Fetal carbon dioxide tension during human labour

Eur. J. Obstet. Gynecol. Reprod. Biol., 22 (1986) 205-216 Elsevier

205

EJO 00346

Fetal carbon dioxide tension during human labour Carsten Department

Nickelsen,

Sten Grove

Thomsen

of Obstetrics and Gynaecologv, Rigshospitalet, Accepted

for publication

and Tom

Weber

University of Copenhagen, Denmark

25 March

1986

Fetal carbon dioxide tension during labour is elevated in both metabolic and respiratory acidosis, but intermittent fetal blood analyses often fail to detect Pco, changes during acute complications. Transcutaneous carbon dioxide monitoring is continuous and the possibility of diagnosing Pco, changes is therefore better. The theoretical background for transcutaneous measurements and methods for clinical monitoring are described. Close correlations with capillary and arterial blood values have been found, and the atraumatic principle with a simple electrode application indicates a promising new method for acid-base assessment during human labour. acid-base;

fetus; fetal blood analyses

Introduction For more than a century the fetal heart rate (FHR) has been used for evaluation of fetal well-being during labour. During the last 20 years FHR and its relationship to uterine contractions have been more exactly described by continuous cardiotocography (CTG), but the CTG patterns are difficult to correlate to perinatal depression. However, animal experiments have demonstrated that the degree of acidosis, especially metabolic acidosis, and hypoxia during labour correlates to the seriousness of later brain damage [l]; this is the theoretical basis for biochemical monitoring of the fetus during labour. During normal placental blood perfusion fetal Pco, changes with alterations in maternal blood Pco, [2]. During decreased placental perfusion hypoxia and acidosis develop in the fetus and the fetal-maternal gap increases. Both respiratory and

Correspondence to: Carsten Nickelsen, Department Blegdamsvej, DK-2100 Copenhagen 0, Denmark.

0028-2243/86/$03.50

of Obstetrics

0 1986 Elsevier Science Publishers

and

Gynaecology,

B.V. (Biomedical

Diesion)

Rigshospitalet,

9

hypoxic metabolic acidosis may cause elevation of Pco, in fetal blood. The respiratory acidosis is caused by reduced CO, excretion by the placenta and the hypoxic metabolic acidosis is usually developed following a stage of respiratory acidosis [3]. Additionally, decreasing pH caused by lactic acid formation alters the carbon dioxide/bicarbonate equilibrium, inducing a Pco, increase [4]. Consequently, both kinds of acidosis should - at least temporarily - induce a Pco, change detectable by continuous monitoring. By fetal blood analysis (FBA), as introduced by Saling in 1961 [5] capillary blood pH and Pco, can be determined. Unfortunately, this only offers intermittent information, and repeated samples are often needed. Furthermore, every collection of blood requires vaginal examination and amnioscopy. Continuous tissue-pH measurements have been possible during the last 9 years, but in spite of promising results [6] this method is not suitable for routine monitoring because of technical problems with the electrode. Transcutaneous continuous Pco, measurements of the human fetus have been performed during the last years. The theoretical background, the practical use of the electrode and the clinical experience obtained by this monitoring until now will be described in this paper. Fetal carbon dioxide tension measured by fetal blood sampling Fetal Pco, can be measured in capillary blood obtained by FBA when the cervical dilatation is more than 2-3 cm and the membranes are ruptured. The fetal head is visualized by amnioscopy and the skin cleaned of amnion fluid, maternal blood and vernix. Following preparation of the skin with silicone an incision of l-2 mm is performed and the capillary blood is collected into heparinized glass tubes. Because of the gradient between Pco, in blood and the carbon dioxide tension of ambient air, it is essential that the blood droplets are collected into the tube as fast as possible. Saling [5] described CO, loss of 2-13s (mean 7.8%) following a delay of 2 s before collection, while Kubli [7] only found a 6% CO, loss after a delay of 10 s.

