con-dition can be obtained.A major problem with regard to determination ofthe acid-base equilibrium in intermittently ob-tained fetal blood samples is the inclusion of airbubbles in the sample. When air bubbles are intro-duced into the electrode cuvettes of an acid-baseanalyzer, the measured values cannot be consid-ered reliable. In the Department of Obstetrics andGynecology of the Vrije Universiteit of Amster-dam, this problem was solved by a "pipe"shapedspecial collecting vessel and the percentage of suc-cessful samplings increased significantly. Thisspecial collecting vessel will be discussed.

2 Transcutaneous surveillance of the fetus

Since years it has been possible to measure Poitranscutaneously (tcPoi) in the neonatal period.A good correlation was found between tcPo2 andsimultaneous Ρθ2 measurements in arterial blood[7]. In the fetus during delivery however, there aresome specific problems that make interpretationof continuous tcPo2 curves more difficult. Duringthe first stage of labor tcPo2 values are close bycorrelated to Ρθ2 values in fetal blood samples, butduring the second stage of labor this correlation islost [16]. During the second stage of labor the

1987 by Walter de Gruyter & Co. Berlin · New York

370 EC Project "Perinatal Monitoring"

fetal head rotates in the pelvis and the tcPo2 elec-trode can experience pressure by the bony pelvis,resulting in too low measured values [10].Secondly, the formation of a caput succedaneumdecreases capillary circulation of the skin beneaththe electrode which results in a falsely reductionof recorded tcPo2 levels [6].

For about 8 years, measurement of tissue pHhas been possible. Studies performed during laborshow a close correlation to the pH of blood ob-tained from the fetus according to the micro-blood sampling technique introduced by SALING,and the pH of umbilical artery blood [9, 23].However, the current status of continuous pHmeasurement of the fetus does not permit wide-spread introduction into clinical practice. Thetechnique is invasive and has risks comparablewith fetal blood sampling. Following applicationof a pH electrode, development of a fetal skinabscess has been described [1]. The distance be-tween the skin surface and the tip of the electrode,as well as the angle under which the electrode isplaced (it should be placed perpendicular), arecritical to obtain reliable measurements [3]. Evenafter extensive training the percentage of success-ful measurements is only 75% [24].

Recently, transcutaneous Pcoz measurement be-came available. The transcutaneously measuredvalues of fetal Pcoi are higher than simultaneouslymeasured values of Pco2 in fetal arterial blood.There are two reasons for the slightly elevatedPco2 levels obtained transcutaneously. In the firstplace, the electrode usually is heated to 39 — 44 °C.Secondly, the upperlayer of the epidermis has itsown C 2 production [17].

A close correlation has been found between thetcPco2 and the Pco2 level in fetal capillary bloodand in umbilical artery blood [4, 19]. In contrastto continuous monitoring of tcPo2, reliable meas-urements of tcPco2 appear to be possible duringthe second stage of labor [16, 19]. The develop-ment of a caput succedaneum appears to decreasereliability of tcPco2 levels slightly, but even in itspresence a statistically significant correlation toPco2 values of fetal scalp blood samples has beendescribed [15]. Continuous transcutaneous Pcoimonitoring in the fetus might therefore be a valu-able method for fetal surveillance. Presently avail-able literature indicates that every acute deteriora-tion of the fetal condition during labor was pre-ceded by a raise of fetal tcPco2 [15].

3 Fetal tcPcoi research and computerizationThe working group "New methods" of the Euro-pean Community project "Perinatal Monitoring"is evaluating fetal tcPco2 monitoring in a multicen-ter study. The Department of Obstetrics and Gy-necology of the Vrije Universiteit of Amsterdamis one of the co-operating centers. Following theintroduction of this multicenter study, we devel-oped a plan to record automatically a multitudeof variables such as fetal heart rate, fetal tcPco2and maternal uterine activity.

As our department has a special interest in factorsinfluencing fetal heart rate patterns, we will com-pare the predictive value of the fetal tcPco2 mea-surements with that of the cardiotachogram. Forthis purpose a quantitative analysis of both vari-ables of the fetal condition is desirable.

Besides, we record maternal tcPco2 levels to studythe influence of the maternal levels on the fetaltcPco2. During pregnancy there is an increase inmaternal alveolar ventilation of 60 to 70% accom-panied by an increased maternal Ρθ2 and a de-creased Pco2 [2, 5]. During labor this hyperventila-tion is more pronounced and because of a reducedmaternal expiratory reserve volume, markedchanges in partial pressures may occur [5]. Hyper-ventilation also causes vasoconstriction resultingin reduced blood supply to different organs. Ex-perimental research in sheep showed a reductionof uterine blood flow after hyperventilation [12].Research in human showed an increased incidenceof metabolic acidosis of both mother and fetusfollowing maternal hyperventilation [14].

To reach our aim we need to record: fetal heartrate, maternal uterine activity, fetal tcPco2, heatproduction of the fetal tcPco2 electrode, maternaltcPco2, heat production of the maternal tcPco2electrode, and events during labor (e. g. rupture ofthe membranes, vaginal examination, fetal bloodsampling etc.).

Under usual circumstances the cardiotocogram isrecorded with a Hewlet Packard 8030 on paper ata speed of 1 or 3 cm/minute. The fetal and ma-ternal tcPco2 are recorded on paper at a speed of1 cm/2 minutes (Radiometer equipment). Devia-tion of the heat production of both electrodes canbe collected by digital read out. Events are writtendown in a note book. Such a situation makesevaluation of data very complex. Computerizationof these data therefore is desirable and has thefollowing advantages:

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Bergmans et al, Measurement and processing of fetal Pco2 levels 371

Synchronization of signalsMany variables can be recorded syn-chronously which solves the problem of speeddifferences between the various recording ma-chines.No limitation of the number of signals thatcan be recordedFrom the Radiometer equipment the twotcPco2 signals of mother and fetus and the twoaccompanying heat deviation curves can berecorded continuously.Data reductionThe current situation creates a disorderly stockof different files with a lot of different papers,making evaluation a tough job. Stocking themin a computer gives a clean desk.Orderly and variable displayIt makes presentation of the material easierand more flexible and provides more insightinto it.Quantitative analysis

We already have some experience with computer-ization of data. The technique was at first used infetal behavioral studies, done from 1981 onwardsin our department [22]. Figure 1 is an example ofa recording of fetal heart rate and a variety offetal movements in relation to fetal behavioralstates.Since the earlier described study on fetal and ma-ternal tcPcoz levels is performed during labor, itis not possible to transmit all data directly intothe computer which we use for further processingand evaluation. For this reason we apply a digitalVT103 terminal with built-in LSI-11/02 processormultifunction board MXV11, 16KV RAM, 8-channel AD-converter, parallel unit DRV 11 anddual TU 58 tape drive for DECTAPE-II cartrid-ges; an interface, developed by our technical ser-vice center, is used to adapt the fetal heart ratesignal of the Hewlet Packard 8030 A to theDRV 11 parallel unit. It is a small, easily trans-portable unit that can be placed in every labor

FETAL HRE B . P . N . l

UTER ACT[ HMHG ]

128

TINE 16 :θθ 16:10 16:15 16:2Θ 16:25 16:30

PEDAL 1EYE HOVH

JAU OPENGRIMACE

YAUN

ROTAT HEADRETROFL HD

GENERAL HVISOL ARM

BREATHINGHICCUP

PEDAL 2

Figure 1.

J. Perinat.

IB I IB l M

-I 1 1

-H • I ·I I II

-H H—I--H h-

Fetal heart rate and a variety of fetal movements in relation to fetal behavioral states.