TABLE I Carbon dioxide tension (kPa) (mean k 2S.D.) measured by fetal blood sampling during uncomplicated labour cervical dilatation l-3 cm Saling [51 Kubli [7] Beard and Morris [50] G%rdmark [8] Wood [51]

caput against pelvic floor

caput in introitus

arteria umbilicalis

vena umbilicalis

6.5~k1.8

6.8f2.8

7.6k3.1 6.9 f 2.2 (n = 33) 6.9 + 2.8 (n=20) 6.1 f 2.2 (n =17)

5.8*1.8 5.7+ 1.8 (n = 35) 5.6+ 1.5 (n=23) 4.8 f 1.9 (n =17)

6-8 cm

5.9 f 2.3 6.0+2.3 5.9kl.7 6.2& 1.5 (n=24) (n=14) 5.9 k 1.6 5.6zlz1.6 (n=8) (n=4) 5.2k1.4 (n =17) 6.3 f ? (n = 21)

6.7 k 2.1 (n=20) 5.3kl.5 (n=lO)

5.9 f 2.4 (n=12) 5.3 f 2.1 (n =18)

207 After collection into a closed tube the CO, alteration until analysis is negligible even during a period of 30 min. The normal range of Pco, in capillary blood increases during labour (Table I). The mean values and upper limits are nearly identical in most studies, but in one study [8] the values are lower. This difference may be explained by the fact that the criteria of normality in the latter study were very strict, and by a different management of labour (psychoprophylaxis with hyperventilation and frequent augmentation of labour involving very short deliveries). In two of the studies [7,8] PCO, was also measured during pathological deliveries. Gardmark found significantly higher Pco, levels during the second stage if bradycardia or meconium-stained amnion fluid was present, but he found no difference between complicated and uncomplicated labours during the first stage. Kubli detected elevated Pco, values during the first stage of labour in most cases when the fetus was acidotic. However, both studies included very few pathological cases. Transcutaneous

measurements

History In 1793 John Abernethy [9] for the first time demonstrated carbon dioxide exchange through the intact skin by submerging his arm into mercury. He analysed the gas bubles accumulating above the mercury and found carbon dioxide. This observation was corroborated by Gerlach in 1851 [lo] by the demonstration of changes in oxygen and carbon dioxide tensions in an air-filled bladder glued to his chest. A century later Baumberger and Goodfriend [ll] demonstrated the value of heating the skin by submerging a finger into a warm phopshate buffer solution. They achieved an oxygen tension in the solution close to the arterial oxygen tension. The first electrochemical CO, electrode was constructed in 1954 by Stow and Randell [12] and improved by Severinghaus and Bradley [13]. Human transcutaneous measurements using this electrode were performed by Johns, Lindsay and Shepard in 1969 [14], but until thermostatted electrodes were constructed by Huch, Lubbers and Huch in 1973 [15] clinical use was difficult. Physiology The transcutaneous technique measures the gas tension of extracellular fluid in the upper layer of the skin. The measured value reflects the arterial gas tension modified by the dermal capillary blood flow and the production or consumption of gas in this area [16]. Transcutaneous oxygen measurements at normal skin temperature result in low values correlating badly to arterial values. Vasodilatation by heating the electrode and the skin below it causes increasing capillary blood flow and extracellular PO,. When the vessels are dilated maximally, a close correlation between arterial and transcutaneous values will be found. In contrast, the transcutaneous carbon dioxide tension is close to the capillary carbon dioxide tension during normal capillary blood flow, because of the much higher transmissibility of CO, through the

208 epidermal layers (CO, is 20-times more soluble than 0, in skin tissues [17]). The difference between the oxygen tension at the arterial end and the venous end of capillaries is 11 kPa (85 mmHg), but the same difference in the carbon dioxide tension is only 1 kPa (8 mmHg) [28]. Consequently, an increase of the capillary blood flow is not necessary for PCO, measurements. Nevertheless, some heating of the skin may be advantageous because of the faster skin diffusion and shorter stabilization time and in vivo response time. In the evaluation of carbon dioxide tension in the extracellular compartment of the skin, three separate layers of skin should be discussed. The deepest layer is formed by the corium with the capillary loops. Carbon dioxide tension in the extracellular fluid in this layer is identical with the capillary Pco,. The heating of the electrode causes an increased tissue temperature and an elevated tissue Pco,, just as Pco, in blood samples increases with temperature [18]. Using the anaerobic temperature coefficient of blood, this elevation can be calculated [19]. The intermediate layer is formed by the deeper part of the epidermis containing respiring cells, but no blood vessels. As carbon dioxide is produced in this layer, the Pco, is further elevated. The superficial layer consists of the non-living upper part of epidermis and PCO, is not altered, when diffusing through this layer. Consequently, the tc-Pco, value can be calculated as the arterial PCO, multiplied by a temperature correction factor. A constant, representing the amount of carbon dioxide produced in the lower layer of epidermis, should be added to this value. The anaerobic temperature coefficient of CO, in blood, (Y,is O.O21”C-’ [18]. The correction for a change of temperature from 37 o C to to C is calculated by the formula: Pco,(t)