Med. 15 (1987)

372 EC Project "Perinatal Monitoring"

room. All signals are transmitted synchronouslyto the datalogger. Events are marked by a pedal.A continuous registration of four hours is easilyobtainable. Afterwards, the information filed onthe tape cartridge is copied to the PDP-11/34computer for proper evaluation, including quanti-tative analysis. The software for the datalogger,also developed by our technical service center, iswritten in fortran IV.

4 Method for de-bubbling fetal scalp blood sam-ples

Blood samples, taken from the fetal scalp andcollected into long glass capillaries, often containair bubbles.

When one introduces the content of such capilla-ries directly into a pH-bloodgas-analyzer, onecauses a disturbance of the measurement becauseblood as well as air bubbles will enter into themeasuring chambers and the connecting capilla-ries. These air bubbles lead to false values for theΡθ2 and the Pco2 of the sample because of theirdifferent 2 and COi content and the air bubblesinfluence the pH when they affect the conductivitybetween the pH electrode and the reference cell.

To prevent these drawbacks we drain, after collec-tion in a regular glass capillary, the fetal scalpblood samples into a special "pipe"shaped collect-ing vessel (figure 2), by which procedure the airbubbles escape from the blood sample. The "pipe"is subsequently connected to the suction device ofthe analyzer.

SILVERMAN already showed that admixture withair as such, does not influence the values for pHand the blood gasses in fetal blood samples [18].

In our laboratory it was investigated whether thedebubbling procedure described above affects thevalues for pH and blood gasses. A venous bloodsample from an adult volunteer was introducedinto a glass syringe of 10 ml. This syringe alreadycontained some glass beads and a heparin solu-tion. The plunger was in the most downwardposition before the blood was introduced. The airwas removed. Next the content of the syringe wasmixed thoroughly by gentle turning the syringeupside down many times. Then 12 hockey stickshaped glass capillaries of about 40 cm of lengthand having a content of about 0.15 ml, were filledanaerobically from the syringe. Four of these ca-pillaries were drained into the "pipe"shapedspecial collecting vessels, just prior to the measure-ment of the sample (one sample was lost). Thenext four samples were directly introduced fromthe glass capillary into the analyzer. The last foursamples again were introduced into the collectingvessels prior to their measurement. The wholeprocedure took 43 minutes.

The results are summarized in table I. The valuesof the table show that the procedure of transfer-

Table I. The effect of transferring blood from glasscapillaries (CAP) into special collecting vessels (SCV)on the values for pH and the blood gasses (KPa). Theinstrument used was a Corning 178 pH/Blood Gas Ana-lyser.

pH PC02 P02

Figure 2. Pipe-shaped special collecting vessel.

SCV

mean

CAP

mean

SCV

mean

123

4567

89

1011

7.3987.3927.4007.397

7.3977.3947.3977.3897.396

7.3897.3887.3947.3897.390

5.695.825.725.76

5.855.735.825.975.82

5.885.915.875.975.91

4.124.164.204.16

4.154.074.104.254.16

4.204.214.244.254.23

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Bergmans et al, Measurement and processing of fetal Pco2 levels 373

ring blood samples from the glass capillaries intothe "pipe"shaped special collecting vessels, has noeffect on the values for pH and the blood gasses.We concluded that blood samples, collected in aglass capillary and contaminated with air bubbles,may validly be analyzed after having been de-bubbled by the method described above.

5 Conclusions

Evaluation of fetal acid-base equilibrium has pro-ven to be beneficial in fetal surveillance, especiallyin cases of suspicious fetal heart rate patterns.

The currently used fetal blood sampling techniquedeveloped by SALING has an invasive characterfor both mother and fetus. Furthermore, it reflectsonly one measuring point in a continuously chang-ing situation. Continuous tcPco2 monitoring doesnot have these disadvantages. Therefore, when themeasurements are proven to be reliable, it will bea valuable addition to our diagnostic possibilitiesregarding prediction of fetal distress.Automatic recording and processing, and quanti-tative analysis of fetal tcPcoi curves in relation toother parameters of fetal and maternal conditionwill provide more insight into the risks of thelabor process for the fetus.