= Pco,(37)

x 10*“-37’

Pco,(t) being the carbon dioxide tension at t”C and Pco,(37) being the carbon dioxide tension at 37°C. At the temperature of 44°C the correction factor will be 1.403. The production of carbon dioxide in the deep layer of the epidermis is modest and only responsible for a gradient of 0.5 kPa (3-4 mmHg) [20,21]. Sufficient dermal capillary blood flow is necessary for a close correlation between tc-Pco, and arterial Pco,. The heated electrode induces local dilatation of vessels, but during hypotension, as in shock and when local ischaemia is caused by mechanical compression of the skin, the dermal blood flow will be reduced in spite of local vasodilatation. During such circumstances the CO,, produced in the tissues under the electrode, may not be removed and large differences between tc-Pco, and the central artery Pco, may appear. Different

types of transducers

Transcutaneous measurements of PCO, have been performed with transducers based on five different technical principles: absorption photometry [22-241, fluorescence photometry [25], gas chromatography [26], mass spectrometry [26,27,52] and electrochemical methods (based on reduction-oxidation reactions) [29] or measurements by pH glass electrodes. While transducers for gas chromatography and mass spectrometry may monitor all gases (e.g. CO,, N,O, N2), the other transducers can

209

only measure one specific gas, e.g. CO,. In the papers describing absorption photometry, stratum corneum was removed from the measuring site before monitoring, as the transducers were not heated. The other transducers were used without any preparation of the skin. At present electrochemical electrodes are preferred in clinical trials, and only sporadic reports of fetal monitoring with prototypes of other transducers have been published [30]. Removal of stratum corneum before fetal monitoring is difficult and should be avoided to obtain a non-invasive monitoring. The transducers for gas chromatography and mass spectrometry are small and applicable for fetal monitoring, but the tubes connecting the transducers to the monitors lack flexibility. The monitors are large and need to be placed close to the parturient. Using electrochemical electrodes these disadvantages are avoided.

The electrochemical

electrode

The electrochemical electrode used in most studies of tc-Pco, monitoring during labour [2,31-33,35,36,43,47,53-551 is a thermostatted Stow-Severinghaus electrode consisting of a heating element, a temperature sensor, a glass pH electrode and a silver/silver chloride reference electrode (Fig. 1). The size of the electrodes from different manufacturers differs only negligibly; the electrode in Fig. 1 (Radiometer E5230) is 11 mm in height, 15 mm in diameter and weighs 3.5 g. The electrode is heated to a chosen temperature between 37 and 45°C. The carbon dioxide released from the skin diffuses through the gas-permeable membrane into the electrolyte, where it reacts with water to form carbonic acid, which immediately dissociates into HCO; and H+ according to the following equation: CO, + H,O

e H&O3

e H+ + HCO,

Fig. 1. Electrochemical tc-Pco, electrode (Radiometer E5230). 1. Electrode housing with epoxy. 2. Glass pH electrode. 3. Temperature sensor. 4. O-ring securing the membrane. 5. Electrolyte chamber. 6. Electrolyte covering the electrode surfase. 7. CO,-permeable membrane. 8. Heating element. 9. Reference electrode.

210

This involves changes in pH and the pH change according to the Henderson-Hasselbalch equation:

is converted

to a Pco,

reading

pH=pK+logz 2

where pK is the dissociation constant of carbonic acid, cHC0, is the concentration of HCO,, a the solubility coefficient of dissolved CO, and PCO, the partial pressure of CO,. Change in pH is measured as the change in potential between the glass electrode and the reference electrode. Consequently, the electrode does not measure the carbon dioxide tension directly, but it monitors pH, which is inversely proportional to log Pco,. In contrast to some other transducers, the electrode does not consume CO, and the monitoring does not influence the measured value [26,38,39]. The electrode is calibrated at 5% and 10% CO,/N, gas mixtures. In this procedure the electrode is corrected for drift and for sensitivity. In consequence of the indirect method of measurements, it is not possible to calibrate at zero. As the sensitivity of the electrode usually is unchanged between measurements, it is often sufficient to calibrate for drift only by a one-point calibration. The calibration is usually performed at the actual measuring temperature, but the composition of the inner electrolyte may be adjusted for calibration at one temperature and measuring at another without correction of the measuring value. Calibration is usually performed in vitro before monitoring. It can be performed in vivo [40], but no advantages seem to be obtained by this procedure. In some papers the correction for the difference between the electrode temperature and the body temperature is incorporated in the calibration procedure, i.e. the Pco, value displayed on the monitor is corrected to 37°C [33,41], making the evaluation of the results more simple for clinical use. For fetal monitoring the electrode must be cleaned and sterilized, and for that reason the electrode-membrane and electrolyte are renewed between each monitoring. Sterilization can be performed by formaldehyde steam [33], by ethylene oxide gas [42] or by fluid mixtures of aldehydes (KorsolinR [2,31,36,37] or UrolicideR [43]). With gas or steam sterilization the calibration must be performed after sterilization under sterile circumstances, while sterilization by fluid aldehydes is usually performed after the calibration. The ethylene oxide sterilization procedure is very slow, and sterilization with formaldehyde steam involves heating to 60°C diminishing the total lifetime of the electrode. Clinical use Tc-PCO, monitoring was originally introduced for neonatal [20,44,45] and anaesthesiological [22-24,461 monitoring in the hope of reducing the number of measurements of blood samples. Only recently, the method has been adopted by obstetricians. For fetal monitoring the electrode is applied on the fetal head head after rupture of membranes, provided the cervical OS is at least 3 cm dilated. The electrode may