Summary

Because asphyxia is not the only factor influencing fetalheart rhythm, a non-optimal cardiotachogram is notnecessarily a sign of fetal distress. It makes furtherevaluation of the fetal condition advisable, especiallydetermination of the acid-base equilibrium.The method of fetal blood sampling, introduced bySALING, has a number of disadvantages for mother andfetus, because of the invasiveness for both. Further, themeasured acid-base equilibrium is only representativefor a very short period of time and often repeated micro-blood sampling is necessary.A major problem with regard to determination of theacid-base equilibrium in intermittently obtained fetalblood samples is the inclusion of air bubbles in thesample. When they are introduced into the electrodecuvettes, the measured values cannot be considered reli-able. The problem was solved in the Department ofObstetrics and Gynecology of the Vrije Universiteit ofAmsterdam with a "pipe"shaped special collecting ves-sel. Similar measuring results were obtained with theformerly used glass capillary method and the specialcollecting vessel method.Continuous, non-invasive methods have been pursuedto avoid the above mentioned problems. Fetal transcu-taneous Ρθ2 measurement has been possible for years,but does not provide adequate information during theimportant second stage of labor because of methodologi-cal problems.Continuous fetal tissue pH surveillance is possible, butit also has an invasive character and is technically diffi-cult to perform, leading to many methodological failu-res.

Recently, continuous transcutaneous Pco2 measurementtcPco2 became available. A good correlation was foundwith simultaneously measured Pco2 levels in fetal bloodsamples and with those of umbilical artery blood.Presently, available literature indicates that acute de-terioration of the fetal condition during labor was al-ways preceded by a raise of fetal tcPco2. Therefore, fetaltcPco2 surveillance potentially is a valuable adjunct tomonitor the fetal condition, if reliable and adequateassessment can be obtained. The method will be evalu-ated in a multicenter study of the working group "NewMethods" of the European Community project "Perin-atal Monitoring".In our department we are comparing the predictive valueof fetal tcPco2 measurement with that of the cardiota-chogram. Besides, we are studying the influence of ma-ternal tcPco2 on fetal tcPco2 levels. To reach our aim weneed to record: fetal heart rate, maternal uterine activity,fetal tcPco2, heat consumption of the fetal tcPco2 elec-trode, maternal tcPco2, heat consumption of the ma-ternal tcPco2 electrode and events during labor. Com-puterization of these data is desirable and has followingadvantages:— synchronisation of signals,— no limitation of the number of signals that can be

recorded,— data reduction,— orderly and variable display,— quantitative analysis.Our technical service center developed a system in whichthe data are collected by a digital VT103 dataloggerand then transmitted to a P. D. P. 11/34 computer forfurther processing and evaluation.

Keywords: Capillary blood analysis, fetal monitoring, measurement error, transcutaneous blood gas monitoring.

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Zusammenfassung

Messung und Auswertung der fetalen transkutanen Pcoz-WerteWeil eine Asphyxie nicht der einzige Faktor ist, derdas fetale Herzfrequenzmuster beeinflußt, ist ein nichtoptimales Kardiotokogramm nicht unbedingt ein Zei-chen eines „fetal distress". Es ist deshalb wünschenswert,eine weitere Untersuchung des fetalen Zustandes zu be-rücksichtigen, insbesondere durch die Bestimmung desSäure-Basen-Haushaltes.Die Methode der Fetalblutanalyse, die SALING entwik-kelte, zeigt eine Reihe von Nachteilen für Mutter undKind, weil sie für beide invasiv ist. Der gemessene Säure-Basen-Haushalt ist weiterhin nur für sehr geringe Zeitaussagekräftig und eine Wiederholung der Fetalblutana-lyse ist oft notwendig.Ein großes Problem angesichts der Bestimmung des Säu-re-Basen-Haushaltes besteht darin, daß in den intermit-tierend gewonnenen fetalen Blutproben Luftblasen auf-tauchen können. Wenn sie in die Elektrodenküvetteneindringen, können die gemessenen Werte nicht zuverläs-sig bewertet werden. Dieses Problem wurde in der Abtei-lung für Geburtshilfe und Gynäkologie der Freien Uni-versität in Amsterdam durch die Einführung eines be-sonderen Sammelgefaßes gelöst. Ähnliche Meßergeb-nisse wurden mit der früher benutzten Glaskapillarme-thode und der besonderen Sammelgefaßmethodeerreicht.Kontinuierliche, nicht-invasive Methoden sind unter-sucht worden, um diese obengenannten Probleme zuvermeiden. Transkutane 2-Messungen sind seit Jahrenmöglich gewesen, aber sie liefern aufgrund methodischerProbleme keine ausreichende Information während derwichtigen zweiten Phase der Geburt.Eine kontinuierliche fetale Gewebe-pH-Überwachungist möglich, aber auch sie hat einen invasiven Charakterund bereitet technische Schwierigkeiten, die zu vielenmethodischen Mißerfolgen führen.

Neuerdings ist die kontinuierliche transkutane Pco2-Messung möglich geworden. Es wurde eine gute Korrela-tion mit den simultan gemessenen Pco2-Werten in fetalenBlutproben und mit denen des Nabelarterienblutes ge-funden.Zur Zeit zeigt die vorhandene Literatur, daß einer akutenVerschlechterung des fetalen Zustandes während der Ge-burt immer ein Anstieg der fetalen tcPco2 vorausging.Daher ist die fetale tcPco2-Überwachung potentiell einwertvoller Zusatz, um den fetalen Zustand zu über-wachen, wenn zuverlässige und hinreichende Registrie-rungen erhalten werden können. Diese Methode wird ineiner Multizenter-Studie der Arbeitsgruppe „New Me-thods" des EC Projektes „Perinatal Monitoring" ausge-wertet.In unserer Abteilung vergleichen wir den vorhergesagtenWert der fetalen tcPco2-Messung mit dem des Kardioto-kogramms. Außerdem studieren wir den Einfluß mater-naler tcPco2- auf fetale tcPco2-Werte. Dazu sind folgendeRegistrierungen erforderlich: fetale Herzfrequenz, müt-terliche uterine Aktivität, fetaler tcPco2, Heizleistung derfetalen tcPco2 Elektrode, mütterlicher tcPco2, Heizlei-stung der mütterlichen tcPco2 Elektrode und Ereignissewährend der Geburt. Die Datenspeicherung im Compu-ter bietet folgende Vorteile:— Synchronisierung der Signale,— Keine Einschränkung der Zahl der Signale, die regi-

striert werden können,— Reduktion der Daten,— Option verschiedener Darstellungsformen der Ergeb-

nisse,— Quantitative Analyse.Unser Institut hat ein System entwickelt, das die Datendurch einen digitalen V 1103 Datenspeicher erfaßt; diesewerden anschließend auf einen P. D. P. 11/34 Computerzur weiteren Verarbeitung und Auswertung übermittelt.

Schlüsselwörter: Fetale Überwachung, kapillare Blutanalyse, Meßfehler, transkutane Blutgasmessung.

Resume

Acquisition et traitement de la mesure de la Pcoz transcu-tanee foetaleComme l'asphyxie n'est pas le seul facteur influencant lerythme cardiaque foetal, un trace de frequence cardiaqueimparfait n'est pas obligatoirement un signe de souf-france foetale. En presence de dernier il est souhaitabled'effectuer une evaluation complementaire de Fetat deFenfant et en particulier une determination de Fequilibreacido-basique.La methode de prelevement de sang foetal mise au pointpar SALING presente Finconvenient d'etre invasive ä lafois pour la mere et pour le foetus. De plus, Fevaluationde Fequilibre acide-base qu'elle fournit n'est representa-tive que d'une tres courte periode et des micropreleve-ments repetes sont souvent necessaires.