Fig. 2. Tc-Pco,

electrode mounted in a suction ring,

be attached by glue (Zbutylcyanoacrylat (Histoacryl)) [34,43] or by suction (- 20 kPa) [2,31,36] (Fig. 2). In contrast to the glue fixation, suction fixation is simple and requires neither amnioscopy nor preparation of the skin. Application by suction is performed during vaginal examination without discomfort to the parturient. Both fixation methods are reliable and the readings correlate well (Nickelsen et Weber, unpublished data). The tc-Pco, value is displayed digitally and graphically on the monitor together with the effect (mW) used for heating the electrode. The effect reflects the capillary blood flow under the electrode. Additionally, the tc-Pco, value can be plotted on the cardiotocogram (Fig. 3). The stabilization time and the in vivo response time depend on the temperature of the electrode. The 95% stabilization time (i.e. the time from application until 95% of the stable value is obtained) is approximately 2 min at 44°C [31]. Monitoring also depends on the in vivo response time (i.e. the time during monitoring from when an alteration in Pco, happens until 95% of the new value is recorded). The in vivo response time cannot be measured in clinical trials, but in animal experiments a reduction of the measuring temperature from 44 to 39°C doubled the response time [34]. Stabilization time and response time are important at the start of monitoring, at sudden changes in Pco,, and at reapplication during monitoring. The in vitro electrode drift is usually less than 1% per hour and the in vivo drift in clinical studies has been less than 8% during the monitoring period [2,33,36]. Calculation of the drift per hour during monitoring has no meaning, as the drift develops stepwise and possibly mostly during application and removal of the electrode. Correction for drift is recommended every fourth hour during neonatal and anaesthesiological monitoring, but this is not feasible during fetal monitoring as it would imply re-sterilization.

Fig. 3. Cardiotocogram with the fetal heart rate (FHR) and the transcutaneous carbon dioxide (bottom of vertical lines) in the upper part and the labour registration in the lower part.

tension

In previous studies (Table II) close correlations were found between tc-Pco, and blood Pco, measured by FBA or in the umbilical artery just after delivery. The degree of caput succedaneum has only little influence on tc-Pco,, as the correlation between transcutaneous and capillary blood values is only insignificantly lessened by pronounced caput succedaneum [47].

TABLE

II

Correlations between blood Pco, and transcutaneous Pco, during labour. The values in the formula regression line are corrected for temperature. All values are expressed in kPa Blood from

Electrode temp.

FBS FBS (l.stadium) FBS (2stadium) Arteria umbilicalis Arteria umbilicalis Arteria umbilicalis Arteria umbilicalis Arteria umbilicalis

44 39 39 44 44 39 44 44

Temp. fact.

No.

Correlation coefficient

Regression-line formula

1.40 1.15 1.15 1.40 1.40 1.15 1.40 1.40

10 45 66 49 15 22 21 64

0.96 0.83 0.66 0.60 0.91 0.68 0.69 0.77

tc-Pco, tc-Pco, tc-Pco, tc-Pco, tc-Pco, tc-Pco, tc-Pco, tc-Pco,

(“C) Hansen 1311 Schmidt‘[33] Schmidt [33] Hansen [31] Thomsen and Weber [2] Schmidt [33] Nickelsen et al. [36] Nickelsen et al.

= = = = = = = =

1.37 0.98 0.87 1.10 0.92 0.64 1.04 0.98

Pco, Pco, Pco, Pco, Pco, Pco, Pco, Pco,

for

- 1.91 + 0.45 + 1.33 -0.95 + 0.55 + 2.77 - 0.07 + 0.41

213 The assumption that PCO, is elevated in all acidotic fetuses has been corroborated by fetal tc-Pco, studies [32], as a correlation between capillary pH (measured by FBA) and tc-Pco, was found, and as tc-Pco, values below 7.5 kPa (at 37°C) always imply capillary pH above 7.25 (upper limit for fetal preacidosis). Transcutaneous carbon dioxide can be monitored together with tissue pH, making it possible to diagnose and analyse an acidosis for the respiratory and metabolic components [36,37]. Using the same electrodes, on line calculation of base excess [48] offers a rapid diagnosis of developing metabolic acidosis. Comparison between transcutaneous/blood correlations in the published studies can only be performed if the electrode temperature and possible corrections during calibration are known. In Table II all values are expressed in kPa, and temperature correction to 37°C is performed by the coefficient suggested by the author. No correction for metabolism in the epidermis is incorporated. In most papers the slope of the regression line is approximately 1.0, the positive intercept being approximately the value of the epidermal CO? production. At prolonged measurements, neonatal monitoring at 44°C may involve erythema at the electrode site [44,49]. The erythema disappears within a few days and no irreversible skin damage has been reported. On some occations, fetal monitoring has led to similar skin reactions (S. Schmidt, personal communication), but this complication never appeared following our measurements. No skin reactions are observed if the electrode position is changed every fourth hour or if lower electrode temperatures are chosen. Using suction fixation, change of electrode position every third or fourth hour is simple, thus avoiding any risk of thermal injuries. No other complications have been observed in connection with the method.

Conclusion Previous studies of fetal PCO, values during labour were based on fetal blood sampling and relatively few observations. PCO, levels during pathological deliveries seem to be elevated, but the intermittent character of FBA impedes the detection of sudden changes caused by acute complications. The present study of continuous tc-Pco, monitoring documents its reliability and the close correlation with arterial Pco,. Electrode application and monitoring are simple, the response time is short, the success rate is high, and no serious complications have been reported. Large numbers of samples are necessary to define the normal range of fetal Pco, as well as its predictive values. Both sensitivity and specificity for predicting fetal acidosis must be evaluated before the method can be used for routine fetal monitoring. The ideal monitoring should probably include both pH and Pco,, as studies have revealed hypoxic metabolic acidosis to be the most dangerous threat to the fetus. Future studies must reveal whether tc-Pco, monitoring alone or combined with cardiotocography is sufficient to detect this condition. At present the method should be reserved for scientific purposes, but the non-invasive character makes it a promising tool in future clinical routine.