La presence de bulles d'air dans les prelevements consti-tue un probleme majeur pour la determination de Fequi-libre acido-basique ä partir des prelevements disconti-nus. Quand elles sont presentes ä Finterieur de Felec-trode, la mesure ne peut pas etre consideree commefiable. Ce probleme a ete resolu, dans le departementde Gynecologie-Obstetrique de FUniversite de Vrije äAmsterdam, au moyen d'un tube collecteur de formespeciale. Des mesures identiques ont ete obtenues avecles anciens tubes capillaires en verre et avec le collecteurspecial.Pour eviter les problemes mentionnes ci-dessus, on arecherche des methodes non invasives de mesure conti-nue. La mesure de la 2 transcutanee foetale est possibledepuis plusieurs annees, mais du fait de problemes met-

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Bergmans et al, Measurement and processing of fetal Pco2 levels 375

hodologiques, ne fournit pas d'information pertinentependant la seconde partie du travail pourtant impor-tante. La surveillance continue du pH tissulaire foetalest possible mais eile a egalement un caractere invasif etdu fait de s difficulte technique entraine beaucoupd'echecs methodologiques.Recemment, des methodes de mesure continue de la Pco2sont devenues disponibles. Une bonne correlation a etetrouvee avec la Pco2 mesuree sur des prelevements simul-tanes effectues simultanement sur le scalp du foetus ousur 1'artere ombilicale.Actuellement, la litterature disponible indique qu'unedeterioration aigue de l'etat de l'enfant au cours dutravail est toujours precedee par une elevation de latcPco2 foetale. La surveillance de la tcPco2 foetale repre-sente done un complement potentiel precieux pour lemonitorage de l'etat foetal, sous reserve d'en permettreune evaluation flable et pertinente. La methode seraevaluee dans une etude multicentrique du groupe detravail «Methodes nouvelles» du projet «MonitoragePerinatal» de la Communaute Europeenne.

Dans notre departement, nous comparons la valeur pre-dictive de la mesure de la tcPco2 foetale et celle des tracesde frequence cardiaque foetale. En outre, nous etudionsTinfluence de la tcPco2 maternelle sur les valeurs detcPco2 foetale. Pour cela, nous devons enregistrer: lafrequence cardiaque foetale, 1'activite uterine, les tcPco2du foetus et de la mere, les consommations de chaleurdes deux electrodes a tcPco2 foetale et maternelle et lesevenements survenant pendant le travail. Un traitementinformatise de ces donnees est souhaitable et presenteles advantages suivants:— synchronisation des signaux,— pas de limitation du nombre de signaux que Γόη

peut enregistrer,— reduction des donnees,— affichage ordonne et variable,— analyse quantitative.Notre centre de service technique a developpe un Systemedans lequel les donnees sont recueillies au moyen d'unenregistreur digital VT103 puis transmises un ordina-teur PDP 11/34 pour traitement ulterieur et evaluation.

Mots-cles: Analyse du sang capillaire, erreur de mesure, mesure de la Pco2 transcutanee, monitorage fetal.

Acknowledgements: We gratefully thank our computer specialist F. J. M. CARON and our technical service centerfor all the work they did to make registrations possible. We thank the study group on fetalbehavioral patterns of the Department of Obstetrics and Gynecology of the Vrije Universiteitof Amsterdam (J. T. J. BRONS, F. J. M. CARON, J. M. SWARTJES, E. E. VAN WOERDEN) for thedisposal of their material.

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Martin G. M. Bergmans, M.D.Department of Obstetrics and GynecologySt Laurentius ZiekenhuisMgr Driessenstraat 66043 CV Roermond, The Netherlands

J. Perinat. Med. 15 (1987)