214

Acknowledgements Some of the tc-Pco, monitorings referred to were performed according to the EEC-project: Perinatal Monitoring - evaluation of transcutaneous carbon dioxide monitoring during labour (project leader H.P. van Geijn). Co-operation between European centres is supported by EEC funds. References 1 Myers RE. Two patterns of perinatal brain damage and their conditions of occurrence. Am J Obstet Gynecol 1972; 112: 246-276. 2 Thomsen SG, Weber T. Fetal transcutaneous carbon dioxide tension during the second stage of labour. Br J Obstet Gynaecol 1984; 91: 1103-1106. 3 Saling E. Das Kind im Bereich der Geburtshilfe. Stuttgart, Georg Thieme Verlag; 1966. 4 Goodlin RC, Kaiser IH. The effect of ammonium chloride induced maternal acidosis on the human fetus at term: I.pH, Hemoglobin, Blood gases. Am Med Sci 1957; 233: 662-674. 5 Saling E. Die Blutgasverhaltnisse und der Satire-Basen-Haushalt des Feten bei ungestiirtem Geburtsablauf. Geburtsh Gynaecol 1964; 161: 262-292. 6 Weber T. Cardiotocography supplemented with continuous fetal pH monitoring during labour. Acta Obstet Gynecol Stand 1982; 61: 351-355. 7 Kubli F. lntrapartale Asphyxia und ihre Diagnose. In: Kubli F, ed. Fetale Gefahrenzustande und ihre Diagnose. Stuttgart, Georg Thieme: 1966: 49-88. 8 Gardmark S. (ed.) Fetal and maternal acid-base balance, carbohydrate and lipid metabolism in relation to different signs of fetal distress during labour. In: Studies on acid-base balance, carbohydrate and lipid metabolism in human fetal and maternal blood in clinical and experimental conditions during labour. Malmii, Studentlitteratur; 1974: 145-183. 9 Abernethy J. An essay of the nature of the matter perspired and absorbed from the skin. In: Surgical and Physiological Essays, part 2. London, 1793; 107-165. 10 Gerlach JV. ijber das Hautathmen. Archiv Anatom Physiol Leipzig 1851; 431-479. 11 Baumberger JP, Goodfriend RB. Determination of arterial oxygen tension in man by equilibration through intact skin. Fed Proc 1951; 10: 10-11. 12 Stow RW, Randall BF. Electrical measurement of the pC0, of blood. Am J Physiol 1954; 179: 678. 13 Severinghaus JW, Bradley AF. Electrodes for blood ~0, and pC0, determination. J Appl Physiol 1958; 13: 515-520. 14 Johns RJ, Lindsay WJ, Shephard RH. A system for monitoring pulmonary ventilation. Biomed Sci Instrum 1969; 5: 119-121. 15 Huch A, Lubbers DW, Huch R. Patientenliberwachung durch transcutane pC0, Messung bei gleichzeitiger Kontrolle der relative lokalen Perfusion. Anaesthesist 1973; 22: 379-380. 16 Beran AV, Huxtable RF, Shigezawa GY, Yeung HN. In vivo evaluation of transcutaneous CO, partial pressure monitoring. Appl Physiol 1981; 50: 1220-1223. 17 Hebrank DR, Mentelos RA. Noninvasive transcutaneous carbon dioxide monitoring. Medical Instrum 1981; 14: 203-206. 18 Siggard-Andersen 0. The acid base status of the blood. 4th edn. Copenhagen, Munksgaard; 1974. 19 Herrell N, Martin RJ, Pultusker M, Lough M, Fanaroff A. Optimal temperature for measurement of transcutaneous carbon dioxide tension in the neonate. Pediat 1980; 97: 114-117. 20 Hazinski TA, Severinghaus JW. Transcutaneous analysis of arterial pC0,. Medical Instrum 1982; 16: 150-153. 21 Severinghaus JW, Stafford M, Bradley AF. TcpCO, electrode design, calibration and temperature gradient problems. Acta Anaesthesiol Stand 1978; suppl 68: 118-122. 22 McLellan PA, Goldstein RS, Ramcharan V, Rebuck AS. Transcutaneous carbon dioxide monitoring. Am Rev Respir Dis 1981; 124: 199-201. 23 Greenspan GH, Block AJ, Haldeman LW, Lindsey S, Martin CS. Transcutaneous noninvasive monitoring of carbon dioxide tension. Chest 1981; 80: 442-446.

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216 47 Langner K, Schmidt S, Saling E. Method&he Besonderheiten der transkutane pC0, - Messung beim Feten sub partu. In: Dudenhausen J, Saling E, eds. Perinatale Medicine, Vol. 10. Stuttgart, Georg Thieme, 1984; 373-375. 48 Nickelsen C, Weber T. Continuous standard base excess monitoring during human labour. Eur J Obstet Gynecol Reprod Biol 1986; 21: 7-14. 49 Martin RJ, Herrell N, Pultusker M. Transcutaneous measurement of carbon dioxide tension: Effect of sleep state in term infants. Pediatrics 1981; 67: 622-625. 50 Beard RW, Morris ED. Foetal and maternal acid-base balance during normal labour. Obstet Gynaecol Br Commonw 1965; 72: 4966506. 51 Wood C. Diagnostic and therapeutic implications of intrapartum fetal pH measurement. Acta Obstet Gynecol Stand 1978; 57: 13-18. 52 Mcllroy MB, Simbruner G, Sonoda Y. Transcutaneous blood gas measurements using a mass spectrometer. Acta Anaesthesiol Stand 1978; suppl 68: 1288130. 53 Schmidt S, Langner K, Rothe J, Saling E. A new combined non-invasive electrode for tcpCO,-measurement and fetal heart rate recording. J Perinat Med 1982; 10: 297-300. 54 Schmidt S, Langner K, Gesche J, Dudenhausen JW, Saling E. Der transkutan gemessene Kohlendioxydpartialdruck beim nichthypoxischen Feten wlhrend der Geburt. Geburtsh Frauenheilk 1983; 43: 538-541. 55 Schmidt S, Langner K, Rothe J, Saling E. Eine neue Elektrode zur kombinierten tcpCO,und CTG-Registrierung beim Feten sub partu. Erste Ergebnisse. Geburtsh Frauenheilk 1983; 43: 59.